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'RFCXXXX' -- Possible downref: Non-RFC (?) normative reference: ref. 'RFCYYYY' ** Obsolete normative reference: RFC 2141 (Obsoleted by RFC 8141) ** Obsolete normative reference: RFC 2434 (Obsoleted by RFC 5226) ** Downref: Normative reference to an Informational RFC: RFC 1737 ** Downref: Normative reference to an Informational RFC: RFC 2276 Summary: 9 errors (**), 0 flaws (~~), 4 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet-Draft L. Daigle 2 URN WG Thinking Cat Enterprises 3 Expires May 11, 2001 D. van Gulik 4 Category: Best Current Practice WebWeaving 5 draft-ietf-urn-rfc2611bis-00.txt R. Iannella 6 DSTC Pty Ltd 7 P. Faltstrom 8 Cisco 9 November 10, 2000 11 URN Namespace Definition Mechanisms 13 Status of this Memo 15 This document is an Internet-Draft and is in full conformance with 16 all provisions of Section 10 of RFC2026. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six 24 months and may be updated, replaced, or obsoleted by other 25 documents at any time. It is inappropriate to use Internet- Drafts 26 as reference material or to cite them other than as "work in 27 progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 Abstract 37 The URN WG has defined a syntax for Uniform Resource Names (URNs) 38 [RFC2141], as well as some proposed mechanisms for their resolution 39 and use in Internet applications ([RFCXXXX], [RFCYYYY]). The whole 40 rests on the concept of individual "namespaces" within the URN 41 structure. Apart from proof-of-concept namespaces, the use of 42 existing identifiers in URNs has been discussed ([RFC2288]), and this 43 document lays out general definitions of and mechanisms for 44 establishing URN "namespaces". 46 This document obsoletes RFC2611. 48 Discussion of this document should be directed to urn-ietf@ietf.org 50 Table of Contents 52 Abstract ........................................................ 1 53 Table of Contents ............................................... 2 54 1.0 Introduction ................................................ 2 55 2.0 What is a URN Namespace? .................................... 3 56 3.0 URN Namespace (Registration) Types .......................... 3 57 3.1 Experimental Namespaces .................................... 4 58 3.2 Informal Namespaces ......................................... 4 59 3.3 Formal Namespaces ........................................... 4 60 4.0 URN Namespace Registration, Update, and NID Assignment 61 Process ..................................................... 5 62 4.1 Experimental ................................................ 5 63 4.2 Informal .................................................... 6 64 4.3 Formal ...................................................... 6 65 5.0 Illustration ................................................ 8 66 5.1 Example Template ............................................ 8 67 5.1 Registration steps in practice .............................. 10 68 6.0 Security Considerations ..................................... 11 69 7.0 IANA Considerations ......................................... 11 70 8.0 References .................................................. 11 71 9.0 Authors' Addresses .......................................... 12 72 10.0 Appendix -- URN Namespace Definition Template .............. 13 74 1.0 Introduction 76 Uniform Resource Names (URNs) are resource identifiers with the 77 specific requirements for enabling location independent 78 identification of a resource, as well as longevity of reference. 79 There are 2 assumptions that are key to this document: 81 Assumption #1: 83 Assignment of a URN is a managed process. 85 I.e., not all strings that conform to URN syntax are necessarily 86 valid URNs. A URN is assigned according to the rules of a 87 particular namespace (in terms of syntax, semantics, and process). 89 Assumption #2: 91 The space of URN namespaces is managed. 93 I.e., not all syntactically correct URN namespaces (per the URN 94 syntax definition) are valid URN namespaces. A URN namespace 95 must have a recognized definition in order to be valid. 97 The purpose of this document is to outline a mechanism and provide a 98 template for explicit namespace definition, along with the mechanism 99 for associating an identifier (called a "Namespace ID", or NID) which 100 is registered with the Internet Assigned Numbers Authority, IANA. 102 Note that this document restricts itself to the description of 103 processes for the creation of URN namespaces. If "resolution" of any 104 so-created URN identifiers is desired, a separate process of 105 registration in a global NID directory, such as that provided by the 106 DDDS system [RFCXXXX], is necessary. See [RFCYYYY] for information 107 on obtaining registration in the DDDS global NID directory. 109 2.0 What is a URN Namespace? 