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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: June 22, 2020 Bibliotheque nationale de France 6 December 20, 2019 8 The ARK Identifier Scheme 9 draft-kunze-ark-23 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 [http://NMAH/]ark:[/]NAAN/Name[Qualifier] 25 an optional and mutable Name Mapping Authority Hostport (usually a 26 hostname), the "ark:" label, the Name Assigning Authority Number 27 (NAAN), the assigned Name, and an optional and possibly mutable 28 Qualifier supported by the NMA. The NAAN and Name together form the 29 immutable persistent identifier for the object independent of the URL 30 hostname. An ARK is a special kind of URL that connects users to 31 three things: the named object, its metadata, and the provider's 32 promise about its persistence. When entered into the location field 33 of a Web browser, the ARK leads the user to the named object. That 34 same ARK, inflected by appending `?info', returns a metadata record 35 that is both human- and machine-readable. The returned record 36 contains core metadata and a commitment statement from the current 37 provider. Tools exist for 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 June 22, 2020. 56 Copyright Notice 58 Copyright (c) 2019 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 . . . . . . . . . . . . . . . . 8 75 2. ARK Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . 8 76 2.1. The Name Mapping Authority Hostport (NMAH) . . . . . . . 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 NMAHs in a Globally Accessible File . . . . . 26 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 NMAHs Distributed via DNS . . . . . . . . 37 106 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. No breaking changes from the 2008 114 spec are envisaged. Some minor changes are being deferred to later 115 in order to make it easier to review more important changes; some of 116 those small changes would result in "noisy diffs" since they are 117 global in scope, for example, converting all instances of http:// and 118 NMAH to https:// and NMA, respectively. ] 120 This document describes a scheme for the high-quality naming of 121 information resources. The scheme, called the Archival Resource Key 122 (ARK), is well suited to long-term access and identification of any 123 information resources that accommodate reasonably regular electronic 124 description. This includes digital documents, databases, software, 125 and websites, as well as physical objects (books, bones, statues, 126 etc.) and intangible objects (chemicals, diseases, vocabulary terms, 127 performances). Hereafter the term "object" refers to an information 128 resource. The term ARK itself refers both to the scheme and to any 129 single identifier that conforms to it. A reasonably concise and 130 accessible overview and rationale for the scheme is available at 131 [ARK]. 133 Schemes for persistent identification of network-accessible objects 134 are not new. In the early 1990's, the design of the Uniform Resource 135 Name [RFC2141] responded to the observed failure rate of URLs by 136 articulating an indirect, non-hostname-based naming scheme and the 137 need for responsible name management. Meanwhile, promoters of the 138 Digital Object Identifier [DOI] succeeded in building a community of 139 providers around a mature software system [Handle] that supports name 140 management. The Persistent Uniform Resource Locator [PURL] was 141 another scheme that had the advantage of working with unmodified web 142 browsers. ARKs represent an approach that attempts to build on the 143 strengths and to avoid the weaknesses of these schemes. 145 A founding principle of the ARK is that persistence is purely a 146 matter of service. Persistence is neither inherent in an object nor 147 conferred on it by a particular naming syntax. Nor is the technique 148 of name indirection -- upon which URNs, Handles, DOIs, and PURLs are 149 founded -- of central importance. Name indirection is an ancient and 150 well-understood practice; new mechanisms for it keep appearing and 151 distracting practitioner attention, with the Domain Name System (DNS) 152 [RFC1034] being a particularly dazzling and elegant example. What is 153 often forgotten is that maintenance of an indirection table is an 154 unavoidable cost to the organization providing persistence, and that 155 cost is equivalent across naming schemes. That indirection has 156 always been a native part of the web while being so lightly utilized 157 for the persistence of web-based objects indicates how unsuited most 158 organizations will probably be to the task of table maintenance and 159 to the much more fundamental challenge of keeping the objects 160 themselves viable. 162 Persistence is achieved through a provider's successful stewardship 163 of objects and their identifiers. The highest level of persistence 164 will be reinforced by a provider's robust contingency, redundancy, 165 and succession strategies. It is further safeguarded to the extent 166 that a provider's mission is shielded from funding and political 167 instabilities. These are by far the major challenges confronting 168 persistence providers, and no identifier scheme has any direct impact 169 on them. In fact, some schemes may actually be liabilities for 170 persistence because they create short- and long-term dependencies for 171 every object access on complex, special-purpose infrastructures, 172 parts of which are proprietary and all of which increase the carry- 173 forward burden for the preservation community. It is for this reason 174 that the ARK scheme relies only on educated name assignment and light 175 use of general-purpose infrastructures that are maintained mostly by 176 the internet community at large (the DNS, web servers, and web 177 browsers). 179 1.1. Reasons to Use ARKs 181 If no persistent identifier scheme contributes directly to 182 persistence, why not just use URLs? A particular URL may be as 183 durable an identifier as it is possible to have, but nothing 184 distinguishes it from an ordinary URL to the recipient who is 185 wondering if it is suitable for long-term reference. An ARK embedded 186 in a URL provides some of the necessary conditions for credible 187 persistence, inviting access to not one, but to three things: to the 188 object, to its metadata, and to a nuanced statement of commitment 189 from the provider in question (the NMA, described below) regarding 190 the object. Existence of the extra service can be probed 191 automatically by appending `?info' to the ARK. 193 The form of the ARK also supports the natural separation of naming 194 authorities into the original name assigning authority and the 195 diverse multiple name mapping (or servicing) authorities that in 196 succession and in parallel will take over custodial responsibilities 197 from the original assigner (assuming the assigner ever held that 198 responsibility) for the large majority of a long-term object's 199 archival lifetime. The name mapping authority, indicated by the 200 hostname part of the URL that contains the ARK, serves to launch the 201 ARK into cyberspace. Should it ever fail (and there is no reason why 202 a well-chosen hostname for a 100-year-old cultural memory institution 203 shouldn't last as long as the DNS), that host name is considered 204 disposeable and replaceable. Again, the form of the ARK helps 205 because it defines exactly how to recover the core immutable object 206 identity, and simple algorithms (one based on the URN model) or even 207 by-hand internet query can be used for for locating another mapping 208 authority. 210 There are tools to assist in generating ARKs and other identifiers, 211 such as [NOID] and "uuidgen", both of which rely for uniqueness on 212 human-maintained registries. This document also contains some 213 guidelines and considerations for managing namespaces and choosing 214 hostnames with persistence in mind. 216 1.2. Three Requirements of ARKs 218 The first requirement of an ARK is to give users a link from an 219 object to a promise of stewardship for it. That promise is a multi- 220 faceted covenant that binds the word of an identified service 221 provider to a specific set of responsibilities. It is critical for 222 the promise to come from a current provider and almost irrelevant, 223 over a long period of time, what the original assigner's intentions 224 were. No one can tell if successful stewardship will take place 225 because no one can predict the future. Reasonable conjecture, 226 however, may be based on past performance. There must be a way to 227 tie a promise of persistence to a provider's demonstrated or 228 perceived ability -- its reputation -- in that arena. Provider 229 reputations would then rise and fall as promises are observed 230 variously to be kept and broken. This is perhaps the best way we 231 have for gauging the strength of any persistence promise. 233 The second requirement of an ARK is to give users a link from an 234 object to a description of it. The problem with a naked identifier 235 is that without a description real identification is incomplete. 236 Identifiers common today are relatively opaque, though some contain 237 ad hoc clues reflecting assertions that were briefly true, such as 238 where in a filesystem hierarchy an object lived during a short stay. 239 Possession of both an identifier and an object is some improvement, 240 but positive identification may still be uncertain since the object 241 itself might not include a matching identifier or might not carry 242 evidence obvious enough to reveal its identity without significant 243 research. In either case, what is called for is a record bearing 244 witness to the identifier's association with the object, as supported 245 by a recorded set of object characteristics. This descriptive record 246 is partly an identification "receipt" with which users and archivists 247 can verify an object's identity after brief inspection and a 248 plausible match with recorded characteristics such as title and size. 250 The final requirement of an ARK is to give users a link to the object 251 itself (or to a copy) if at all possible. High quality access is the 252 central duty of an ARK. Persistent identification plays a vital 253 supporting role but, strictly speaking, it can be construed as no 254 more than a record attesting to the original assignment of a never- 255 reassigned identifier. Object access may not be feasible for various 256 reasons, such as a transient service outage, a catastrophic loss, a 257 licensing agreement that keeps an archive "dark" for a period of 258 years, or when an object's own lack of tangible existence confuses 259 normal concepts of access (e.g., a vocabulary term might be 260 "accessed" through its definition). In such cases the ARK's 261 identification role assumes a much higher profile. But attempts to 262 simplify the persistence problem by decoupling access from 263 identification and concentrating exclusively on the latter are of 264 questionable utility. A perfect system for assigning forever unique 265 identifiers might be created, but if it did so without reducing 266 access failure rates, no one would be interested. The central issue 267 -- which may be summed up as the "HTTP 404 Not Found" problem -- 268 would not have been addressed. 