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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group G. Scott, Editor 3 INTERNET DRAFT Defense Information Systems Agency 4 December 1997 6 Guide for Internet Standards Writers 7 9 Status of this Memo 11 This document is an Internet Draft. Internet Drafts are working 12 documents of the Internet Engineering Task Force (IETF), its areas, 13 and its working groups. Note that other groups may also distribute 14 working documents as Internet Drafts. 16 Internet Drafts are draft documents valid for a maximum of six months 17 and may be updated, replaced, or obsoleted by other documents at any 18 time. It is not appropriate to use Internet Drafts as reference 19 material or to cite them other than as a "working draft" or "work in 20 progress." 22 To learn the current status of any Internet-Draft, please check the 23 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 24 Directories on ds.internic.net (US East Coast), nic.nordu.net 25 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific 26 Rim). 28 Distribution of this document is unlimited. 30 This Internet Draft expires on 30 June 1997. 32 Abstract 34 This document is a guide for Internet standard writers. It defines 35 those characteristics that make standards coherent, unambiguous, and 36 easy to interpret. Also, it singles out usage believed to have led 37 to unclear specifications, resulting in non-interoperable 38 interpretations in the past. These guidelines are to be used with 39 RFC 1543, ''Instructions to RFC Authors.'' 41 This version of the document is a draft. 43 CHANGES FROM PREVIOUS DRAFT 45 A paragraph pointing to a pending document that further addresses 46 security was updated. 48 References to RFC 1583 were changed to RFC 2178 which obsoleted it. 50 Editorial changes and corrections were also made. 52 Table of Contents 54 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2 General Guidelines . . . . . . . . . . . . . . . . . . . . . . 3 56 2.1 Discussion of Security . . . . . . . . . . . . . . . . . . . . 3 57 2.2 Protocol Description . . . . . . . . . . . . . . . . . . . . . 5 58 2.3 Target Audience . . . . . . . . . . . . . . . . . . . . . . . 6 59 2.4 Level of Detail . . . . . . . . . . . . . . . . . . . . . . . 6 60 2.5 Protocol Versions . . . . . . . . . . . . . . . . . . . . . . 6 61 2.6 Decision History . . . . . . . . . . . . . . . . . . . . . . . 7 62 2.7 Response to Out of Specification Behavior . . . . . . . . . . 7 63 2.8 The Liberal/Conservative Rule . . . . . . . . . . . . . . . . 7 64 2.9 Handling of Protocol Options . . . . . . . . . . . . . . . . . 8 65 2.10 Indicating Requirement Levels . . . . . . . . . . . . . . . . 9 66 2.11 Notational Conventions . . . . . . . . . . . . . . . . . . . . 10 67 2.12 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 68 2.13 Network Management Considerations . . . . . . . . . . . . . . 11 69 2.14 Scalability Considerations . . . . . . . . . . . . . . . . . . 11 70 2.15 Network Stability . . . . . . . . . . . . . . . . . . . . . . 11 71 2.16 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 72 3 Specific Guidelines . . . . . . . . . . . . . . . . . . . . . 12 73 3.1 Packet Diagrams . . . . . . . . . . . . . . . . . . . . . . . 12 74 3.2 Summary Tables . . . . . . . . . . . . . . . . . . . . . . . . 13 75 3.3 State Machine Descriptions . . . . . . . . . . . . . . . . . . 14 76 3.4 Character Sets . . . . . . . . . . . . . . . . . . . . . . . . 16 77 4 Document Checklist . . . . . . . . . . . . . . . . . . . . . . 16 78 5 Security Considerations . . . . . . . . . . . . . . . . . . . 17 79 6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 80 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 81 8 Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 19 83 1 Introduction 85 This document is a guide for Internet standard writers. It offers 86 guidelines on how to write a standards-track document with clarity, 87 precision, and completeness. These guidelines are based on both 88 prior successful and unsuccessful IETF specification experiences. 89 These guidelines are to be used with RFC 1543, "Instructions to RFC 90 Authors," or its update. Note that some guidelines may not apply in 91 certain situations. 93 The process for standardizing protocols and procedures is given in 94 BCP 9/RFC 2026, "The Internet Standards Process -- Revision 3." Some 95 considerations for protocol design are given in RFC 1958, 96 Architectural Principles of the Internet." 97 The goal is to increase the possibility that multiple implementations 98 of a protocol will interoperate. Writing specifications to these 99 guidelines will not guarantee interoperability. However, a 100 recognized barrier to the creation of interoperable protocol 101 implementations is unclear specifications. 103 Many will benefit from having well-written protocol specifications. 104 Implementors will have a better chance to conform to the protocol 105 specification. Protocol testers can use the specification to derive 106 unambiguous testable statements. Purchasers and users of the 107 protocol will have a better understanding of its capabilities. 