111 For the purposes of URNs, a "namespace" is a collection of uniquely- 112 assigned identifiers. A URN namespace itself has an identifier in 113 order to 115 - ensure global uniqueness of URNs 116 - (where desired) provide a cue for the structure of the 117 identifier 119 For example, ISBNs and ISSNs are both collections of identifiers used 120 in the traditional publishing world; while there may be some number 121 (or numbers) that is both a valid ISBN identifier and ISSN 122 identifier, using different designators for the two collections 123 ensures that no two URNs will be the same for different resources. 125 The development of an identifier structure, and thereby a collection 126 of identifiers, is a process that is inherently dependent on the 127 requirements of the community defining the identifier, how they will 128 be assigned, and the uses to which they will be put. All of these 129 issues are specific to the individual community seeking to define a 130 namespace (e.g., publishing community, association of booksellers, 131 protocol developers, etc); they are beyond the scope of the IETF URN 132 work. 134 This document outlines the processes by which a collection of 135 identifiers satisfying certain constraints (uniqueness of assignment, 136 etc) can become a bona fide URN namespace by obtaining a NID. In a 137 nutshell, a template for the definition of the namespace is completed 138 for deposit with IANA, and a NID is assigned. The details of the 139 process and possibilities for NID strings are outlined below; first, 140 a template for the definition is provided. 142 3.0 URN Namespace (Registration) Types 144 There are 3 categories of URN namespaces defined here, 146 distinguished by expected level of service and required procedures 147 for registration. Registration processes for each of these namespace 148 types are given in Section 4.0. 150 3.1 Experimental Namespaces 152 These are not explicitly registered with IANA. They take the form 154 X- 156 No provision is made for avoiding collision of experimental NIDs; 157 they are intended for use within internal or limited experimental 158 contexts. 160 3.2 Informal Namespaces 162 These are fully fledged URN namespaces, with all the rights and 163 requirements associated thereto. Informal namespaces can be 164 registered in global registration services. They are required to 165 uphold the general principles of a well-managed URN namespace -- 166 providing persistent, unique identification of resources. Informal 167 and formal namespaces (described below) differ in the NID assignment. 168 IANA will assign an alphanumeric NID to registered informal 169 namespaces, per the process outlined in Section 4.0. 171 3.3 Formal Namespaces 173 A formal namespace may be requested, and IETF review sought, in cases 174 where the publication of the NID proposal and the underlying 175 namespace will provide benefit to an open and broad base of the 176 Internet community. That is, as in any open standards outcome, 177 publication of the NID proposal would allow persons not immediately 178 associated with the proposer to create new software, or services, or 179 otherwise better carry out their own activities than if the NID 180 publication had not been made. Benefits are expected to be in the 181 form of open accessibility, interoperability, etc. 183 It is expected that Formal NIDs may be applied to namespaces where 184 some aspects are not fully open. For example, a namespace may make 185 use of an externally managed (proprietary) registry (as, e.g., ISBN 186 does), for assignment of URNs in the namespace, but it may still 187 provide broad community benefit if the services associated have 188 openly-published access protocols. 190 In addition to the basic registration information defined in the 191 registration template (in the Appendix), a formal namespace request 192 must be accompanied by documented considerations of the need for a 193 new namespace and the community benefit of formally establishing the 194 proposed URN namespace. 196 Additionally, since the goal of URNs is to provide persistent 197 identification, some consideration as to the longevity and 198 maintainability of the namespace must be given. The URN WG discussed 199 at length the issue of finding objective measures for predicting (a 200 priori) the continued success of a namespace. No conclusion was 201 reached -- much depends on factors that are completely beyond the 202 technical scope of the namespace. However, the collective experience 203 of the IETF community does contain a wealth of information on 204 technical factors that will prevent longevity of identification. The 205 IESG may elect not to publish a proposed namespace RFC if the IETF 206 community consensus is that it contains technical flaws that will 207 prevent (or seriously impair the possibility of) persistent 208 identification. 210 4.0 URN Namespace Registration, Update, and NID Assignment Process 212 Different levels of disclosure are expected/defined for namespaces. 