270 ARK resolvers must support the `?info' inflection for requesting 271 metadata. Older versions of this specification distinguished between 272 two minimal inflections: `?' (brief metadata) and `??' (more 273 metadata). While these older inflections are still reserved, because 274 they have proven hard to recognize in some environments, supporting 275 them is optional. 277 1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff 279 An organization and the user community it serves can often be seen to 280 struggle with two different areas of persistent identification: the 281 Our Stuff problem and the Their Stuff problem. In the Our Stuff 282 problem, we in the organization want our own objects to acquire 283 persistent names. Since we possess or control these objects, our 284 organization tackles the Our Stuff problem directly. Whether or not 285 the objects are named by ARKs, our organization is the responsible 286 party, so it can plan for, maintain, and make commitments about the 287 objects. 289 In the Their Stuff problem, we in the organization want others' 290 objects to acquire persistent names. These are objects that we do 291 not own or control, but some of which are critically important to us. 292 But because they are beyond our influence as far as support is 293 concerned, creating and maintaining persistent identifiers for Their 294 Stuff is not especially purposeful or feasible for us to engage in. 295 There is little that we can do about someone else's stuff except 296 encourage their uptake or adoption of persistence services. 298 Co-location of persistent access and identification services is 299 natural. Any organization that undertakes ongoing support of true 300 persistent identification (which includes description) is well-served 301 if it controls, owns, or otherwise has clear internal access to the 302 identified objects, and this gives it an advantage if it wishes also 303 to support persistent access to outsiders. Conversely, persistent 304 access to outsiders requires orderly internal collection management 305 procedures that include monitoring, acquisition, verification, and 306 change control over objects, which in turn requires object 307 identifiers persistent enough to support auditable record keeping 308 practices. 310 Although organizing ARK support under one roof thus tends to make 311 sense, object hosting can successfully be separated from name 312 mapping. An example is when a name mapping authority centrally 313 provides uniform resolution services via a protocol gateway on behalf 314 of organizations that host objects behind a variety of access 315 protocols. It is also reasonable to build value-added description 316 services that rely on the underlying services of a set of mapping 317 authorities. 319 Supporting ARKs is not for every organization. By requiring 320 specific, revealed commitments to preservation, to object access, and 321 to description, the bar for providing ARK services is higher than for 322 some other identifier schemes. On the other hand, it would be hard 323 to grant credence to a persistence promise from an organization that 324 could not muster the minimum ARK services. Not that there isn't a 325 business model for an ARK-like, description-only service built on top 326 of another organization's full complement of ARK services. For 327 example, there might be competition at the description level for 328 abstracting and indexing a body of scientific literature archived in 329 a combination of open and fee-based repositories. The description- 330 only service would have no direct commitment to the objects, but 331 would act as an intermediary, forwarding commitment statements from 332 object hosting services to requestors. 334 1.4. Definition of Identifier 336 An identifier is not a string of character data -- an identifier is 337 an association between a string of data and an object. This 338 abstraction is necessary because without it a string is just data. 339 It's nonsense to talk about a string's breaking, or about its being 340 strong, maintained, and authentic. But as a representative of an 341 association, a string can do, metaphorically, the things that we 342 expect of it. 344 Without regard to whether an object is physical, digital, or 345 conceptual, to identify it is to claim an association between it and 346 a representative string, such as "Jane" or "ISBN 0596000278". What 347 gives a claim credibility is a set of verifiable assertions, or 348 metadata, about the object, such as age, height, title, or number of 349 pages. In other words, the association is made manifest by a record 350 (e.g., a cataloging or other metadata record) that vouches for it. 352 In the complete absence of any testimony (metadata) regarding an 353 association, a would-be identifier string is a meaningless sequence 354 of characters. To keep an externally visible but otherwise internal 355 string from being perceived as an identifier by outsiders, for 356 example, it suffices for an organization not to disclose the nature 357 of its association. For our immediate purpose, actual existence of 358 an association record is more important than its authenticity or 359 verifiability, which are outside the scope of this specification. 361 It is a gift to the identification process if an object carries its 362 own name as an inseparable part of itself, such as an identifier 363 imprinted on the first page of a document or embedded in a data 364 structure element of a digital document header. In cases where the 365 object is large, unwieldy, or unavailable (such as when licensing 366 restrictions are in effect), a metadata record that includes the 367 identifier string will usually suffice. That record becomes a 368 conveniently manipulable object surrogate, acting as both an 369 association "receipt" and "declaration". 371 Note that our definition of identifier extends the one in use for 372 Uniform Resource Identifiers [RFC3986]. The present document still 373 sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for 374 the string part of an identifier, but the context should make the 375 meaning clear. 377 2. ARK Anatomy 379 An ARK is represented by a sequence of characters (a string) that 380 contains the label, "ark:", optionally preceded by the beginning part 381 of a URL. Here is a diagrammed example. 383 ARK ANATOMY Core Immutable Identity 384 =========== _______________________________ 385 / \ 386 Resolver Service Base Object Name Qualifiers 387 _________________ ________________ _____________ 388 / \/ \/ \ 389 http://example.org/ark:12025/654xz321/s3/f8.05v.tiff 390 \_________/ \__/\___/ \______/\____/\_______/ 391 | | | | | | 392 | Label | | Sub-parts Variants 393 | | | 394 Name Mapping Authority | Assigned Name 395 Hostport (NMAH) | 396 Name Assigning Authority Number (NAAN) 398 The ARK syntax can be summarized, 400 [http://NMAH/]ark:[/]NAAN/Name[Qualifier] 402 where the NMAH, '/', and Qualifier parts are in brackets to indicate 403 that they are optional. The Base Object Name is the substring 404 comprising the "ark:" label, the NAAN and the assigned Name. The 405 Resolver Service is replaceable and makes the ARK actionable for a 406 period of time. Without the Resolver Service part, what remains is 407 the Core Immutable Identity (the "persistible") part of the ARK. 409 2.1. The Name Mapping Authority Hostport (NMAH) 411 Before the "ark:" label may appear an optional Name Mapping Authority 412 Hostport (NMAH) that is a temporary address where ARK service 413 requests may be sent. Preceded by a URI-type protocol designation 414 such as "https://", it specifies a Resolver Service. The NMAH itself 415 is an Internet hostname or hostport combination having the same 416 format and semantics as the hostport part of a URL. The most 417 important thing about the NMAH is that it is "identity inert" from 418 the point of view of object identification. In other words, ARKs 419 that differ only in the optional NMAH part identify the same object. 420 Thus, for example, the following three ARKs are synonyms for just one 421 information object: 423 http://loc.gov/ark:12025/654xz321 424 http://rutgers.edu/ark:12025/654xz321 425 ark:12025/654xz321 427 Strictly speaking, in the realm of digital objects, these ARKs may 428 lead over time to somewhat different or diverging instances of the 429 originally named object. In an ideal world, divergence of persistent 430 objects is not desirable, but it is widely believed that digital 431 preservation efforts will inevitably lead to alterations in some 432 original objects (e.g, a format migration in order to preserve the 433 ability to display a document). If any of those objects are held 434 redundantly in more than one organization (a common preservation 435 strategy), chances are small that all holding organizations will 436 perform the same precise transformations and all maintain the same 437 object metadata. More significant divergence would be expected when 438 the holding organizations serve different audiences or compete with 439 each other. 441 The NMAH part makes an ARK into an actionable URL. As with many 442 internet parameters, it is helpful to approach the NMAH being liberal 443 in what you accept and conservative in what you propose. From the 444 recipient's point of view, the NMAH part should be treated as 445 temporary, disposable, and replaceable. From the NMA's point of 446 view, it should be chosen with the greatest concern for longevity. A 447 carefully chosen NMAH should be at least as permanent as the 448 providing organization's own hostname. In the case of a national or 449 university library, for example, there is no reason why the NMAH 450 should not be considerably more permanent than soft-funded proxy 451 hostnames such as hdl.handle.net, dx.doi.org, and purl.org. In 452 general and over time, however, it is not unexpected for an NMAH 453 eventually to stop working and require replacement with the NMAH of a 454 currently active service provider. 456 This replacement relies on a mapping authority "resolver" discovery 457 process, of which two alternate methods are outlined in a later 458 section. The ARK, URN, Handle, and DOI schemes all use a resolver 459 discovery model that sooner or later requires matching the original 460 assigning authority with a current provider servicing that 461 authority's named objects; once found, the resolver at that provider 462 performs what amounts to a redirect to a place where the object is 463 currently held. All the schemes rely on the ongoing functionality of 464 currently mainstream technologies such as the Domain Name System 465 [RFC1034] and web browsers. The Handle and DOI schemes in addition 466 require that the Handle protocol layer and global server grid be 467 available at all times. 469 The practice of prepending "http://" and an NMAH to an ARK is a way 470 of creating an actionable identifier by a method that is itself 471 temporary. Assuming that infrastructure supporting [RFC2616] 472 information retrieval will no longer be available one day, ARKs will 473 then have to be converted into new kinds of actionable identifiers. 