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rfc:bcp:bcp22

Network Working Group G. Scott, Editor Request for Comments: 2360 Defense Information Systems Agency BCP: 22 June 1998 Category: Best Current Practice

                Guide for Internet Standards Writers

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

 This document is a guide for Internet standard writers.  It defines
 those characteristics that make standards coherent, unambiguous, and
 easy to interpret.  In addition, it singles out usage believed to
 have led to unclear specifications, resulting in non-interoperable
 interpretations in the past.  These guidelines are to be used with
 RFC 2223, "Instructions to RFC Authors".

Table of Contents

 1     Introduction   . . . . . . . . . . . . . . . . . . . . . . . 2
 2     General Guidelines . . . . . . . . . . . . . . . . . . . . . 2
 2.1   Discussion of Security . . . . . . . . . . . . . . . . . . . 3
 2.2   Protocol Description   . . . . . . . . . . . . . . . . . . . 4
 2.3   Target Audience  . . . . . . . . . . . . . . . . . . . . . . 5
 2.4   Level of Detail  . . . . . . . . . . . . . . . . . . . . . . 5
 2.5   Change Logs  . . . . . . . . . . . . . . . . . . . . . . . . 6
 2.6   Protocol Versions  . . . . . . . . . . . . . . . . . . . . . 6
 2.7   Decision History   . . . . . . . . . . . . . . . . . . . . . 6
 2.8   Response to Out of Specification Behavior  . . . . . . . . . 6
 2.9   The Liberal/Conservative Rule  . . . . . . . . . . . . . . . 7
 2.10  Handling of Protocol Options   . . . . . . . . . . . . . . . 8
 2.11  Indicating Requirement Levels  . . . . . . . . . . . . . . . 9
 2.12  Notational Conventions . . . . . . . . . . . . . . . . . . . 9
 2.13  IANA Considerations  . . . . . . . . . . . . . . . . . . .  10
 2.14  Network Management Considerations  . . . . . . . . . . . .  10
 2.15  Scalability Considerations . . . . . . . . . . . . . . . .  10
 2.16  Network Stability  . . . . . . . . . . . . . . . . . . . .  11
 2.17  Internationalization . . . . . . . . . . . . . . . . . . .  11

Scott Best Current Practice [Page 1] RFC 2360 Guide for Internet Standards Writers June 1998

 2.18  Glossary   . . . . . . . . . . . . . . . . . . . . . . . .  11
 3     Specific Guidelines  . . . . . . . . . . . . . . . . . . .  12
 3.1   Packet Diagrams  . . . . . . . . . . . . . . . . . . . . .  12
 3.2   Summary Tables   . . . . . . . . . . . . . . . . . . . . .  13
 3.3   State Machine Descriptions . . . . . . . . . . . . . . . .  13
 4     Document Checklist . . . . . . . . . . . . . . . . . . . .  15
 5     Security Considerations  . . . . . . . . . . . . . . . . .  16
 6     References . . . . . . . . . . . . . . . . . . . . . . . .  16
 7     Acknowledgments  . . . . . . . . . . . . . . . . . . . . .  18
 8     Editor's Address . . . . . . . . . . . . . . . . . . . . .  18
 9     Appendix . . . . . . . . . . . . . . . . . . . . . . . . .  19
 10    Full Copyright Statement . . . . . . . . . . . . . . . . .  20

1 Introduction

 This document is a guide for Internet standard writers.  It offers
 guidelines on how to write a standards-track document with clarity,
 precision, and completeness.  These guidelines are based on both
 prior successful and unsuccessful IETF specification experiences.
 These guidelines are to be used with RFC 2223, "Instructions to RFC
 Authors", or its update.  Note that some guidelines may not apply in
 certain situations.
 The goal is to increase the possibility that multiple implementations
 of a protocol will interoperate.  Writing specifications to these
 guidelines will not guarantee interoperability.  However, a
 recognized barrier to the creation of interoperable protocol
 implementations is unclear specifications.
 Many will benefit from having well-written protocol specifications.
 Implementers will have a better chance to conform to the protocol
 specification.  Protocol testers can use the specification to derive
 unambiguous testable statements.  Purchasers and users of the
 protocol will have a better understanding of its capabilities.
 For further information on the process for standardizing protocols
 and procedures please refer to BCP 9/RFC 2026, "The Internet
 Standards Process -- Revision 3".  In addition, some considerations
 for protocol design are given in RFC 1958, "Architectural Principles
 of the Internet".