109 2 General Guidelines 111 It is important that multiple readers and implementors of a standard 112 have the same understanding of a document. To this end, information 113 should be orderly and detailed. The following are general guidelines 114 intended to help in the production of such a document. The IESG may 115 require that all or some of the following sections appear in a 116 standards track document. 118 2.1 Discussion of Security 120 If the Internet is to achieve its full potential in commercial, 121 governmental, and personal affairs, it must assure users that their 122 information transfers are free from tampering or compromise. Well- 123 written security sections in standards-track documents can help 124 promote the confidence level required. For an implementor will find 125 it easier to provide the security measures specified. While users 126 will understand the security measures, and so have a higher level of 127 trust in the Internet. Above all, new protocols and practices must 128 not worsen overall Internet security. 130 A significant threat to the Internet are those individuals who are 131 motivated and capable of exploiting circumstances, events, or 132 vulnerabilities of the system to cause harm. Also, deliberate or 133 inadvertent user behavior may expose the system to attack or 134 exploitation. The harm could range from disrupting or denying 135 network service, to damaging user systems. Additionally, information 136 disclosure could provide the means to attack another system, or 137 reveal patterns of behavior that could be used to harm an individual, 138 organization, or network. This is a particular concern with 139 standards that define a portion of the Management Information Base 140 (MIB). 142 Standards authors must accept that the protocol they specify will be 143 subject to attack. They are responsible for determining what attacks 144 are possible, and for detailing the nature of the attacks in the 145 document. Otherwise, they must convincingly argue that attack is not 146 realistic in a specific environment, and restrict the use of the 147 protocol to that environment. 149 This discussion of the threat model and other assumptions should 150 appear early in the standard. Doing so will establish a basis for 151 the further discussion of security throughout the document. 153 After the document has exhaustively identified the security risks the 154 protocol is exposed to, the authors must formulate and detail a 155 defense against those attacks. They must discuss the applicable 156 countermeasures employed, or the risk the user is accepting by using 157 the protocol. The countermeasures may be provided by a protocol 158 mechanism or by reliance on external mechanisms. Authors should be 159 knowledgeable of existing security mechanisms, and reuse them if 160 practical. When cryptographic algorithms are use, the protocol 161 should be written to permit its substitution with another algorithm 162 in the future. Finally, the authors should discuss implementation 163 hints or guidelines, e.g., how to deal with untrustworthy data or 164 peer systems. 166 Additionally, the effects the security measures have on the 167 protocol's use and performance should be discussed. Security 168 measures will have an impact on the environment they are used in. 169 Perhaps users will now be locked out of portions of the Internet 170 previously open to them, or users will experience a degradation in 171 the speed of service. The user may decided to accept a greater risk 172 in exchange for improved access or service. But the user must be 173 able to make an informed decision. They need to understand the risks 174 they are facing and the costs of reducing their risk. 176 The discussion of security can be concentrated in the Security 177 Considerations section of the document, or throughout the document 178 where it is relevant to particular parts of the specification. An 179 advantage of the second approach is that it ensures security is an 180 integral part of the protocol's development, rather than something 181 that is a follow-on or secondary effort. If security is discussed 182 throughout the document, the Security Considerations section must 183 summarized and make reference to the appropriate specification 184 sections. This will insure that the protocol's security measures are 185 emphasized to implementor and user both. 187 Within the Security Considerations section a discussion of the path 188 not taken may be appropriate. There may be several security 189 mechanisms that were not selected for a variety of reasons: cost or 190 difficulty of implementation; ineffectiveness for a given network 191 environment; or export control. By listing the mechanisms they did 192 not use and the reasons, editors can demonstrate that the protocol's 193 WG gave security the necessary thought. Also, this gives the 194 protocol's users the information they need to consider whether one of 195 the non-selected mechanisms would be better suited to their 196 particular requirements. 