474 By that time, if ARKs see widespread use, web browsers would 475 presumably evolve to perform this (currently simple) transformation 476 automatically. 478 2.2. The ARK Label Part (ark:) 480 The label part distinguishes an ARK from an ordinary identifier. 481 There is a new form of the label, "ark:", and an old form, "ark:/", 482 both of which must be recognized in perpetuity. Implementations 483 should generate new ARKs in the new form (without the "/") and 484 resolvers must always treat received ARKs as equivalent if they 485 differ only in regard to new form versus old form labels. Thus these 486 two ARKs are equivalent: 488 ark:/12025/654xz321 489 ark:12025/654xz321 491 In a URL found in the wild, the label indicates that the URL stands a 492 reasonable chance of being an ARK. If the context warrants, 493 verification that it actually is an ARK can be done by testing it for 494 existence of the three ARK services. 496 Since nothing about an identifier syntax directly affects 497 persistence, the "ark:" label (like "urn:", "doi:", and "hdl:") 498 cannot tell you whether the identifier is persistent or whether the 499 object is available. It does tell you that the original Name 500 Assigning Authority (NAA) had some sort of hopes for it, but it 501 doesn't tell you whether that NAA is still in existence, or whether a 502 decade ago it ceased to have any responsibility for providing 503 persistence, or whether it ever had any responsibility beyond naming. 505 Only a current provider can say for certain what sort of commitment 506 it intends, and the ARK label suggests that you can query the NMAH 507 directly to find out exactly what kind of persistence is promised. 508 Even if what is promised is impersistence (i.e., a short-term 509 identifier), saying so is valuable information to the recipient. 510 Thus an ARK is a high-functioning identifier in the sense that it 511 provides access to the object, the metadata, and a commitment 512 statement, even if the commitment is explicitly very weak. 514 2.3. The Name Assigning Authority Number (NAAN) 516 Recalling that the general form of the ARK is, 518 [http://NMAH/]ark:[/]NAAN/Name[Qualifier] 520 the part of the ARK directly following the "ark:" (or older "ark:/") 521 label is the Name Assigning Authority Number (NAAN), up to but not 522 including the next `/' (slash) character. This part is always 523 required, as it identifies the organization that originally assigned 524 the Name of the object. xxx It is used to discover a currently valid 525 NMAH and to provide top-level partitioning of the space of all ARKs. 527 xxx define organization to be an institution, a department, or 528 possibly any organized and stable name assigning effort An 529 organization may request a NAAN from the ARK Maintenance Agency 530 [ARKagency] (described in Appendix A) by filling out the form at 531 [NAANrequest]. NAANs are opaque strings of one or more characters 532 drawn from this set, 534 0123456789bcdfghjkmnpqrstvwxz 536 which consists of digits and consonants, minus the letter 'l'. 537 Restricting NAANs to this set serves two goals. It reduces the 538 chances that words -- past, present, and future -- will appear in 539 NAANs and carry unintended semantics. It also helps usability by not 540 mixing commonly confused characters ('0' and 'O', '1' and 'l') and by 541 being compatible with strong transcription error detection (eg, the 542 [NOID] check digit algorithm). Since 2001, every assigned NAAN has 543 consisted of exactly five digits, and no immediate change in that 544 practice is foreseen. 546 The NAAN designates a top-level ARK namespace. Once registered for a 547 namespace, a NAAN is never re-registered. It is possible, however, 548 for there to be a succession of organizations that manage an ARK 549 namespace. 551 2.4. The Name Part 553 The part of the ARK just after the NAAN is the Name assigned by the 554 NAA, and it is also required. Semantic opaqueness in the Name part 555 is strongly encouraged in order to reduce an ARK's vulnerability to 556 era- and language-specific change. Identifier strings containing 557 linguistic fragments can create support difficulties down the road. 558 No matter how appropriate or even meaningless they are today, such 559 fragments may one day create confusion, give offense, or infringe on 560 a trademark as the semantic environment around us and our communities 561 evolves. 563 Names that look more or less like numbers avoid common problems that 564 defeat persistence and international acceptance. The use of digits 565 is highly recommended. Mixing in non-vowel alphabetic characters a 566 couple at a time is a relatively safe and easy way to achieve a 567 denser namespace (more possible names for a given length of the name 568 string). Such names have a chance of aging and traveling well. 569 Tools exists that mint, bind, and resolve opaque identifiers, with or 570 without check characters [NOID]. More on naming considerations is 571 given in a subsequent section. 573 2.5. The Qualifier Part 575 The part of the ARK following the NAA-assigned Name is an optional 576 Qualifier. It is a string that extends the base ARK in order to 577 create a kind of service entry point into the object named by the 578 NAA. At the discretion of the providing NMA, such a service entry 579 point permits an ARK to support access to individual hierarchical 580 components and subcomponents of an object, and to variants (versions, 581 languages, formats) of components. A Qualifier may be invented by 582 the NAA or by any NMA servicing the object. 584 In form, the Qualifier is a ComponentPath, or a VariantPath, or a 585 ComponentPath followed by a VariantPath. A VariantPath is introduced 586 and subdivided by the reserved character `.', and a ComponentPath is 587 introduced and subdivided by the reserved character `/'. In this 588 example, 590 http://example.org/ark:12025/654xz321/s3/f8.05v.tiff 592 the string "/s3/f8" is a ComponentPath and the string ".05v.tiff" is 593 a VariantPath. The ARK Qualifier is a formalization of some 594 currently mainstream URL syntax conventions. This formalization 595 specifically reserves meanings that permit recipients to make strong 596 inferences about logical sub-object containment and equivalence based 597 only on the form of the received identifiers; there is great 598 efficiency in not having to inspect metadata records to discover such 599 relationships. NMAs are free not to disclose any of these 600 relationships merely by avoiding the reserved characters above. 601 Hierarchical components and variants are discussed further in the 602 next two sections. 604 The Qualifier, if present, differs from the Name in several important 605 respects. First, a Qualifier may have been assigned either by the 606 NAA or later by the NMA. The assignment of a Qualifier by an NMA 607 effectively amounts to an act of publishing a service entry point 608 within the conceptual object originally named by the NAA. For our 609 purposes, an ARK extended with a Qualifier assigned by an NMA will be 610 called an NMA-qualified ARK. 612 Second, a Qualifier assignment on the part of an NMA is made in 613 fulfillment of its service obligations and may reflect changing 614 service expectations and technology requirements. NMA-qualified ARKs 615 could therefore be transient, even if the base, unqualified ARK is 616 persistent. For example, it would be reasonable for an NMA to 617 support access to an image object through an actionable ARK that is 618 considered persistent even if the experience of that access changes 619 as linking, labeling, and presentation conventions evolve and as 620 format and security standards are updated. For an image "thumbnail", 621 that NMA could also support an NMA-qualified ARK that is considered 622 impersistent because the thumbnail will be replaced with higher 623 resolution images as network bandwidth and CPU speeds increase. At 624 the same time, for an originally scanned, high-resolution master, the 625 NMA could publish an NMA-qualfied ARK that is itself considered 626 persistent. Of course, the NMA must be able to return its separate 627 commitments to unqualified, NAA-assigned ARKs, to NMA-qualified ARKs, 628 and to any NAA-qualified ARKs that it supports. 630 A third difference between a Qualifier and a Name concerns the 631 semantic opaqueness constraint. When an NMA-qualified ARK is to be 632 used as a transient service entry point into a persistent object, the 633 priority given to semantic opaqueness observed by the NAA in the Name 634 part may be relaxed by the NMA in the Qualifier part. If service 635 priorities in the Qualifier take precedence over persistence, short- 636 term usability considerations may recommend somewhat semantically 637 laden Qualifier strings. 639 Finally, not only is the set of Qualifiers supported by an NMA 640 mutable, but different NMAs may support different Qualifier sets for 641 the same NAA-identified object. In this regard the NMAs act 642 independently of each other and of the NAA. 644 The next two sections describe how ARK syntax may be used to declare, 645 or to avoid declaring, certain kinds of relatedness among qualified 646 ARKs. 648 2.5.1. ARKs that Reveal Object Hierarchy 650 An NAA or NMA may choose to reveal the presence of a hierarchical 651 relationship between objects using the `/' (slash) character after 652 the Name part of an ARK. Some authorities will choose not to 653 disclose this information, while others will go ahead and disclose so 654 that manipulators of large sets of ARKs can infer object 655 relationships by simple identifier inspection; for example, this 656 makes it possible for a system to present a collapsed view of a large 657 search result set. 659 If the ARK contains an internal slash after the NAAN, the piece to 660 its left indicates a containing object. For example, publishing an 661 ARK of the form, 663 ark:12025/654/xz/321 665 is equivalent to publishing three ARKs, 666 ark:12025/654/xz/321 667 ark:12025/654/xz 668 ark:12025/654 670 together with a declaration that the first object is contained in the 671 second object, and that the second object is contained in the third. 673 Revealing the presence of hierarchy is completely up to the assigner 674 (NMA or NAA). It is hard enough to commit to one object's name, let 675 alone to three objects' names and to a specific, ongoing relatedness 676 among them. Thus, regardless of whether hierarchy was present 677 initially, the assigner, by not using slashes, reveals no shared 678 inferences about hierarchical or other inter-relatedness in the 679 following ARKs: 681 ark:12025/654_xz_321 682 ark:12025/654_xz 683 ark:12025/654xz321 684 ark:12025/654xz 685 ark:12025/654 687 Note that slashes around the ARK's NAAN (/12025/ in these examples) 688 are not part of the ARK's Name and therefore do not indicate the 689 existence of some sort of NAAN super object containing all objects in 690 its namespace. A slash must have at least one non-structural 691 character (one that is neither a slash nor a period) on both sides in 692 order for it to separate recognizable structural components. So 693 initial or final slashes may be removed, and double slashes may be 694 converted into single slashes. 