213 According to the level of open-forum discussion surrounding the 214 disclosure, a URN namespace may be assigned or may request a 215 particular identifier. The "IANA Considerations" document [RFC2434] 216 suggests the need to specify update mechanisms for registrations -- 217 who is given the authority to do so, from time to time, and what are 218 the processes. Since URNs are meant to be persistently useful, few 219 (if any) changes should be made to the structural interpretation of 220 URN strings (e.g., adding or removing rules for lexical equivalence 221 that might affect the interpretation of URN IDs already assigned). 222 However, it may be important to introduce clarifications, expand the 223 list of authorized URN assigners, etc, over the natural course of a 224 namespace's lifetime. Specific processes are outlined below. 226 URN namespace registrations will be posted in the anonymous FTP 227 directory "ftp://ftp.isi.edu/in-notes/iana/assignments/URN- 228 namespaces/". 230 The registration and maintenance procedures vary slightly from one 231 namespace type (as defined in Section 3.0) to another. 233 4.1 Experimental 235 These are not explicitly registered with IANA. They take the form 236 X- 238 No provision is made for avoiding collision of experimental NIDs; 239 they are intended for use within internal or limited experimental 240 contexts. 242 As there is no registration, no registration maintenance procedures 243 are needed. 245 4.2 Informal 247 These are registered with IANA and are assigned a number sequence as 248 an identifier, in the format: 250 "urn-" 252 where is chosen by the IANA on a First Come First Served 253 basis (see [RFC2434]). 255 Registrants should send a copy of the registration template (see the 256 Appendix), duly completed, to the 258 urn-nid@apps.ietf.org 260 mailing and allow for a 2 week discussion period for clarifying the 261 expression of the registration information and suggestions for 262 improvements to the namespace proposal. 264 After suggestions for clarification of the registration information 265 have been incorporated, the template may be submitted to: 267 iana@iana.org 269 for assignment of a NID. 271 The only restrictions on are that it consist strictly of 272 digits and that it not cause the NID to exceed length limitations 273 outlined in the URN syntax ([RFC2141]). 275 Registrations may be updated by the original registrant, or an entity 276 designated by the registrant, by updating the registration template, 277 submitting it to the discussion list for a further 2 week discussion 278 period, and finally resubmitting it to IANA, as described above. 280 4.3 Formal 281 Formal NIDs are assigned via IETF Consensus, as defined in [RFC2434]: 283 "IETF Consensus - New values are assigned through the IETF 284 consensus process. Specifically, new assignments are made via 285 RFCs approved by the IESG. Typically, the IESG will seek 286 input on prospective assignments from appropriate persons 287 (e.g., a relevant Working Group if one exists)." 289 Thus, the Formal NID application is made via publication of an RFC 290 through standard IETF processes. The RFC need not be standards- 291 track, but it will be subject to IESG review and acceptance pursuant 292 to the guidelines written here (as well as standard RFC publication 293 guidelines). The template defined in the Appendix may be included as 294 part of an RFC defining some other aspect of the namespace, or it may 295 be put forward as an RFC in its own right. The proposed template 296 should be sent to the 298 urn-nid@apps.ietf.org 300 mailing list to allow for a 2 week discussion period for clarifying 301 the expression of the registration information, before the IESG 302 reviews the document. 304 The RFC must include a "Namespace Considerations" section, which 305 outlines the perceived need for a new namespace (i.e., where existing 306 namespaces fall short of the proposer's requirements). 307 Considerations might include: 309 - URN assignment procedures - URN resolution/delegation 310 - type of resources to be identified - type of services to 311 be supported 313 NOTE: It is expected that more than one namespace may serve the same 314 "functional" purpose; the intent of the "Namespace Considerations" 315 section is to provide a record of the proposer's "due diligence" in 316 exploring existing possibilities, for the IESG's consideration. 318 The RFC must also include a "Community Considerations" section, which 319 indicates the dimensions upon which the proposer expects the Internet 320 community to be able to benefit by publication of this namespace. 