198 A document giving further guidance on security topics is in 199 developmemnt. Authors should obtain a copy of the completed RFC to 200 help them prepare the security portion of the standard. 202 Finally, it is no longer acceptable that Security Considerations 203 sections consist solely of statements to the effect that security was 204 not considered in preparing the standard. 206 Some examples of Security Considerations sections are found in 207 STD 33/RFC 1350, STD 51/RFC 1662, and STD 53/RFC 1939. 209 2.2 Protocol Description 211 Standards track documents must include a description of the protocol. 212 This description must address the protocol's purpose, intended 213 functions, services it provides, and, the arena, circumstances, or 214 any special considerations of the protocol's use. 216 The authors of a protocol specification will have a great deal of 217 knowledge as to the reason for the protocol. However, the reader is 218 more likely to have general networking knowledge and experience, 219 rather than expertise in a particular protocol. An explanation of 220 it's purpose and use will give the reader a reference point for 221 understanding the protocol, and where it fits in the Internet. The 222 Draft Standard RFC 2178 was recommended to the STDGUIDE working guide 223 as providing a good example of this in it's "Protocol Overview" 224 section. 226 The protocol's general description must also provide information on 227 the relationship between the different parties to the protocol. 228 This can be done by showing typical packet sequences. 230 This also applies to the algorithms used by a protocol. A detailed 231 description of the algorithms or citation of readily available 232 references that give such a description is necessary. 234 2.3 Target Audience 236 RFCs have been written with many different purposes, ranging from the 237 technical to the administrative. Those written as standards should 238 clearly identify the intended audience, for example, designers, 239 implementors, testers, help desk personnel, educators, end users, or 240 others. If there are multiple audiences being addressed in the 241 document, what sections are for each audience needs to be identified. 242 The goal is to help the reader discover and focus on what they have 243 turned to the document for, and avoid what they may find confusing, 244 diverting, or extraneous. 246 2.4 Level of Detail 248 The author should consider what level of descriptive detail best 249 conveys the protocol's intent. Concise text has several advantages. 250 It makes the document easier to read. Such text reduces the chance 251 for conflict between different portions of the specification. The 252 reader can readily identify the required protocol mechanisms in the 253 standard. Also, it makes it easier to identify the requirements for 254 protocol implementation. A disadvantage of concise descriptions is 255 that a reader may not fully comprehend the reasoning behind the 256 protocol, and thus make assumptions that will lead to implementation 257 errors. 259 Longer descriptions may be necessary to explain purpose, background, 260 rationale, implementation experience, or to provide tutorial 261 information. This helps the reader understand the protocol. Yet 262 several dangers exist with lengthy text. Finding the protocol 263 requirements in the text is difficult or confusing. The same 264 mechanism may have multiple descriptions, which leads to 265 misinterpretations or conflict. Finally, it is more difficult to 266 comprehend, a consideration as English is not the native language of 267 the many worldwide readers of IETF standards. 269 One approach is to divide the standard into sections: one describing 270 the protocol concisely, while another section consists of explanatory 271 text. The STD 3/RFC 1122/RFC 1123 and Draft Standard RFC 2178 272 provides examples of this method. 274 2.5 Protocol Versions 276 Often the standard is specifying a new version of an existing 277 protocol. In such a case, the authors should detail the differences 278 between the previous version and the new version. This should 279 include the rationale for the changes, for example, implementation 280 experience, changes in technology, responding to user demand, etc. 282 2.6 Decision History 284 In standards development, reaching consensus requires making 285 difficult choices. These choices are made through working group 286 discussions or from implementation experience. By including the 287 basis for a contentious decision, the author can prevent future 288 revisiting of these disagreements when the original parties have 289 moved on. Also, the knowledge of the "why" is as useful to an 290 implementor as the description of "how." For example, the 291 alternative not taken may have been simpler to implement, so 292 including the reasons behind the choice may prevent future 293 implementors from taking nonstandard shortcuts. 