696 2.5.2. ARKs that Reveal Object Variants 698 An NAA or NMA may choose to reveal the possible presence of variant 699 objects or object components using the `.' (period) character after 700 the Name part of an ARK. Some authorities will choose not to 701 disclose this information, while others will go ahead and disclose so 702 that manipulators of large sets of ARKs can infer object 703 relationships by simple identifier inspection; for example, this 704 makes it possible for a system to present a collapsed view of a large 705 search result set. 707 If the ARK contains an internal period after Name, the piece to its 708 left is a root name and the piece to its right, and up to the end of 709 the ARK or to the next period is a suffix. A Name may have more than 710 one suffix, for example, 711 ark:12025/654.24 712 ark:12025/xz4/654.24 713 ark:12025/654.20v.78g.f55 715 There are two main rules. First, if two ARKs share the same root 716 name but have different suffixes, the corresponding objects were 717 considered variants of each other (different formats, languages, 718 versions, etc.) by the assigner (NMA or NAA). Thus, the following 719 ARKs are variants of each other: 721 ark:12025/654.20v.78g.f55 722 ark:12025/654.321xz 723 ark:12025/654.44 725 Second, publishing an ARK with a suffix implies the existence of at 726 least one variant identified by the ARK without its suffix. The ARK 727 otherwise permits no further assumptions about what variants might 728 exist. So publishing the ARK, 730 ark:12025/654.20v.78g.f55 732 is equivalent to publishing the four ARKs, 734 ark:12025/654.20v.78g.f55 735 ark:12025/654.20v.78g 736 ark:12025/654.20v 737 ark:12025/654 739 Revealing the possibility of variants is completely up to the 740 assigner. It is hard enough to commit to one object's name, let 741 alone to multiple variants' names and to a specific, ongoing 742 relatedness among them. The assigner is the sole arbiter of what 743 constitutes a variant within its namespace, and whether to reveal 744 that kind of relatedness by using periods within its names. 746 A period must have at least one non-structural character (one that is 747 neither a slash nor a period) on both sides in order for it to 748 separate recognizable structural components. So initial or final 749 periods may be removed, and adjacent periods may be converted into a 750 single period. Multiple suffixes should be arranged in sorted order 751 (pure ASCII collating sequence) at the end of an ARK. 753 2.6. Character Repertoires 755 The Name and Qualifier parts are strings of visible ASCII characters. 756 For received ARKs, implementations must support a minimum length of 757 255 octets for the string composed of the Base ARK plus Qualifier. 758 Implementations generating strings exceeding this length should 759 understand that receiving implementations may not be able to index 760 such ARKs properly. Characters may be letters, digits, or any of 761 these seven characters: 763 = ~ * + @ _ $ 765 The following characters may also be used, but their meanings are 766 reserved: 768 % - . / 770 The characters `/' and `.' are ignored if either appears as the last 771 character of an ARK. If used internally, they allow a name assigner 772 to reveal object hierarchy and object variants as previously 773 described. 775 Hyphens are considered to be insignificant and are always ignored in 776 ARKs. A `-' (hyphen) may appear in an ARK for readability, or it may 777 have crept in during the formatting and wrapping of text, but it must 778 be ignored in lexical comparisons. As in a telephone number, hyphens 779 have no meaning in an ARK. It is always safe for an NMA that 780 receives an ARK to remove any hyphens found in it. As a result, like 781 the NMAH, hyphens are "identity inert" in comparing ARKs for 782 equivalence. For example, the following ARKs are equivalent for 783 purposes of comparison and ARK service access: 785 ark:12025/65-4-xz-321 786 http://sneezy.dopey.com/ark:12025/654--xz32-1 787 ark:12025/654xz321 789 The `%' character is reserved for %-encoding all other octets that 790 would appear in the ARK string, in the same manner as for URIs 791 [RFC3986]. A %-encoded octet consists of a `%' followed by two hex 792 digits; for example, "%7d" stands in for `}'. Lower case hex digits 793 are preferred to reduce the chances of false acronym recognition; 794 thus it is better to use "%acT" instead of "%ACT". The character `%' 795 itself must be represented using "%25". As with URNs, %-encoding 796 permits ARKs to support legacy namespaces (e.g., ISBN, ISSN, SICI) 797 that have less restricted character repertoires [RFC2288]. 799 2.7. Normalization and Lexical Equivalence 801 To determine if two or more ARKs identify the same object, the ARKs 802 are compared for lexical equivalence after first being normalized. 803 Since ARK strings may appear in various forms (e.g., having different 804 NMAHs), normalizing them minimizes the chances that comparing two ARK 805 strings for equality will fail unless they actually identify 806 different objects. In a specified-host ARK (one having an NMAH), the 807 NMAH never participates in such comparisons. Normalization described 808 here serves to define lexical equivalence but does not restrict how 809 implementors normalize ARKs locally for storage. 811 Normalization of a received ARK for the purpose of octet-by-octet 812 equality comparison with another ARK consists of the following steps. 814 1. The NMAH part (eg, everything from an initial "http://" up to the 815 next slash), if present is removed. 817 2. Any URI query string is removed (everything from the first 818 literal '?' to the end of the string). 820 3. The first case-insensitive match on "ark:/" or "ark:" is 821 converted to "ark:" (replacing any upper case letters and 822 removing any terminal '/'). 824 4. In the string that remains, the two characters following every 825 occurrence of `%' are converted to lower case. The case of all 826 other letters in the ARK string must be preserved. 828 5. All hyphens are removed. 830 6. If normalization is being done as part of a resolution step, and 831 if the end of the remaining string matches a known inflection, 832 the inflection is noted and removed. 834 7. Structural characters (slash and period) are normalized: initial 835 and final occurrences are removed, and two structural characters 836 in a row (e.g., // or ./) are replaced by the first character, 837 iterating until each occurrence has at least one non-structural 838 character on either side. 840 8. If there are any components with a period on the left and a slash 841 on the right, either the component and the preceding period must 842 be moved to the end of the Name part or the ARK must be thrown 843 out as malformed. 845 9. The final step is to arrange the suffixes in ASCII collating 846 sequence (that is, to sort them) and to remove duplicate 847 suffixes, if any. It is also permissible to throw out ARKs for 848 which the suffixes are not sorted. 850 The resulting ARK string is now normalized. Comparisons between 851 normalized ARKs are case-sensitive, meaning that upper case letters 852 are considered different from their lower case counterparts. 854 To keep ARK string variation to a minimum, no reserved ARK characters 855 should be %-encoded unless it is deliberately to conceal their 856 reserved meanings. No non-reserved ARK characters should ever be 857 %-encoded. Finally, no %-encoded character should ever appear in an 858 ARK in its decoded form. 860 3. Naming Considerations 862 The most important threats faced by persistence providers include 863 such things as funding loss, natural disaster, political and social 864 upheaval, processing faults, and errors in human oversight. There is 865 nothing that an identifer scheme can do about such things. Still, a 866 few observed identifier failures and inconveniences can be traced 867 back to naming practices that we now know to be less than optimal for 868 persistence. 870 3.1. ARKS Embedded in Language 872 The ARK has different goals from the URI, so it has different 873 character set requirements. Because linguistic constructs imperil 874 persistence, for ARKs non-ASCII character support is unimportant. 875 ARKs and URIs share goals of transcribability and transportability 876 within web documents, so characters are required to be visible, non- 877 conflicting with HTML/XML syntax, and not subject to tampering during 878 transmission across common transport gateways. Add the goal of 879 making an undelimited ARK recognizable in running prose, as in 880 ark:12025/=@_22*$, and certain punctuation characters (e.g., comma, 881 period) end up being excluded from the ARK lest the end of a phrase 882 or sentence be mistaken for part of the ARK. 884 This consideration has more direct effect on ARK usability in a 885 natural language context than it has on ARK persistence. The same is 886 true of the rule preventing hyphens from having lexical significance. 887 It is fine to publish ARKs with hyphens in them (e.g., such as the 888 output of UUID/GUID generators), but the uniform treatment of hyphens 889 as insignificant reduces the possibility of users transcribing 890 identifiers that will have been broken through unpredictable 891 hyphenation by word processors. Any measure that reduces user 892 irritation with an identifier will increase its chances of survival. 894 3.2. Objects Should Wear Their Identifiers 896 A valuable technique for provision of persistent objects is to try to 897 arrange for the complete identifier to appear on, with, or near its 898 retrieved object. An object encountered at a moment in time when its 899 discovery context has long since disappeared could then easily be 900 traced back to its metadata, to alternate versions, to updates, etc. 901 This has seen reasonable success, for example, in book publishing and 902 software distribution. An identifier string only has meaning when 903 its association is known, and this a very sure, simple, and low-tech 904 method of reminding everyone exactly what that association is. 906 3.3. Names are Political, not Technological 908 If persistence is the goal, a deliberate local strategy for 909 systematic name assignment is crucial. Names must be chosen with 910 great care. Poorly chosen and managed names will devastate any 911 persistence strategy, and they do not discriminate by identifier 912 scheme. Whether a mistakenly re-assigned name is a URN, DOI, PURL, 913 URL, or ARK, the damage -- failed access and confusion -- is not 914 mitigated more in one scheme than in another. Conversely, in-house 915 efforts to manage names responsibly will go much further towards 916 safeguarding persistence than any choice of naming scheme or name 917 resolution technology. 