2 General Guidelines

 It is important that multiple readers and implementers of a standard
 have the same understanding of a document.  To this end, information
 should be orderly and detailed.  The following are general guidelines
 intended to help in the production of such a document.  The IESG may
 require that all or some of the following sections appear in a

Scott Best Current Practice [Page 2] RFC 2360 Guide for Internet Standards Writers June 1998

 standards track document.

2.1 Discussion of Security

 If the Internet is to achieve its full potential in commercial,
 governmental, and personal affairs, it must assure users that their
 information transfers are free from tampering or compromise.  Well-
 written security sections in standards-track documents can help
 promote the confidence level required.  Above all, new protocols and
 practices must not worsen overall Internet security.
 A significant threat to the Internet comes from those individuals who
 are motivated and capable of exploiting circumstances, events, or
 vulnerabilities of the system to cause harm.  In addition, deliberate
 or inadvertent user behavior may expose the system to attack or
 exploitation.  The harm could range from disrupting or denying
 network service, to damaging user systems.  Additionally, information
 disclosure could provide the means to attack another system, or
 reveal patterns of behavior that could be used to harm an individual,
 organization, or network.  This is a particular concern with
 standards that define a portion of the Management Information Base
 (MIB).
 Standards authors must accept that the protocol they specify will be
 subject to attack.  They are responsible for determining what attacks
 are possible, and for detailing the nature of the attacks in the
 document.  Otherwise, they must convincingly argue that attack is not
 realistic in a specific environment, and restrict the use of the
 protocol to that environment.
 After the document has exhaustively identified the security risks the
 protocol is exposed to, the authors must formulate and detail a
 defense against those attacks.  They must discuss the applicable
 countermeasures employed, or the risk the user is accepting by using
 the protocol.  The countermeasures may be provided by a protocol
 mechanism or by reliance on external mechanisms.  Authors should be
 knowledgeable of existing security mechanisms, and reuse them if
 practical.  When a cryptographic algorithm is used, the protocol
 should be written to permit its substitution with another algorithm
 in the future.  Finally, the authors should discuss implementation
 hints or guidelines, e.g., how to deal with untrustworthy data or
 peer systems.
 Security measures will have an impact within the environment that
 they are used.  Perhaps users will now be constrained on what they
 can do in the Internet, or will experience degradation in the speed
 of service.  The effects the security measures have on the protocol's
 use and performance should be discussed.

Scott Best Current Practice [Page 3] RFC 2360 Guide for Internet Standards Writers June 1998

 The discussion of security can be concentrated in the Security
 Considerations section of the document, or throughout the document
 where it is relevant to particular parts of the specification.  An
 advantage of the second approach is that it ensures security is an
 integral part of the protocol's development, rather than something
 that is a follow-on or secondary effort.  If security is discussed
 throughout the document, the Security Considerations section must
 summarize and refer to the appropriate specification sections.  This
 will insure that the protocol's security measures are emphasized to
 implementer and user both.
 Within the Security Considerations section, a discussion of the path
 not taken may be appropriate.  There may be several security
 mechanisms that were not selected for a variety of reasons: cost or
 difficulty of implementation, or ineffectiveness for a given network
 environment.  By listing the mechanisms they did not use and the
 reasons, editors can demonstrate that the protocol's WG gave security
 the necessary thought.  In addition, this gives the protocol's users
 the information they need to consider whether one of the non-selected
 mechanisms would be better suited to their particular requirements.
 A document giving further guidance on security topics is in
 development.  Authors should obtain a copy of the completed RFC to
 help them prepare the security portion of the standard.
 Finally, it is no longer acceptable that Security Considerations
 sections consist solely of statements to the effect that security was
 not considered in preparing the standard.
 Some examples of Security Considerations sections are found in STD
 33/RFC 1350, STD 51/RFC 1662, and STD 53/RFC 1939.  RFC 2316, "Report
 of the IAB Security Architecture Workshop", provides additional
 information in this topic area.