321 Potential considerations include: 323 - open assignment and use of identifiers within the namespace 324 - open operation of resolution servers for the namespace 325 (server) - creation of software that can meaningfully resolve 326 and access services for the namespace (client) 327 A particular NID string is requested, and is assigned by IETF 328 consensus (as defined in [RFC2434]), with the additional constraints 329 that the NID string must 331 - not be an already-registered NID - not start with "x-" 332 (see Type I above) - not start with "urn-" (see Type II above) 333 - not start with "XY-", where XY is any combination of 2 334 ASCII letters (see NOTE, below) - be more than 2 letters long 336 NOTE: ALL two-letter combinations, and two-letter combinations 337 followed by "-" and any sequence of valid NID characters, are 338 reserved for potential use as countrycode- based NIDs for eventual 339 national registrations of URN namespaces. The definition and 340 scoping of rules for allocation of responsibility for such namespaces 341 is beyond the scope of this document. 343 Registrations may be revised by updating the RFC through standard 344 IETF RFC update mechanisms. Thus, proposals for updates may be made 345 by the original authors, other IETF participants, or the IESG. In 346 any case, the proposed updated template must be circulated on the 347 urn-nid discussion list, allowing for a 2 week review period. 349 5.0 Illustration 351 5.1 Example Template 353 The following example is provided for the purposes of illustration of 354 the URN NID template described in the Appendix. Although it is based 355 on a hypothetical "generic Internet namespace" that has been 356 discussed informally within the URN WG, there are still technical and 357 infrastructural issues that would have to be resolved before such a 358 namespace could be properly and completely described. 360 Namespace ID: 361 To be assigned 363 Registration Information: 365 Version 1 366 Date: 368 Declared registrant of the namespace: 370 Required: Name and e-mail address. 371 Recommended: Affiliation, address, etc. 373 Declared registrant of the namespace: 375 Name: T. Cat 376 E-mail: leslie@thinkingcat.com 377 Affiliation: Thinking Cat Enterprises 378 Address: 1 ThinkingCat Way 379 Trupville, NewCountry 381 Declaration of structure: 383 The identifier structure is as follows: 385 URN::: 387 where FQDN is a fully-qualified domain name, and the assigned 388 string is conformant to URN syntax requirements. 390 Relevant ancillary documentation: 392 Definition of domain names, found in: 394 P. Mockapetris, "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION", 395 RFC1035, November 1987. 397 Identifier uniqueness considerations: 399 Uniqueness is guaranteed as long as the assigned string is never 400 reassigned for a given FQDN, and that the FQDN is never 401 reassigned. 403 N.B.: operationally, there is nothing that prevents a domain name 404 from being reassigned; indeed, it is not an uncommon occurrence. 405 This is one of the reasons that this example makes a poor URN 406 namespace in practice, and is therefore not seriously being 407 proposed as it stands. 409 Identifier persistence considerations: 411 Persistence of identifiers is dependent upon suitable delegation 412 of resolution at the level of "FQDN"s, and persistence of FQDN 413 assignment. 415 Same note as above. 417 Process of identifier assignment: 419 Assignment of these URNs delegated to individual domain name 420 holders (for FQDNs). The holder of the FQDN registration is 421 required to maintain an entry (or delegate it) in the DDDS. 422 Within each of these delegated name partitions, the string may be 423 assigned per local requirements. 425 e.g. urn::thinkingcat.com:001203 427 Process for identifier resolution: 429 Domain name holders are responsible for operating or delegating 430 resolution servers for the FQDN in which they have assigned URNs. 432 Rules for Lexical Equivalence: 434 FQDNs are case-insensitive. Thus, the portion of the URN 436 urn::: 438 is case-insenstive for matches. The remainder of the identifier 439 must be considered case-sensitve. 441 Conformance with URN Syntax: 443 No special considerations. 445 Validation mechanism: 447 None specified. 449 Scope: 451 Global. 453 5.1 Registration steps in practice 455 The key steps for registration of informal or formal namespaces 456 typically play out as follows: 458 Informal NID: 460 1. Complete the registration template. This may be done as part 461 of an Internet-Draft. 463 2. Communicate the registration template to urn-nid@apps.ietf.org 464 for technical review -- as a published I-D, or text e-mail message 465 containing the template. 467 3. Update the registration template as necessary from comments, and 468 repeat steps 2 and 3 as necessary. 470 4. Once comments have been addressed (and the review period has 471 expired) end a request to IANA with the revised registration 472 template. 474 Formal NID: 476 1. Write an Internet-Draft describing the namespace and including 477 the registration template, duly completed. 479 2. Send the Internet-Draft to the I-D editor, and send a copy to 480 urn-nid@apps.ietf.org for technical review. 482 3. Update the Internet-Draft as necessary from comments, and repeat 483 steps 2 and 3 as needed. 