919 Branding (e.g., at the corporate or departmental level) is important 920 for funding and visibility, but substrings representing brands and 921 organizational names should be given a wide berth except when 922 absolutely necessary in the hostname (the identity-inert) part of the 923 ARK. These substrings are not only unstable because organizations 924 change frequently, but they are also dangerous because successor 925 organizations often have political or legal reasons to actively 926 suppress predecessor names and brands. Any measure that reduces the 927 chances of future political or legal pressure on an identifier will 928 decrease the chances that our descendants will be obliged to 929 deliberately break it. 931 3.4. Choosing a Hostname or NMA 933 Hostnames appearing in any identifier meant to be persistent must be 934 chosen with extra care. The tendency in hostname selection has 935 traditionally been to choose a token with recognizable attributes, 936 such as a corporate brand, but that tendency wreaks havoc with 937 persistence that is supposed to outlive brands, corporations, subject 938 classifications, and natural language semantics (e.g., what did the 939 three letters "gay" mean in 1958, 1978, and 1998?). Today's 940 recognized and correct attributes are tomorrow's stale or incorrect 941 attributes. In making hostnames (any names, actually) long-term 942 persistent, it helps to eliminate recognizable attributes to the 943 extent possible. This affects selection of any name based on URLs, 944 including PURLs and the explicitly disposable NMAHs. 946 There is no excuse for a provider that manages its internal names 947 impeccably not to exercise the same care in choosing what could be an 948 exceptionally durable hostname, especially if it would form the 949 prefix for all the provider's URL-based external names. Registering 950 an opaque hostname in the ".org" or ".net" domain would not be a bad 951 start. Another way is to publish your ARKs with an organizational 952 domain name that will be mapped by DNS to an appropriate NMA host. 953 This makes for shorter names with less branding vulnerability. 955 It is a mistake to think that hostnames are inherently unstable. If 956 you require brand visibility, that may be a fact of life. But things 957 are easier if yours is the brand of long-lived cultural memory 958 institution such as a national or university library or archive. 959 Well-chosen hostnames from organizations that are sheltered from the 960 direct effects of a volatile marketplace can easily provide longer- 961 lived global resolvers than the domain names explicitly or implicitly 962 used as starting points for global resolution by indirection-based 963 persistent identifier schemes. For example, it is hard to imagine 964 circumstances under which the Library of Congress' domain name would 965 disappear sooner than, say, "handle.net". 967 For smaller libraries, archives, and preservation organizations, 968 there is a natural concern about whether they will be able to keep 969 their web servers and domain names in the face of uncertain funding. 970 One option is to form or join a consortium [N2T] of like-minded 971 organizations with the purpose of providing mutual preservation 972 support. The first goal of such a consortium would be to perpetually 973 rent a hostname on which to establish a web server that simply 974 redirects incoming member organization requests to the appropriate 975 member server; using ARKs, for example, a 150-member consortium could 976 run a very small server (24x7) that contained nothing more than 150 977 rewrite rules in its configuration file. Even more helpful would be 978 additional consortial support for a member organization that was 979 unable to continue providing services and needed to find a successor 980 archival organization. This would be a low-cost, low-tech way to 981 publish ARKs (or URLs) under highly persistent hostnames. 983 There are no obvious reasons why the organizations registering DNS 984 names, URN Namespaces, and DOI publisher IDs should have among them 985 one that is intrinsically more fallible than the next. Moreover, it 986 is a misconception that the demise of DNS and of HTTP need adversely 987 affect the persistence of URLs. At such a time, certainly URLs from 988 the present day might not then be actionable by our present-day 989 mechanisms, but resolution systems for future non-actionable URLs are 990 no harder to imagine than resolution systems for present-day non- 991 actionable URNs and DOIs. There is no more stable a namespace than 992 one that is dead and frozen, and that would then characterize the 993 space of names bearing the "http://" prefix. It is useful to 994 remember that just because hostnames have been carelessly chosen in 995 their brief history does not mean that they are unsuitable in NMAHs 996 (and URLs) intended for use in situations demanding the highest level 997 of persistence available in the Internet environment. A well-planned 998 name assignment strategy is everything. 1000 3.5. Assigners of ARKs 1002 A Name Assigning Authority (NAA) is an organization that creates (or 1003 delegates creation of) long-term associations between identifiers and 1004 information objects. Examples of NAAs include national libraries, 1005 national archives, and publishers. An NAA may arrange with an 1006 external organization for identifier assignment. The US Library of 1007 Congress, for example, allows OCLC (the Online Computer Library 1008 Center, a major world cataloger of books) to create associations 1009 between Library of Congress call numbers (LCCNs) and the books that 1010 OCLC processes. A cataloging record is generated that testifies to 1011 each association, and the identifier is included by the publisher, 1012 for example, in the front matter of a book. 1014 An NAA does not so much create an identifier as create an 1015 association. The NAA first draws an unused identifier string from 1016 its namespace, which is the set of all identifiers under its control. 1017 It then records the assignment of the identifier to an information 1018 object having sundry witnessed characteristics, such as a particular 1019 author and modification date. A namespace is usually reserved for an 1020 NAA by agreement with recognized community organizations (such as 1021 IANA and ISO) that all names containing a particular string be under 1022 its control. In the ARK an NAA is represented by the Name Assigning 1023 Authority Number (NAAN). 1025 The ARK namespace reserved for an NAA is the set of names bearing its 1026 particular NAAN. For example, all strings beginning with 1027 "ark:12025/" are under control of the NAA registered under 12025, 1028 which might be the National Library of Finland. Because each NAA has 1029 a different NAAN, names from one namespace cannot conflict with those 1030 from another. Each NAA is free to assign names from its namespace 1031 (or delegate assignment) according to its own policies. These 1032 policies must be documented in a manner similar to the declarations 1033 required for URN Namespace registration [RFC2611]. 1035 Organizations can request or update a NAAN by filling out a form 1036 [NAANrequest]. 1038 3.6. NAAN Namespace Management 1040 Every NAA must have a namespace management strategy. A time-honored 1041 technique is to hierarchically partition a namespace into 1042 subnamespaces using prefixes that guarantee non-collision of names in 1043 different partition. This practice is strongly encouraged for all 1044 NAAs, especially when subnamespace management will be delegated to 1045 other departments, units, or projects within an organization. For 1046 example, with a NAAN that is assigned to a university and managed by 1047 its main library, care should be taken to reserve semantically opaque 1048 prefixes that will set aside large parts of the unused namespace for 1049 future assignments. Prefix-based partition management is an 1050 important responsibility of the NAA. 1052 This sort of delegation by prefix is well-used in the formation of 1053 DNS names and ISBN identifiers. An important difference is that in 1054 the former, the hierarchy is deliberately exposed and in the latter 1055 it is hidden. Rather than using lexical boundary markers such as the 1056 period (`.') found in domain names, the ISBN uses a publisher prefix 1057 but doesn't disclose where the prefix ends and the publisher's 1058 assigned name begins. This practice of non-disclosure, borrowed from 1059 the ISBN and ISSN schemes, is encouraged in assigning ARKs, because 1060 it reduces the visibility of an assertion that is probably not 1061 important now and may become a vulnerability later. 1063 Reasonable prefixes for assigned names usually consist of consonants 1064 and digits and are 1-5 characters in length. For example, the 1065 constant prefix "x9t" might be delegated to a book digitization 1066 project that creates identifiers such as 1068 http://444.berkeley.edu/ark:28722/x9t38rk45c 1070 If longevity is the goal, it is important to keep the prefixes free 1071 of recognizable semantics; for example, using an acronym representing 1072 a project or a department is discouraged. At the same time, you may 1073 wish to set aside a subnamespace for testing purposes under a prefix 1074 such as "fk..." that can serve as a visual clue and reminder to 1075 maintenance staff that this "fake" identifier was never published. 1077 There are other measures one can take to avoid user confusion, 1078 transcription errors, and the appearance of accidental semantics when 1079 creating identifiers. If you are generating identifiers 1080 automatically, pure numeric identifiers are likeley to be 1081 semantically opaque enough, but it's probably useful to avoid leading 1082 zeroes because some users mistakenly treat them as optional, thinking 1083 (arithmetically) that they don't contribute to the "value" of the 1084 identifier. 1086 If you need lots of identifiers and you don't want them to get too 1087 long, you can mix digits with consonants (but avoid vowels since they 1088 might accidentally spell words) to get more identifiers without 1089 increasing the string length. In this case you may not want more 1090 than a two letters in a row because it reduces the chance of 1091 generating acronyms. Generator tools such as [NOID] provide support 1092 for these sorts of identifiers, and can also add a computed check 1093 character as a guarantee against the most common transcription 1094 errors. 1096 3.7. Sub-Object Naming 1098 As mentioned previously, semantically opaque identifiers are very 1099 useful for long-term naming of abstract objects, however, it may be 1100 appropriate to extend these names with less opaque extensions that 1101 reference contemporary service entry points (sub-objects) in support 1102 of the object. Sub-object extensions beginning with a digit or 1103 underscore (`_') are reserved for the possibilty of developing a 1104 future registry of canonical service points (e.g., numeric references 1105 to versions, formats, languages, etc). 1107 4. Finding a Name Mapping Authority 1109 In order to derive an actionable identifier (these days, a URL) from 1110 an ARK, a hostport (hostname or hostname plus port combination) for a 1111 working Name Mapping Authority (NMA) must be found. An NMA is a 1112 service that is able to respond to basic ARK service requests. 1113 Relying on registration and client-side discovery, NMAs make known 1114 which NAAs' identifiers they are willing to service. 1116 Upon encountering an ARK, a user (or client software) looks inside it 1117 for the optional NMAH part (the hostport of the NMA's ARK service). 