2.2 Protocol Description

 Standards track documents must include a description of the protocol.
 This description must address the protocol's purpose, intended
 functions, services it provides, and, the arena, circumstances, or
 any special considerations of the protocol's use.
 The authors of a protocol specification will have a great deal of
 knowledge as to the reason for the protocol.  However, the reader is
 more likely to have general networking knowledge and experience,
 rather than expertise in a particular protocol.  An explanation of
 it's purpose and use will give the reader a reference point for

Scott Best Current Practice [Page 4] RFC 2360 Guide for Internet Standards Writers June 1998

 understanding the protocol, and where it fits in the Internet.  The
 STD 54/RFC 2328 was recommended to the STDGUIDE working group as
 providing a good example of this in its "Protocol Overview" section.
 The protocol's general description must also provide information on
 the relationship between the different parties to the protocol. This
 can be done by showing typical packet sequences.
 This also applies to the algorithms used by a protocol.  A detailed
 description of the algorithms or citation of readily available
 references that give such a description is necessary.

2.3 Target Audience

 RFCs have been written with many different purposes, ranging from the
 technical to the administrative.  Those written as standards should
 clearly identify the intended audience, for example, designers,
 implementers, testers, help desk personnel, educators, end users, or
 others.  If there are multiple audiences being addressed in the
 document, the section for each audience needs to be identified.  The
 goal is to help the reader discover and focus on what they have
 turned to the document for, and avoid what they may find confusing,
 diverting, or extraneous.

2.4 Level of Detail

 The author should consider what level of descriptive detail best
 conveys the protocol's intent.  Concise text has several advantages.
 It makes the document easier to read.  Such text reduces the chance
 for conflict between different portions of the specification.  The
 reader can readily identify the required protocol mechanisms in the
 standard.  In addition, it makes it easier to identify the
 requirements for protocol implementation.  A disadvantage of concise
 descriptions is that a reader may not fully comprehend the reasoning
 behind the protocol, and thus make assumptions that will lead to
 implementation errors.
 Longer descriptions may be necessary to explain purpose, background,
 rationale, implementation experience, or to provide tutorial
 information.  This helps the reader understand the protocol.  Yet,
 several dangers exist with lengthy text.  Finding the protocol
 requirements in the text is difficult or confusing.  The same
 mechanism may have multiple descriptions, which leads to
 misinterpretation or conflict.  Finally, it is more difficult to
 comprehend, a consideration as English is not the native language of
 the many worldwide readers of IETF standards.

Scott Best Current Practice [Page 5] RFC 2360 Guide for Internet Standards Writers June 1998

 One approach is to divide the standard into sections: one describing
 the protocol concisely, while another section consists of explanatory
 text.  The STD 3/RFC 1122/RFC 1123 and STD 54/RFC 2328 provides
 examples of this method.

2.5 Change Logs

 As a document moves along the standards track, from Proposed to Draft
 or Draft to Full, or cycles in level, it will undergo changes due to
 better understanding of the protocol or implementation experience. To
 help implementers track the changes being made a log showing what has
 changed from the previous version of the specification is required
 (see Appendix).

2.6 Protocol Versions

 Often the standard is specifying a new version of an existing
 protocol.  In such a case, the authors should detail the differences
 between the previous version and the new version.  This should
 include the rationale for the changes, for example, implementation
 experience, changes in technology, responding to user demand, etc.

2.7 Decision History

 In standards development, reaching consensus requires making
 difficult choices.  These choices are made through working group
 discussions or from implementation experience.  By including the
 basis for a contentious decision, the author can prevent future
 revisiting of these disagreements when the original parties have
 moved on.  In addition, the knowledge of the "why" is as useful to an
 implementer as the description of "how".  For example, the
 alternative not taken may have been simpler to implement, so
 including the reasons behind the choice may prevent future
 implementers from taking nonstandard shortcuts.

2.8 Response to Out of Specification Behavior

 A detail description of the actions taken in case of behavior that is
 deviant from or exceeds the specification is useful.  This is an area
 where implementers often differ in opinion as to the appropriate
 response.  By specifying a common response, the standard author can
 reduce the risk that different implementations will come in to
 conflict.
 The standard should describe responses to behavior explicitly
 forbidden or out of the boundaries defined by the specification.  Two
 possible approaches to such cases are discarding, or invoking error-
 handling mechanisms.  If discarding is chosen, detailing the

Scott Best Current Practice [Page 6] RFC 2360 Guide for Internet Standards Writers June 1998

 disposition may be necessary.  For instance, treat dropped frames as
 if they were never received, or reset an existing connection or
 adjacency state.
 The specification should describe actions taken when a critical
 resource or a performance-scaling limit is exceeded.  This is
 necessary for cases where a risk of network degradation or
 operational failure exists.  In such cases, a consistent behavior
 between implementations is necessary.