295 2.7 Response to Out of Specification Behavior 297 The STDGUIDE working group recommends that detail description of the 298 actions taken in case of behavior that is deviant from or exceeds the 299 specification be included. This is an area where implementors often 300 differ in opinion as to the appropriate response. By specifying a 301 common response, the standard author can reduce the risk that 302 different implementations will come in to conflict. 304 The standard should describe responses to behavior explicitly 305 forbidden or out of the boundaries defined by the specification. Two 306 possible approaches to such cases are discarding, or invoking 307 error-handling mechanisms. If discarding is chosen, detailing the 308 disposition may be necessary. For instance, treat dropped frames as 309 if they were never received, or reset an existing connection or 310 adjacency state. 312 The specification should describe actions taken when critical 313 resource or performance scaling limits are exceeded. This is not 314 necessary for every case. It is necessary for cases where a risk of 315 network degradation or operational failure exists. In such cases, a 316 consistent behavior between implementations is necessary. 318 2.8 The Liberal/Conservative Rule 320 A rule, first stated in STD 5/RFC 791, recognized as having benefits 321 in implementation robustness and interoperability is: 323 "Be liberal in what you accept, and 324 conservative in what you send." 326 Or establish restrictions on what a protocol transmits, but be able 327 to deal with every conceivable error received. Caution is urged in 328 applying this approach in standards track protocols. It has in the 329 past lead to conflicts between vendors when interoperability fails. 330 The sender accuses the receiver of failing to be liberal enough, and 331 the receiver accuses the sender of not being conservative enough. 332 Therefore, the author is obligated to provide extensive detail on 333 send and receive behavior. 335 To avoid any confusion between the two, recommend that standard 336 authors specify send and receive behavior separately. The 337 description of reception will require the most detailing. For 338 implementations will be expected to accept any packet from the 339 network without failure or malfunction. Therefore, the actions taken 340 to achieve that result, need to be laid out in the protocol 341 specification. Standard authors should consider not just how to 342 survive on the network, but achieve the highest level of cooperation 343 possible to limit the amount of network disruption. The appearance 344 of undefined information or conditions must not cause a network or 345 host failure. This requires specification on how to attempt 346 acceptance of most of the packets. Two approaches are available, 347 either using as much of the packet's content as possible, or invoking 348 error procedures. The author should specify a dividing line on when 349 to take which approach. 351 A case for consideration is that of a routing protocol, where 352 acceptance of flawed information can cause network failure. For 353 protocols such as this, the specification should identify packets 354 that could have differing interpretations and mandate that they be 355 either rejected completely or the nature of the attempt to recover 356 some information from them. For example, routing updates that 357 contain more data than the tuple count shows. The protocol authors 358 should consider whether some trailing data can be accepted as 359 additional routes, or to reject the entire packet as suspect because 360 it is non-conformant. 362 2.9 Handling of Protocol Options 364 Specifications with many optional features increase the complexity of 365 the implementation and the chance of non-interoperable 366 implementations. The danger is that different implementations may 367 specify some combination of options that are unable to interoperate 368 with each other. 370 As the document moves along the standard track, implementation 371 experience shall determine the need for each option. Implementation 372 shall show whether the option should be a mandatory part of the 373 protocol or remains an option. If an option is not implemented as 374 the document advances, it must be removed from the protocol before it 375 reaches draft standard status. 