1118 If it contains an NMAH that is working, this NMAH discovery step may 1119 be skipped; the NMAH effectively uses the beginning of an ARK to 1120 cache the results of a prior mapping authority discovery process. If 1121 a new 1123 NMAH needs to found, the client looks inside the ARK again for the 1124 NAAN (Name Assigning Authority Number). Querying a global database, 1125 it then uses the NAAN to look up all current NMAHs that service ARKs 1126 issued by the identified NAA. 1128 The global database is key, and ideally the lookup would be automatic 1129 and transparent to the user. For this, the most promising method is 1130 probably the Name-to-Thing (N2T) Resolver [N2T] at n2t.net. It is a 1131 proposed low-cost, highly reliable, consortially maintained NMAH that 1132 simply exists to support actionable HTTP-based URLs for as long as 1133 HTTP is used. One of its big advantages over the other two methods 1134 and the URN, Handle, DOI, and PURL methods, is that N2T addresses the 1135 namespace splitting problem. When objects maintained by one NMA are 1136 inherited by more than one successor NMA, until now one of those 1137 successors would be required to maintain forwarding tables on behalf 1138 of the other successors. 1140 There are two other ways to discover an NMAH, one of them described 1141 in a subsection below. Another way, described in an appendix, is 1142 based on a simplification of the URN resolver discovery method, 1143 itself very similar in principle to the resolver discovery method 1144 used by Handles and DOIs. None of these methods does more than what 1145 can be done with a very small, consortially maintained web server 1146 such as [N2T]. 1148 In the interests of long-term persistence, however, ARK mechanisms 1149 are first defined in high-level, protocol-independent terms so that 1150 mechanisms may evolve and be replaced over time without compromising 1151 fundamental service objectives. Either or both specific methods 1152 given here may eventually be supplanted by better methods since, by 1153 design, the ARK scheme does not depend on a particular method, but 1154 only on having some method to locate an active NMAH. 1156 At the time of issuance, at least one NMAH for an ARK should be 1157 prepared to service it. That NMA may or may not be administered by 1158 the Name Assigning Authority (NAA) that created it. Consider the 1159 following hypothetical example of providing long-term access to a 1160 cancer research journal. The publisher wishes to turn a profit and 1161 the National Library of Medicine wishes to preserve the scholarly 1162 record. An agreement might be struck whereby the publisher would act 1163 as the NAA and the national library would archive the journal issue 1164 when it appears, but without providing direct access for the first 1165 six months. During the first six months of peak commercial 1166 viability, the publisher would retain exclusive delivery rights and 1167 would charge access fees. Again, by agreement, both the library and 1168 the publisher would act as NMAs, but during that initial period the 1169 library would redirect requests for issues less than six months old 1170 to the publisher. At the end of the waiting period, the library 1171 would then begin servicing requests for issues older than six months 1172 by tapping directly into its own archives. Meanwhile, the publisher 1173 might routinely redirect incoming requests for older issues to the 1174 library. Long-term access is thereby preserved, and so is the 1175 commercial incentive to publish content. 1177 Although it will be common for an NAA also to run an NMA service, it 1178 is never a requirement. Over time NAAs and NMAs will come and go. 1179 One NMA will succeed another, and there might be many NMAs serving 1180 the same ARKs simultaneously (e.g., as mirrors or as competitors). 1181 There might also be asymmetric but coordinated NMAs as in the 1182 library-publisher example above. 1184 4.1. Looking Up NMAHs in a Globally Accessible File 1186 This subsection describes a way to look up NMAHs using a simple name 1187 authority table represented as a plain text file. For efficient 1188 access the file may be stored in a local filesystem, but it needs to 1189 be reloaded periodically to incorporate updates. It is not expected 1190 that the size of the file or frequency of update should impose an 1191 undue maintenance or searching burden any time soon, for even 1192 primitive linear search of a file with ten-thousand NAAs is a 1193 subsecond operation on modern server machines. The proposed file 1194 strategy is similar to the /etc/hosts file strategy that supported 1195 Internet host address lookup for a period of years before the advent 1196 of DNS. 1198 The name authority table file is updated on an ongoing basis and is 1199 available for copying over the internet from a number of mirror sites 1200 [NAANregistry]. The file contains comment lines (lines that begin 1201 with `#') explaining the format and giving the file's modification 1202 time, reloading address, and NAA registration instructions. 1204 5. Generic ARK Service Definition 1206 An ARK request's output is delivered information; examples include 1207 the object itself, a policy declaration (e.g., a promise of support), 1208 a descriptive metadata record, or an error message. The experience 1209 of object delivery is expected to be an evolving mix of information 1210 that reflects changing service expectations and technology 1211 requirements; contemporary examples include such things as an object 1212 summary and component links formatted for human consumption. ARK 1213 services must be couched in high-level, protocol-independent terms if 1214 persistence is to outlive today's networking infrastructural 1215 assumptions. The high-level ARK service definitions listed below are 1216 followed in the next section by a concrete method (one of many 1217 possible methods) for delivering these services with today's 1218 technology. Note that some services may be invoked in one operation, 1219 such as when an '?info' inflection returns both a description and a 1220 permanence declaration for an object. 1222 5.1. Generic ARK Access Service (access, location) 1224 Returns (a copy of) the object or a redirect to the same, although a 1225 sensible object proxy may be substituted. Examples of sensible 1226 substitutes include, 1228 o a table of contents instead of a large complex document, 1230 o a home page instead of an entire web site hierarchy, 1231 o a rights clearance challenge before accessing protected data, 1233 o directions for access to an offline object (e.g., a book), 1235 o a description of an intangible object (a disease, an event), or 1237 o an applet acting as "player" for a large multimedia object. 1239 May also return a discriminated list of alternate object locators. 1240 If access is denied, returns an explanation of the object's current 1241 (perhaps permanent) inaccessibility. 1243 5.1.1. Generic Policy Service (permanence, naming, etc.) 1245 Returns declarations of policy and support commitments for given 1246 ARKs. Declarations are returned in either a structured metadata 1247 format or a human readable text format; sometimes one format may 1248 serve both purposes. Policy subareas may be addressed in separate 1249 requests, but the following areas should be covered: object 1250 permanence, object naming, object fragment addressing, and 1251 operational service support. 1253 The permanence declaration for an object is a rating defined with 1254 respect to an identified permanence provider (guarantor), which will 1255 be the NMA. It may include the following aspects. 1257 (a) "object availability" -- whether and how access to the object 1258 is supported (e.g., online 24x7, or offline only), 1260 (b) "identifier validity" -- under what conditions the identifier 1261 will be or has been re-assigned, 1263 (c) "content invariance" -- under what conditions the content of 1264 the object is subject to change, and 1266 (d) "change history" -- access to corrections, migrations, and 1267 revisions, whether through links to the changed objects themselves 1268 or through a document summarizing the change history 1270 A recent approach to persistence statements, conceived independently 1271 from ARKs, can be found at [PStatements], with ongoing work available 1272 at [ARKagency]. An older approach to a permanence rating framework 1273 is given in [NLMPerm], which identified the following "permanence 1274 levels": 1276 Not Guaranteed: No commitment has been made to retain this 1277 resource. It could become unavailable at any time. Its 1278 identifier could be changed. 1280 Permanent: Dynamic Content: A commitment has been made to keep 1281 this resource permanently available. Its identifier will always 1282 provide access to the resource. Its content could be revised or 1283 replaced. 1285 Permanent: Stable Content: A commitment has been made to keep this 1286 resource permanently available. Its identifier will always 1287 provide access to the resource. Its content is subject only to 1288 minor corrections or additions. 1290 Permanent: Unchanging Content: A commitment has been made to keep 1291 this resource permanently available. Its identifier will always 1292 provide access to the resource. Its content will not change. 1294 Naming policy for an object includes an historical description of the 1295 NAA's (and its successor NAA's) policies regarding differentiation of 1296 objects. Since it is the NMA who responds to requests for policy 1297 statements, it is useful for the NMA to be able to produce or 1298 summarize these historical NAA documents. Naming policy may include 1299 the following aspects. 1301 (i) "similarity" -- (or "unity") the limit, defined by the NAA, to 1302 the level of dissimilarity beyond which two similar objects 1303 warrant separate identifiers but before which they share one 1304 single identifier, and 1306 (ii) "granularity" -- the limit, defined by the NAA, to the level 1307 of object subdivision beyond which sub-objects do not warrant 1308 separately assigned identifiers but before which sub-objects are 1309 assigned separate identifiers. 1311 Subnaming policy for an object describes the qualifiers that the NMA, 1312 in fulfilling its ongoing and evolving service obligations, allows as 1313 extensions to an NAA-assigned ARK. To the conceptual object that the 1314 NAA named with an ARK, the NMA may add component access points and 1315 derivatives (e.g., format migrations in aid of preservation) in order 1316 to provide both basic and value-added services. 1318 Addressing policy for an object includes a description of how, during 1319 access, object components (e.g., paragraphs, sections) or views 1320 (e.g., image conversions) may or may not be "addressed", in other 1321 words, how the NMA permits arguments or parameters to modify the 1322 object delivered as the result of an ARK request. If supported, 1323 these sorts of operations would provide things like byte-ranged 1324 fragment delivery and open-ended format conversions, or any set of 1325 possible transformations that would be too numerous to list or to 1326 identify with separately assigned ARKs. 1328 Operational service support policy includes a description of general 1329 operational aspects of the NMA service, such as after-hours staffing 1330 and trouble reporting procedures. 