485 4. Send a request to the IESG to publish the I-D as an RFC. The 486 IESG may request further changes (published as I-D revisions) 487 and/or direct discussion to designated working groups, area 488 experts, etc. 490 5. If the IESG approves the document for publication as an RFC, 491 send a request to IANA to register the requested NID. 493 6.0 Security Considerations 495 This document largely focuses on providing mechanisms for the 496 declaration of public information. Nominally, these declarations 497 should be of relatively low security profile, however there is always 498 the danger of "spoofing" and providing mis-information. Information 499 in these declarations should be taken as advisory. 501 7.0 IANA Considerations 503 This document outlines the processes for registering URN namespaces, 504 and has implications for the IANA in terms of registries to be 505 maintained. In all cases, the IANA should assign the appropriate NID 506 (informal or formal), as described above, once an IESG-designated 507 expert has confirmed that the requisite registration process steps 508 have been completed. 510 8.0 References 512 [ISO8601] ISO 8601 : 1988 (E), "Data elements and interchange 513 formats - Information interchange - Representation of 514 dates and times" 516 [RFC2288] Lynch, C., Preston, C. and R. Daniel, "Using Existing 517 Bibliographic Identifiers as Uniform Resource Names", RFC 518 2288, February 1998. 520 [RFCXXXX] Mealling, M., "URI Resolution using the Dynamic 521 Delegation Discovery System", RFCXXXX. 523 [RFCYYYY] Mealling, M., "Assignment Procedures for URI Resolution 524 Using DNS", RFCYYYY. 526 [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. 528 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 529 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 530 October 1998. 532 [RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for 533 Uniform Resource Names", RFC 1737, December 1994. 535 [RFC2276] Sollins, K., "Architectural Principles of Uniform 536 Resource Name Resolution", RFC 2276, January 1998. 538 9.0 Authors' Addresses 540 Leslie L. Daigle 541 Thinking Cat Enterprises 543 EMail: leslie@thinkingcat.com 545 Dirk-Willem van Gulik 546 WebWeaving 547 Plein 1813 - 5a 548 8545 HX Arnhem 549 The Netherlands 551 Phone: +39 0332 78 0014 (Phone and Fax) 552 EMail: Dirkx@webweaving.org 554 Renato Iannella 555 DSTC Pty Ltd 556 Gehrmann Labs, The Uni of Queensland 557 AUSTRALIA, 4072 559 Phone: +61 7 3365 4310 560 Fax: +61 7 3365 4311 561 EMail: renato@dstc.edu.au 563 Patrik Faltstrom 564 Cisco Systems Inc 565 170 W Tasman Drive SJ-13/2 566 San Jose CA 95134 567 USA 569 EMail: paf@cisco.com 570 URL: http://www.cisco.com 572 10.0 Appendix -- URN Namespace Definition Template 574 Definition of a URN namespace is accomplished by completing the 575 following information template. Apart from providing a mechanism for 576 disclosing structure of the URN namespace, this information is 577 designed to be useful for 579 - entities seeking to have a URN assigned in a namespace (if 580 applicable) 581 - entities seeking to provide URN resolvers for a namespace (if 582 applicable) 584 This is particularly important for communities evaluating the 585 possibility of using a portion of an existing URN namespace rather 586 than creating their own. 588 Information in the template is as follows: 590 Namespace ID: 591 Assigned by IANA. In some contexts, a particular one may be 592 requested (see below). 594 Registration Information: 596 This is information to identify the particular version of 597 registration information: 599 - registration version number: starting with 1, incrementing by 1 600 with each new version 601 - registration date: date submitted to the IANA, using the format 602 YYYY-MM-DD 603 as outlined in [ISO8601]. 605 Declared registrant of the namespace: 607 Required: Name and e-mail address. 608 Recommended: Affiliation, address, etc. 610 Declaration of syntactic structure: 612 This section should outline any structural features of identifiers 613 in this namespace. At the very least, this description may be 614 used to introduce terminology used in other sections. This 615 structure may also be used for determining realistic 616 caching/shortcuts approaches; suitable caveats should be provided. 617 If there are any specific character encoding rules (e.g., which 618 character should always be used for single-quotes), these should 619 be listed here. 621 Answers might include, but are not limited to: 623 - the structure is opaque (no exposition) - a regular expression 624 for parsing the identifier into components, including naming 625 authorities 627 Relevant ancillary documentation: 629 This section should list any RFCs, standards, or other published 630 documentation that defines or explains all or part of the 631 namespace structure. 633 Answers might include, but are not limited to: 635 - RFCs outlining syntax of the namespace 636 - Other of the defining community's (e.g., ISO) documents 637 outlining syntax of the identifiers in the namespace 638 - Explanatory material introducing the namespace 640 Identifier uniqueness considerations: This section should address the 641 requirement that URN identifiers be assigned uniquely -- they are 642 assigned to at most one resource, and are not reassigned. 