377 Therefore, options shall only be present in a protocol to address a 378 real requirement. For example, options can support future 379 extensibility of the protocol, a particular market, e.g., the 380 financial industry, or a specific network environment, e.g., a 381 network constrained by limited bandwidth. They shall not be included 382 as a means to "buy-off" a minority opinion. Omission of the optional 383 item shall have no interoperability consequences for the 384 implementation that does so. 386 One possible approach is to document protocol options in a separate 387 document. Doing so would make it clear that the options are not 388 integral to the implementation of the protocol, and would keep the 389 main protocol specification clean. Regardless of whether they appear 390 within the specification or in a separate document, the text shall 391 discuss the full implications of either using the option or not, and 392 the case for choosing either course. As part of this, the author 393 needs to consider and describe how the options are intended to be 394 used alongside other protocols. The text must also specify the 395 default conditions of all options. For security checking options the 396 default condition is on or enabled. 398 There may be occasions when mutually exclusive options appear within 399 a protocol. That is, the implementation of an optional feature 400 precludes the implementation of the other optional feature. For 401 clarity, the author needs to state when to implement one or the 402 other, what the effect of choosing one over the other is, and what 403 problems the implementor or user may face. The choice of one or the 404 other options shall have no interoperability consequences between 405 multiple implementations. 407 2.10 Indicating Requirement Levels 409 The BCP 14/RFC 2119, "Key words for use in RFCs to Indicate 410 Requirement Level," defines several words that are necessary for 411 writing a standards track document. Editors of standards track 412 documents must not deviate from the definitions provided as they are 413 intended to identify interoperability requirements or limit 414 potentially harmful behavior. The capitalization of these words is 415 the accepted norm, and can help in identifying an unintentional or 416 unreasonable requirement. These words have been used in several RFCs 417 the first instances being STD 3/RFC 1122/RFC 1123. 419 2.11 Notational Conventions 421 Formal syntax notations can be used to define complicated protocol 422 concepts or data types, and to specify values of these data types. 423 This permits the protocol to be written without concern on how the 424 implementation is constructed, or how the data type is represented 425 during transfer. The specification is simplified because it can be 426 presented as "axioms" that will be proven by implementation. 428 The formal specification of the syntax used should be referenced in 429 the text of the standard. Any extensions, subsets, alterations, or 430 exceptions to that formal syntax should be defined within the 431 standard. 433 The STD 11/RFC 822 provides an example of this. In RFC 822 (Section 434 2 and Appendix D) the Backus-Naur Form (BNF) meta-language was 435 extended to make its representation smaller and easier to understand. 436 Another example is STD 16/RFC 1155 (Section 3.2) where a subset of 437 the Abstract Syntax Notation One (ASN.1) is defined. 439 The author of a standards track protocol needs to consider several 440 things before they use a formal syntax notation. Is the formal 441 specification language being used parseable by an existing machine? 442 If no parser exists, is there enough information provided in the 443 specification to permit the building of a parser? If not, it is 444 likely the reader will not have enough information to decide what the 445 notation means. Also, the author should remember machine parseable 446 syntax is often unreadable by humans, and can make the specification 447 excessive in length. Therefore, syntax notations cannot take the 448 place of a clearly written protocol description. 450 2.12 IANA Considerations 452 The common use of the Internet standard track protocols by the 453 Internet community requires that the unique values be assigned to the 454 parameter fields. An IETF WG does not have the authority to assign 455 these values for the protocol it is working on. The Internet 456 Assigned Numbers Authority (IANA) is the central coordinator for the 457 assignment of unique parameter values for Internet protocols, and is 458 responsible for establishing the procedures by which it does so. The 459 authors of a developing protocol that use a link, socket, port, 460 protocol, etc., need to coordinate with the IANA the rules and 461 procedures to be used to register constants and tags. This 462 coordination needs to be completed prior to submitting the internet 463 draft to the standards track. For further information on parameter 464 assignment and current assignments, authors can reference STD 2/RFC 465 1700, "Assigned Numbers." 467 2.13 Network Management Considerations 469 When relevant, each standard needs to discuss how to manage the 470 protocol being specified. This management process should be 471 compatible with the current IETF Standard management protocol. Also 472 a MIB must be defined within the standard or in a companion document. 473 The MIB must be compatible with current SMI and parseable using a 474 tool such as SMICng. Where management or a MIB is not necessary this 475 section of the standard should explain the reason it is not relevant 476 to the protocol. 