1332 5.1.2. Generic Description Service 1334 Returns a description of the object. Descriptions are returned in a 1335 structured metadata format, human readable text format, or in one 1336 format that serves both purposes (such as human-readable HTML with 1337 embedded machine-readable metadata). A description must at a minimum 1338 answer the who, what, when, and where questions concerning an 1339 expression of the object. Standalone descriptions should be 1340 accompanied by the modification date and source of the description 1341 itself. May also return discriminated lists of ARKs that are related 1342 to the given ARK. 1344 5.2. Overview of The HTTP URL Mapping Protocol (THUMP) 1346 The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any 1347 identifier) and asking such questions as, what information does this 1348 identify and how permanent is it? [THUMP] is in fact one specific 1349 method under development for delivering ARK services. The protocol 1350 runs over HTTP to exploit the web browser's current pre-eminence as 1351 user interface to the Internet. THUMP is designed so that a person 1352 can enter ARK requests directly into the location field of current 1353 browser interfaces. Because it runs over HTTP, THUMP can be 1354 simulated and tested via keyboard-based interactions [RFC0854]. 1356 The asker (a person or client program) starts with an identifier, 1357 such as an ARK or a URL. The identifier reveals to the asker (or 1358 allows the asker to infer) the Internet host name and port number of 1359 a server system that responds to questions. Here, this is just the 1360 NMAH that is obtained by inspection and possibly lookup based on the 1361 ARK's NAAN. The asker then sets up an HTTP session with the server 1362 system, sends a question via a THUMP request (contained within an 1363 HTTP request), receives an answer via a THUMP response (contained 1364 within an HTTP response), and closes the session. That concludes the 1365 connected portion of the protocol. 1367 A THUMP request is a string of characters beginning with a `?' 1368 (question mark) that is appended to the identifier string. The 1369 resulting string is sent as an argument to HTTP's GET command. 1370 Request strings too long for GET may be sent using HTTP's POST 1371 command. The two most common requests correspond to two degenerate 1372 special cases. First, a simple key with no request at all is the 1373 same as an ordinary access request. Thus a plain ARK entered into a 1374 browser's location field behaves much like a plain URL, and returns 1375 access to the primary identified object, for instance, an HTML 1376 document. 1378 The second special case is a minimal ARK description request string 1379 consisting of just "?info". For example, entering the string, 1381 ark.nlm.nih.gov/12025/psbbantu?info 1383 into the browser's location field directly precipitates a request for 1384 a metadata record describing the object identified by ark:12025/ 1385 psbbantu. The browser, unaware of THUMP, prepares and sends an HTTP 1386 GET request in the same manner as for a URL. THUMP is designed so 1387 that the response (indicated by the returned HTTP content type) is 1388 normally displayed, whether the output is structured for machine 1389 processing (text/plain) or formatted for human consumption (text/ 1390 html). In addition to '?info', this specification reserves both '?' 1391 and '??' (originally older forms) for future use. 1393 In the following example THUMP session, each line has been annotated 1394 to include a line number and whether it was the client or server that 1395 sent it. Without going into much depth, the session has four pieces 1396 separated from each other by blank lines: the client's piece (lines 1397 1-3), the server's HTTP/THUMP response headers (4-7), and the body of 1398 the server's response (8-13). The first and last lines (1 and 13) 1399 correspond to the client's steps to start the TCP session and the 1400 server's steps to end it, respectively. 1402 1 C: [opens session] 1403 C: GET http://ark.nlm.nih.gov/ark:12025/psbbantu?info HTTP/1.1 1404 C: 1405 S: HTTP/1.1 200 OK 1406 5 S: Content-Type: text/plain 1407 S: THUMP-Status: 0.6 200 OK 1408 S: 1409 S: erc: 1410 S: who: Lederberg, Joshua 1411 10 S: what: Studies of Human Families for Genetic Linkage 1412 S: when: 1974 1413 S: where: http://profiles.nlm.nih.gov/BB/A/N/T/U/_/bbantu.pdf 1414 S: erc-support: 1415 S: who: USNLM 1416 15 S: what: Permanent, Unchanging Content 1417 S: when: 20010421 1418 S: where: http://ark.nlm.nih.gov/yy22948 1419 S: [closes session] 1421 The first two server response lines (4-5) above are typical of HTTP. 1422 The next line (6) is peculiar to THUMP, and indicates the THUMP 1423 version and a normal return status. 1425 The balance of the response consists of a single metadata record 1426 (8-17) that comprises the ARK description service response. The 1427 returned record is in the format of an Electronic Resource Citation 1428 [ERC], which is discussed in overview in the next section. For now, 1429 note that it contains four elements that answer the top priority 1430 questions regarding an expression of the object: who played a major 1431 role in expressing it, what the expression was called, when is was 1432 created, and where the expression may be found. This quartet of 1433 elements comes up again and again in ERCs. Lines 13-17 contain a 1434 minimal persistence statement. 1436 Each segment in an ERC tells a different story relating to the 1437 object, so although the same four questions (elements) appear in 1438 each, the answers depend on the segment's story type. While the 1439 first segment tells the story of an expression of the object, the 1440 second segment tells the story of the support commitment made to it: 1441 who made the commitment, what the nature of the commitment was, when 1442 it was made, and where a fuller explanation of the commitment may be 1443 found. 1445 5.3. The Electronic Resource Citation (ERC) 1447 An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a 1448 kind of object description that uses Dublin Core Kernel metadata 1449 elements [DCKernel]. The ERC with Kernel elements provides a simple, 1450 compact, and printable record for holding data associated with an 1451 information resource. As originally designed [Kernel], Kernel 1452 metadata balances the needs for expressive power, very simple machine 1453 processing, and direct human manipulation. 1455 The previous section shows two limited examples of what is fully 1456 described elsewhere [ERC]. The rest of this short section provides 1457 some of the background and rationale for this record format. 1459 A founding principle of Kernel metadata is that direct human contact 1460 with metadata will be a necessary and sufficient condition for the 1461 near term rapid development of metadata standards, systems, and 1462 services. Thus the machine-processable Kernel elements must only 1463 minimally strain people's ability to read, understand, change, and 1464 transmit ERCs without their relying on intermediation with 1465 specialized software tools. The basic ERC needs to be succinct, 1466 transparent, and trivially parseable by software. 1468 Borrowing from the data structuring format that underlies the 1469 successful spread of email and web services, the ERC format uses 1470 [ANVL], which is based on email and HTTP headers [RFC2822]. There is 1471 a naturalness to ANVL's label-colon-value format (seen in the 1472 previous section) that barely needs explanation to a person beginning 1473 to enter ERC metadata. 1475 While ANVL elements are expected at the top level and don't 1476 themselves support hierarchy, the value of an ANVL element may be an 1477 arbitrary encoded hierarchy of JSON or XML. Typically, the name of 1478 such an ANVL element ends in "json" or "xml", for example, "json" or 1479 "geojson". Care should be taken to escape structural characters that 1480 appear in element names and values, specifically, line terminators 1481 (both newlines ("\n") and carriage returns ("\r")) and, in element 1482 names, colons (":"). 1484 Besides simplicity of ERC system implementation and data entry 1485 mechanics, ERC semantics (what the record and its constituent parts 1486 mean) must also be easy to explain. ERC semantics are based on a 1487 reformulation and extension of the Dublin Core [RFC5013] hypothesis, 1488 which suggests that the fifteen Dublin Core metadata elements have a 1489 key role to play in cross-domain resource description. The ERC 1490 design recognizes that the Dublin Core's primary contribution is the 1491 international, interdisciplinary consensus that identified fifteen 1492 semantic buckets (element categories), regardless of how they are 1493 labeled. The ERC then adds a definition for a record and some 1494 minimal compliance rules. In pursuing the limits of simplicity, the 1495 ERC design combines and relabels some Dublin Core buckets to isolate 1496 a tiny kernel (subset) of four elements for basic cross-domain 1497 resource description. 1499 For the cross-domain kernel, the ERC uses the four basic elements -- 1500 who, what, when, and where -- to pretend that every object in the 1501 universe can have a uniform minimal description. Each has a name or 1502 other identifier, a location, some responsible person or party, and a 1503 date. It doesn't matter what type of object it is, or whether one 1504 plans to read it, interact with it, smoke it, wear it, or navigate 1505 it. Of course, this approach is flawed because uniformity of 1506 description for some object types requires more semantic contortion 1507 and sacrifice than for others. That is why at the beginning of this 1508 document, the ARK was said to be suited to objects that accommodate 1509 reasonably regular electronic description. 1511 While insisting on uniformity at the most basic level provides 1512 powerful cross-domain leverage, the semantic sacrifice is great for 1513 many applications. So the ERC also permits a semantically rich and 1514 nuanced description to co-exist in a record along with a basic 1515 description. In that way both sophisticated and naive recipients of 1516 the record can extract the level of meaning from it that best suits 1517 their needs and abilities. Key to unlocking the richer description 1518 is a controlled vocabulary of ERC record types (not explained in this 1519 document) that permit knowledgeable recipients to apply defined sets 1520 of additional assumptions to the record. 