644 (Note that the definition of "resource" is fairly broad; for example, 645 information on "Today's Weather" might be considered a single 646 resource, although the content is dynamic.) 648 Possible answers include, but are not limited to: 650 - exposition of the structure of the identifiers, and partitioning 651 of the space of identifiers amongst assignment authorities which 652 are individually responsible for respecting uniqueness rules 653 - identifiers are assigned sequentially 654 - information is withheld; the namespace is opaque 656 Identifier persistence considerations: 658 Although non-reassignment of URN identifiers ensures that a URN 659 will persist in identifying a particular resource even after the 660 "lifetime of the resource", some consideration should be given to 661 the persistence of the usability of the URN. This is particularly 662 important in the case of URN namespaces providing global 663 resolution. 665 Possible answers include, but are not limited to: 667 - quality of service considerations 669 Process of identifier assignment: 671 This section should detail the mechanisms and/or authorities for 672 assigning URNs to resources. It should make clear whether 673 assignment is completely open, or if limited, how to become an 674 assigner of identifiers, and/or get one assigned by existing 675 assignment authorities. Answers could include, but are not 676 limited to: 678 - assignment is completely open, following a particular algorithm 679 - assignment is delegated to authorities recognized by a 680 particular organization (e.g., the Digital Object Identifier 681 Foundation controls the DOI assignment space and its delegation) 682 - assignment is completely closed (e.g., for a private 683 organization) 685 Process for identifier resolution: 687 If a namespace is intended to be accessible for global resolution, 688 it must be registerd in an RDS (Resolution Discovery System, see 689 [RFC2276]) such as DDDS. Resolution then proceeds according to 690 standard URI resolution processes, and the mechanisms of the RDS. 691 What this section should outline is the requirements for becoming 692 a recognized resolver of URNs in this namespace (and being so- 693 listed in the RDS registry). 695 Answers may include, but are not limited to: 697 - the namespace is not listed with an RDS; this is not relevant 698 - resolution mirroring is completely open, with a mechanism for 699 updating an appropriate RDS 700 - resolution is controlled by entities to which assignment has 701 been delegated 703 Rules for Lexical Equivalence: 705 If there are particular algorithms for determining equivalence 706 between two identifiers in the underlying namespace (hence, in the 707 URN string itself), rules can be provided here. 709 Some examples include: 711 - equivalence between hyphenated and non-hyphenated groupings in 712 the identifier string 713 - equivalence between single-quotes and double-quotes 714 - Namespace-defined equivalences between specific characters, such 715 as "character X with or without diacritic marks". 717 Note that these are not normative statements for any kind of best 718 practice for handling equivalences between characters; they are 719 statements limited to reflecting the namespace's own rules. 721 Conformance with URN Syntax: 723 This section should outline any special considerations required 724 for conforming with the URN syntax. This is particularly 725 applicable in the case of legacy naming systems that are used in 726 the context of URNs. 728 For example, if a namespace is used in contexts other than URNs, 729 it may make use of characters that are reserved in the URN syntax. 730 This section should flag any such characters, and outline 731 necessary mappings to conform to URN syntax. Normally, this will 732 be handled by hex encoding the symbol. 734 For example, see the section on SICIs in [RFC2288]. 736 Validation mechanism: 738 Apart from attempting resolution of a URN, a URN namespace may 739 provide mechanism for "validating" a URN -- i.e., determining 740 whether a given string is currently a validly-assigned URN. For 741 example, even if an ISBN URN namespace is created, it is not clear 742 that all ISBNs will translate directly into "assigned URNs". 744 A validation mechanims might be: 746 - a syntax grammar 747 - an on-line service 748 - an off-line service 750 Scope: 752 This section should outline the scope of the use of the 753 identifiers in this namespace. Apart from considerations of 754 private vs. public namespaces, this section is critical in 755 evaluating the applicability of a requested NID. For example, a 756 namespace claiming to deal in "social security numbers" should 757 have a global scope and address all social security number 758 structures (unlikely). On the other hand, at a national level, it 759 is reasonable to propose a URN namespace for "this nation's social 760 security numbers".