478 2.14 Scalability Considerations 480 The standard should establish the limitations on the scale of use, 481 e.g., tens of millions of sessions, gigabits per second, etc., and 482 establish limits on the resources used, e.g, round trip time, 483 computing resources, etc. This is important because it establishes 484 the ability of the network to accommodate the number of users and the 485 complexity of their relations. The STD 53/RFC 1939 has an example of 486 such a section. If this is not applicable to the protocol an 487 explanation of why not should be included. 489 2.15 Network Stability 491 A standard should discuss the relationship between network topology 492 and convergence behavior. As part of this, any topology which would 493 be troublesome for the protocol should be identified. Additionally, 494 the specification should address any possible destablizing events, 495 and how the protocol resists or recovers from them. The purpose is 496 to insure that the network will stabilize, in a timely fashion, after 497 a change, and that a combination of errors or events will not plunge 498 the network into chaos. The STD 34/RFC 1058, as an example, has 499 sections which discuss how that protocol handles the affects of 500 changing topology. 502 The obvious case this would apply to is a routing protocol. However, 503 an application protocol could also have dynamic behavior that would 504 affect the network. For example, a messaging protocol could suddenly 505 dump a large number of messages onto the network. Therefore, editors 506 of an application protocol will have to consider possible impacts to 507 network stability and convergence behavior. 509 2.16 Glossary 511 Every standards track RFC should have a glossary, as words can have 512 many meanings. By defining any new words introduced, the author can 513 avoid confusing or misleading the implementer. The definition should 514 appear on the word's first appearance within the text of the protocol 515 specification, and in a separate glossary section. 517 It is likely that definition of the protocol will rely on many words 518 frequently used in IETF documents. All authors must be knowledgeable 519 of the common accepted definitions of these frequently used words. 520 FYI 18/RFC 1983, "Internet Users' Glossary," provides definitions 521 that are specific to the Internet. Any deviation from these 522 definitions by authors is strongly discouraged. If circumstances 523 require deviation, an author should state that he is altering the 524 commonly accepted definition, and provide rationale as to the 525 necessity of doing so. The altered definition must be included in 526 the Glossary section. 528 If the author uses the word as commonly defined, she does not have to 529 include the definition in the glossary. As a minimum, FYI 18/RFC 530 1983 should be referenced as a source. 532 3 Specific Guidelines 534 The following are guidelines on how to present specific technical 535 information in standards. 537 3.1 Packet Diagrams 539 Most link, network, and transport layer protocols have packet 540 descriptions. The STDGUIDE working group recommends that packet 541 diagrams be included in the standard, as they are very helpful to the 542 reader. The preferred form for packet diagrams is a sequence of long 543 words in network byte order, with each word horizontal on the page 544 and bit numbering at the top: 546 0 1 2 3 547 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 548 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 549 |Version| Prio. | Flow Label | 550 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 552 In cases where a packet is strongly byte-aligned rather than 553 word-aligned (e.g., when byte-boundary variable-length fields are 554 used), display packet diagrams in a byte-wide format. The author can 555 use different height boxes for short and long words, and broken boxes 556 for variable-length fields: 558 0 1 2 3 4 5 6 7 559 +-+-+-+-+-+-+-+-+ 560 | Length N | 561 +-+-+-+-+-+-+-+-+ 562 | | 563 + Address + 564 ... 565 + (N bytes) + 566 | | 567 +-+-+-+-+-+-+-+-+ 568 | | 569 + 2-byte field + 570 | | 571 +-+-+-+-+-+-+-+-+ 573 3.2 Summary Tables 575 The specifications of some protocols are particularly lengthy, 576 sometimes covering a hundred pages or more. In such cases the 577 inclusion of a summary table can reduce the risk of conformance 578 failure by an implementation through oversight. A summary table 579 itemizes what in a protocol is mandatory, optional, or prohibited. 580 Summary tables do not guarantee conformance, but serve to assist an 581 implementor in checking that they have addressed all protocol 582 features. 584 The summary table will consist of, as a minimum, four (4) columns: 585 Protocol Feature, Section Reference, Status, and 586 References/Footnotes. The author may add columns if they further 587 explain or clarify the protocol. 589 In the Protocol Feature column describe the feature, for example, a 590 command word. We recommend grouping series of related transactions 591 under descriptive headers, for example, RECEPTION. 