1522 5.4. Advice to Web Clients 1524 ARKs are envisaged to appear wherever durable object references are 1525 planned. Library cataloging records, literature citations, and 1526 bibliographies are important examples. In many of these places URLs 1527 (Uniform Resource Locators) are currently used, and inside some of 1528 those URLs are embedded URNs, Handles, and DOIs. Unfortunately, 1529 there's no suggestion of a way to probe for extra services that would 1530 build confidence in those identifiers; in other words, there's no way 1531 to tell whether any of those identifiers is any better managed than 1532 the average URL. 1534 ARKs are also envisaged to appear in hypertext links (where they are 1535 not normally shown to users) and in rendered text (displayed or 1536 printed). A normal HTML link for which the URL is not displayed 1537 looks like this. 1539 Click Here 1541 A URL with an embedded ARK invites access (via `?info') to extra 1542 services: 1544 Click Here 1546 Using the [N2T] resolver to provide identifier-scheme-agnostic 1547 protection against hostname instability, this ARK could be published 1548 as: 1550 Click Here 1552 An NAA will typically make known the associations it creates by 1553 publishing them in catalogs, actively advertizing them, or simply 1554 leaving them on web sites for visitors (e.g., users, indexing 1555 spiders) to stumble across in browsing. 1557 5.5. Security Considerations 1559 The ARK naming scheme poses no direct risk to computers and networks. 1560 Implementors of ARK services need to be aware of security issues when 1561 querying networks and filesystems for Name Mapping Authority 1562 services, and the concomitant risks from spoofing and obtaining 1563 incorrect information. These risks are no greater for ARK mapping 1564 authority discovery than for other kinds of service discovery. For 1565 example, recipients of ARKs with a specified hostport (NMAH) should 1566 treat it like a URL and be aware that the identified ARK service may 1567 no longer be operational. 1569 Apart from mapping authority discovery, ARK clients and servers 1570 subject themselves to all the risks that accompany normal operation 1571 of the protocols underlying mapping services (e.g., HTTP, Z39.50). 1572 As specializations of such protocols, an ARK service may limit 1573 exposure to the usual risks. Indeed, ARK services may enhance a kind 1574 of security by helping users identify long-term reliable references 1575 to information objects. 1577 6. References 1579 [ANVL] Kunze, J. and B. Kahle, "A Name-Value Language", 2008, 1580 . 1582 [ARK] Kunze, J., "Towards Electronic Persistence Using ARK 1583 Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd, 1584 August 2003, 1585 . 1587 [ARKagency] 1588 ARKs-in-the-Open, "ARK Maintenance Agency", 2019, 1589 . 1591 [DCKernel] 1592 Initiative, D. C. M., "Kernel Metadata Working Group", 1593 2001-2008, . 1595 [DOI] Foundation, I. D., "The Digital Object Identifier (DOI) 1596 System", February 2001, . 1598 [ERC] Kunze, J. and A. Turner, "Kernel Metadata and Electronic 1599 Resource Citations", October 2007, 1600 . 1602 [Handle] Lannom, L., "Handle System Overview", ICSTI Forum No. 30, 1603 April 1999, . 1605 [Kernel] Kunze, J., "A Metadata Kernel for Electronic Permanence", 1606 Journal of Digital Information Vol 2, Issue 2, 1607 ISSN 1368-7506, January 2002, 1608 . 1610 [N2T] Library, C. D., "Name-to-Thing Resolver", August 2006, 1611 . 1613 [NAANregistry] 1614 ARKs.org, "NAAN Registry", 2019, 1615 . 1617 [NAANrequest] 1618 ARKs.org, "NAAN Request Form", 2018, 1619 . 1621 [NLMPerm] Byrnes, M., "Defining NLM's Commitment to the Permanence 1622 of Electronic Information", ARL 212:8-9, October 2000, 1623 . 1625 [NOID] Kunze, J., "Nice Opaque Identifiers", February 2005, 1626 . 1628 [PStatements] 1629 Kunze, J., "Persistence statements: describing digital 1630 stickiness", October 2016, 1631 . 1633 [PURL] Shafer, K., "Introduction to Persistent Uniform Resource 1634 Locators", 1996, . 1636 [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol 1637 Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May 1638 1983, . 1640 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1641 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1642 . 1644 [RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141, 1645 May 1997, . 1647 [RFC2288] Lynch, C., Preston, C., and R. Daniel, "Using Existing 1648 Bibliographic Identifiers as Uniform Resource Names", 1649 RFC 2288, DOI 10.17487/RFC2288, February 1998, 1650 . 1652 [RFC2611] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, 1653 "URN Namespace Definition Mechanisms", BCP 33, RFC 2611, 1654 DOI 10.17487/RFC2611, June 1999, 1655 . 1657 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1658 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1659 Transfer Protocol -- HTTP/1.1", RFC 2616, 1660 DOI 10.17487/RFC2616, June 1999, 1661 . 1663 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 1664 DOI 10.17487/RFC2822, April 2001, 1665 . 1667 [RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer 1668 (NAPTR) DNS Resource Record", RFC 2915, 1669 DOI 10.17487/RFC2915, September 2000, 1670 . 1672 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1673 Resource Identifier (URI): Generic Syntax", STD 66, 1674 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1675 . 1677 [RFC5013] Kunze, J. and T. Baker, "The Dublin Core Metadata Element 1678 Set", RFC 5013, DOI 10.17487/RFC5013, August 2007, 1679 . 1681 [THUMP] Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol", 1682 August 2007, 1683 . 1685 Appendix A. ARK Maintenance Agency: arks.org 1687 The ARK Maintenance Agency [ARKagency] at arks.org has several 1688 functions. 1690 o To manage the registry of organizations that will be assigning 1691 ARKs. Organizations can request or update a NAAN by filling out a 1692 form [NAANrequest]. 1694 o To be a clearinghouse for information about ARKs, such as best 1695 practices, introductory documentation, tutorials, community 1696 forums, etc. These supplemental resources help ARK implementor in 1697 high-level applications across different sectors and disciplines, 1698 and with a variety of metadata standards. 1700 o To be a locus of discussion about future versions of the ARK 1701 specification. 1703 Appendix B. Looking up NMAHs Distributed via DNS 1705 This subsection introduces an older method for looking up NMAHs that 1706 is based on the method for discovering URN resolvers described in 1707 [RFC2915]. It relies on querying the DNS system already installed in 1708 the background infrastructure of most networked computers. A query 1709 is submitted to DNS asking for a list of resolvers that match a given 1710 NAAN. DNS distributes the query to the particular DNS servers that 1711 can best provide the answer, unless the answer can be found more 1712 quickly in a local DNS cache as a side-effect of a recent query. 1713 Responses come back inside Name Authority Pointer (NAPTR) records. 1714 The normal result is one or more candidate NMAHs. 1716 In its full generality the [RFC2915] algorithm ambitiously 1717 accommodates a complex set of preferences, orderings, protocols, 1718 mapping services, regular expression rewriting rules, and DNS record 1719 types. This subsection proposes a drastic simplification of it for 1720 the special case of ARK mapping authority discovery. The simplified 1721 algorithm is called Maptr. It uses only one DNS record type (NAPTR) 1722 and restricts most of its field values to constants. The following 1723 hypothetical excerpt from a DNS data file for the NAAN known as 12026 1724 shows three example NAPTR records ready to use with the Maptr 1725 algorithm. 1727 12026.ark.arpa. 1728 ;; US Library of Congress 1729 ;; order pref flags service regexp replacement 1730 IN NAPTR 0 0 "h" "ark" "USLC" lhc.nlm.nih.gov:8080 1731 IN NAPTR 0 0 "h" "ark" "USLC" foobar.zaf.org 1732 IN NAPTR 0 0 "h" "ark" "USLC" sneezy.dopey.com 1734 All the fields are held constant for Maptr except for the "flags", 1735 "regexp", and "replacement" fields. The "service" field contains the 1736 constant value "ark" so that NAPTR records participating in the Maptr 1737 algorithm will not be confused with other NAPTR records. The "order" 1738 and "pref" fields are held to 0 (zero) and otherwise ignored for now; 1739 the algorithm may evolve to use these fields for ranking decisions 1740 when usage patterns and local administrative needs are better 1741 understood. 1743 When a Maptr query returns a record with a flags field of "h" (for 1744 hostport, a Maptr extension to the NAPTR flags), the replacement 1745 field contains the NMAH (hostport) of an ARK service provider. When 1746 a query returns a record with a flags field of "" (the empty string), 1747 the client needs to submit a new query containing the domain name 1748 found in the replacement field. This second sort of record exploits 1749 the distributed nature of DNS by redirecting the query to another 1750 domain name. It looks like this. 1752 12345.ark.arpa. 1753 ;; Digital Library Consortium 1754 ;; order pref flags service regexp replacement 1755 IN NAPTR 0 0 "" "ark" "" dlc.spct.org. 1757 Here is the Maptr algorithm for ARK mapping authority discovery. In 1758 it replace with the NAAN from the ARK for which an NMAH is 1759 sought. 1761 1. Initialize the DNS query: type=NAPTR, query=.ark.arpa. 1763 2. Submit the query to DNS and retrieve (NAPTR) records, discarding 1764 any record that does not have "ark" for the service field. 1766 3. All remaining records with a flags fields of "h" contain 1767 candidate NMAHs in their replacement fields. Set them aside, if 1768 any. 1770 4. Any record with an empty flags field ("") has a replacement field 1771 containing a new domain name to which a subsequent query should 1772 be redirected. For each such record, set query= 1773 then go to step (2). When all such records have been recursively 1774 exhausted, go to step (5). 1776 5. All redirected queries have been resolved and a set of candidate 1777 NMAHs has been accumulated from steps (3). If there are zero 1778 NMAHs, exit -- no mapping authority was found. If there is one 1779 or more NMAH, choose one using any criteria you wish, then exit. 1781 A Perl script that implements this algorithm is included here. 1783 #!/depot/bin/perl 1785 use Net::DNS; # include simple DNS package 1786 my $qtype = "NAPTR"; # initialize query type 1787 my $naa = shift; # get NAAN script argument 1788 my $mad = new Net::DNS::Resolver; # mapping authority discovery 1790 &maptr("$naa.ark.arpa"); # call maptr - that's it 1792 sub maptr { # recursive maptr algorithm 1793 my $dname = shift; # domain name as argument 1794 my ($rr, $order, $pref, $flags, $service, $regexp, 1795 $replacement); 1796 my $query = $mad->query($dname, $qtype); 1797 return # non-productive query 1798 if (! $query || ! $query->answer); 1799 foreach $rr ($query->answer) { 1800 next # skip records of wrong type 1801 if ($rr->type ne $qtype); 1802 ($order, $pref, $flags, $service, $regexp, 1803 $replacement) = split(/\s/, $rr->rdatastr); 1804 if ($flags eq "") { 1805 &maptr($replacement); # recurse 1806 } elsif ($flags eq "h") { 1807 print "$replacement\n"; # candidate NMAH 1808 } 1809 } 1810 } 1812 The global database thus distributed via DNS and the Maptr algorithm 1813 can easily be seen to mirror the contents of the Name Authority 1814 Table file described in the previous section. 1816 Authors' Addresses 1818 John A. Kunze 1819 California Digital Library 1820 415 20th St, 4th Floor 1821 Oakland, CA 94612 1822 USA 1824 Email: jak@ucop.edu 1825 Emmanuelle Bermes 1826 Bibliotheque nationale de France 1827 Quai Francois Mauriac 1828 Paris, Cedex 13 75706 1829 France 1831 Email: emmanuelle.bermes@bnf.fr