593 Section reference directs the implementor to the section, paragraph, 594 or page that describes the protocol feature in detail. 596 Status indicates whether the feature is mandatory, optional, or 597 prohibited. The author can either use a separate column for each 598 possibility, or a single column with appropriate codes. These codes 599 need to be defined at the start of the summary table to avoid 600 confusion. Possible status codes: 602 M - must or mandatory 603 MN - must not 604 O - optional 605 S - should 606 SN - should not 607 X - prohibited 609 In the References/Footnotes column authors can point to other RFCs 610 that are necessary to consider in implementing this protocol feature, 611 or any footnotes necessary to explain the implementation further. 613 The STD 3/RFC 1122/RFC 1123 provides examples of summary tables. 615 3.3 State Machine Descriptions 617 A convenient method of presenting a protocol's behavior is as a 618 state-machine model. That is, a protocol can be described by a 619 series of states resulting from a command, operation, or transaction. 620 State-machine models define the variables and constants that 621 establish a state, the events that cause state transitions, and the 622 actions that result from those transitions. Through these models, an 623 understanding of the protocol's dynamic operation as sequence of 624 state transitions that occur for any given event is possible. 625 State transitions can be detailed by diagrams, tables, or time lines. 627 Note that state-machine models are never to take the place of 628 detailed text description of the specification. They are adjuncts to 629 the text. The protocol specification shall always take precedence in 630 the case of a conflict. 632 When using a state transition diagram, show each possible protocol 633 state as a box connected by state transition arcs. The author should 634 label each arc with the event that causes the transition, and, in 635 parentheses, any actions taken during the transition. The STD 5/RFC 636 1112 provides an example of such a diagram. As ASCII text is the 637 preferred storage format for RFCs, only simple diagrams are possible. 638 Tables can summarize more complex or extensive state transitions. 640 In a state transition table, read events vertically and states 641 horizontally. The form, action/new state, represents state 642 transitions and actions. Commas separate multiple actions, and 643 succeeding lines are used as required. The authors should present 644 multiple actions in the order they must be executed, if relevant. 645 Letters that follow the state indicate an explanatory footnote. The 646 dash ('-') indicates an illegal transition. The STD 51/RFC 1661 647 provides an example of such a state transition table. The initial 648 columns and rows of that table follow as an example: 650 | State 651 | 0 1 2 3 4 5 652 Events| Initial Starting Closed Stopped Closing Stopping 653 ------+----------------------------------------------------------- 654 Up | 2 irc,scr/6 - - - - 655 Down | - - 0 tls/1 0 1 656 Open | tls/1 1 irc,scr/6 3r 5r 5r 657 Close| 0 tlf/0 2 2 4 4 658 | 659 TO+ | - - - - str/4 str/5 660 TO- | - - - - tlf/2 tlf/3 662 The STD 18/RFC 904 also presents state transitions in table format. 663 However, it lists transitions in the form n/a, where n is the next 664 state and a represents the action. The method in RFC 1661 is 665 preferred as new-state logically follows action. Also, this RFC's 666 Appendix C models transitions as the Cartesian product of two state 667 machines. This is a more complex representation that may be 668 difficult to comprehend for those readers that are unfamiliar with 669 the format. The working group recommends that authors present tables 670 as defined in the previous paragraph. 672 A final method of representing state changes is by a time line. The 673 two sides of the time line represent the machines involved in the 674 exchange. The author lists the states the machines enter as time 675 progresses (downward) along the outside of time line. Within the 676 time line, show the actions that cause the state transitions. An 677 example: 679 client server 681 | | 682 | | LISTEN 683 SYN_SENT |----------------------- | 684 | \ syn j | 685 | ----------------->| SYN_RCVD 686 | | 687 | ------------------| 688 | syn k, ack j+1 / | 689 ESTABLISHED |<---------------------- | 690 | | 692 3.4 Character Sets 694 At one time the Internet had a geographic boundary and was English 695 only. Since the Internet now extends internationally, application 696 protocols must assume that the contents of any text string may be in 697 a language other than English. Therefore, new or updated protocols 698 which transmit text must use ISO 10646 as the default Coded Character 699 Set, and RFC 2044, "UTF-8, a transformation format of Unicode and ISO 700 10646" as the default Character Encoding Scheme. An exception is the 701 use of US-ASCII for a protocol's controlling commands and replies. 702 Protocols that have a backwards compatibility requirement should use 703 the default of the existing protocol. This is in keeping with the 704 recommendations of RFC 2130, "The Report of the IAB Character Set 705 Workshop held 29 February - 1 March 1996." 707 4 Document Checklist 709 The following is a checklist based on these guidelines that can be 710 applied to a document: 712 o Does it identify the security risks? Are countermeasures for each 713 potential attack provided? Are the effects of the security 714 measures on the operating environment detailed? 715 o Does it explain the purpose of the protocol or procedure? Are the 716 intended functions and services addressed? Does it describe how it 717 relates to existing protocols? 718 o Does it consider scaling and stability issues? 719 o Are procedures for assigning numbers provided as guidance for IANA. 720 o Does it discuss how to manage the protocol being specified. Is a 721 MIB defined? 722 o Is a target audience defined? 723 o Does it reference or explain the algorithms used in the protocol? 724 o Does it give packet diagrams in recommended form, if applicable? 725 o Does it describe differences from previous versions, if applicable? 726 o Does it separate explanatory portions of the document from 727 requirements? 728 o Does it give examples of protocol operation? 729 o Does it specify behavior in the face of incorrect operation by 730 other implementations? 731 o Does it delineate which packets should be accepted for processing 732 and which should be ignored? 733 o If multiple descriptions of a requirement are given, does it 734 identify one as binding? 735 o How many optional features does it specify? Does it separate them 736 into option classes? 737 o Have all combinations of options or option classes been examined 738 for incompatibility? 739 o Does it explain the rationale and use of options? 740 o Have all mandatory and optional requirements be identified and 741 documented by the accepted key words that define Internet 742 requirement levels? 743 o Does it use the recommended Internet meanings for any terms use to 744 specify the protocol? 745 o Are new or altered definitions for terms given in a glossary? 747 5 Security Considerations 749 This document does not define a protocol or procedure that could be 750 subject to an attack. It establishes guidelines for the information 751 that should be included in RFCs that are to be submitted to the 752 standards track. In the area of security, IETF standards authors are 753 called on to define clearly the the threats faced by the protocol and 754 the way the protocol does or does not provide security assurances to the 755 user. 757 6 References 759 RFC 791 "Internet Protocol (IP)," J. Postel, September 1981. 761 RFC 904 "Exterior Gateway Protocol formal specification," D. Mills, 762 April 1984 764 RFC 1058 "Routing Information Protocol," C. Hedrick, June 1988 766 RFC 1112 "Host extensions for IP multicasting," S. Deering, 767 August 1989 769 RFC 1122 "Requirements for Internet Hosts -- Communication Layers," 770 R. Braden, October 1989 772 RFC 1123 "Requirements for Internet hosts -- Application and 773 Support," R. Braden, October 1989 775 RFC 1311 "Introduction to the STD Notes," J. Postel, March 1992 777 RFC 1350 "The TFTP Protocol (Revision 2)," K. Sollins, July 1992 779 RFC 1661 "The Point-to-Point Protocol (PPP)," W. Simpson, July 1994 781 RFC 1662 "PPP in HLDC-like Framing," W. Simpson, July 1994 783 RFC 1700 "Assigned Numbers," J. Reynolds, J. Postel, October 1994 785 RFC 1983 "Internet Users' Glossary," G. Malkin, August 1996 787 RFC 1939 "Post Office Protocol - Version 3," J. Meyers, M. Rose, 788 May 1996 790 RFC 2026 "The Internet Standards Process -- Revision 3," S. Bradner, 791 October 1996 793 RFC 2044 "UTF-8, a transformation format of Unicode and ISO 10646," 794 F. Yergeau, October 1996 796 RFC 2119 "Key words for use in RFCs to Indicate Requirement Level," 797 S. Bradner, March 1997 799 RFC 2130 "The Report of the IAB Character Set Workshop held 29 800 February - 1 March 1996," C. Weider, C. Preston, 801 K. Simonsen, H. Alvestrand, R. Atkinson, M. Crispin, 802 P. Svanberg, April 1997 804 RFC 2178 "OSPF Version 2," J. Moy, July 1997 806 7 Acknowledgments 808 Peter Desnoyers and Art Mellor began the work on this document. The 809 area directors that oversaw the STDGUIDE WG's efforts were 810 Scott Bradner, Mike O'Dell, and John Curran. Others that contributed 811 to this document were: 813 Bernard Aboba 814 Harald T. Alvestrand 815 Fred Baker 816 Brian Carpenter 817 Robert Elz 818 Dirk Fieldhouse 819 Dale Francisco 820 Gary Malkin 821 Neal McBurnett 822 Craig Partridge 823 Henning Schulzrinne 824 Kurt Starsinic 825 James Watt 827 8 Editor's Address 829 Gregor D. Scott 830 Director, Defense Information Systems Agency 831 ATTN: JIEO-JEBBC 832 Ft. Monmouth, NJ 07703-5613 833 USA 835 Phone: (732) 427-6856 836 Fax: (732) 532-0853 837 EMail: scottg@ftm.disa.mil 839 This Internet Draft expires on 30 June 1997.