2.9 The Liberal/Conservative Rule

 A rule, first stated in STD 5/RFC 791, recognized as having benefits
 in implementation robustness and interoperability is:
                  "Be liberal in what you accept, and
                    conservative in what you send".
 Or establish restrictions on what a protocol transmits, but be able
 to deal with every conceivable error received.  Caution is urged in
 applying this approach in standards track protocols.  It has in the
 past lead to conflicts between vendors when interoperability fails.
 The sender accuses the receiver of failing to be liberal enough, and
 the receiver accuses the sender of not being conservative enough.
 Therefore, the author is obligated to provide extensive detail on
 send and receive behavior.
 To avoid any confusion between the two, recommend that standard
 authors specify send and receive behavior separately.  The
 description of reception will require the most detailing.  For
 implementations are expected to continue operating regardless of
 error received.  Therefore, the actions taken to achieve that result,
 need to be laid out in the protocol specification.  Standard authors
 should concern themselves on achieving a level of cooperation that
 limits network disruption, not just how to survive on the network.
 The appearance of undefined information or conditions must not cause
 a network or host failure.  This requires specification on how to
 attempt acceptance of most of the packets.  Two approaches are
 available, either using as much of the packet's content as possible,
 or invoking error procedures.  The author should specify a dividing
 line on when to take which approach.
 A case for consideration is that of a routing protocol, where
 acceptance of flawed information can cause network failure.  For
 protocols such as this, the specification should identify packets
 that could have different interpretations and mandate that they be
 rejected completely or the nature of the attempt to recover some
 information from them.  For example, routing updates that contain

Scott Best Current Practice [Page 7] RFC 2360 Guide for Internet Standards Writers June 1998

 more data than the tuple count shows.  The protocol authors should
 consider whether some trailing data can be accepted as additional
 routes, or to reject the entire packet as suspect because it is non-
 conformant.

2.10 Handling of Protocol Options

 Specifications with many optional features increase the complexity of
 the implementation and the chance of non-interoperable
 implementations.  The danger is that different implementations may
 specify some combination of options that are unable to interoperate
 with each other.
 As the document moves along the standard track, implementation
 experience shall determine the need for each option.  Implementation
 shall show whether the option should be a mandatory part of the
 protocol or remain an option.  If an option is not implemented as the
 document advances, it must be removed from the protocol before it
 reaches draft standard status.
 Therefore, options shall only be present in a protocol to address a
 real requirement.  For example, options can support future
 extensibility of the protocol, a particular market, e.g., the
 financial industry, or a specific network environment, e.g., a
 network constrained by limited bandwidth.  They shall not be included
 as a means to "buy-off" a minority opinion.  Omission of the optional
 item shall have no interoperability consequences for the
 implementation that does so.
 One possible approach is to document protocol options in a separate
 specification.  This keeps the main protocol specification clean and
 makes it clear that the options are not required to implement the
 protocol.  Regardless of whether they appear within the specification
 or in a separate document, the text shall discuss the full
 implications of either using the option or not, and the case for
 choosing either course.  As part of this, the author needs to
 consider and describe how the options are used alongside other
 protocols.  The text must also specify the default conditions of all
 options.  For security checking options the default condition is on
 or enabled.
 There are occasions when mutually exclusive options appear within the
 protocol.  That is, the implementation of an optional feature
 precludes the implementation of the other optional feature.  For
 clarity, the author needs to state when to implement one or the
 other, what the effect of choosing one over the other is, and what

Scott Best Current Practice [Page 8] RFC 2360 Guide for Internet Standards Writers June 1998

 problems the implementer or user may face.  The choice of one or the
 other options shall have no interoperability consequences between
 multiple implementations.

2.11 Indicating Requirement Levels

 The BCP 14/RFC 2119, "Key words for use in RFCs to Indicate
 Requirement Level", defines several words that are necessary for
 writing a standards track document.  Editors of standards track
 documents must not deviate from the definitions provided as they are
 intended to identify interoperability requirements or limit
 potentially harmful behavior.  The capitalization of these words is
 the accepted norm, and can help in identifying an unintentional or
 unreasonable requirement.  These words have been used in several RFCs
 the first instances being STD 3/RFC 1122/RFC 1123.

2.12 Notational Conventions

 Formal syntax notations can be used to define complicated protocol
 concepts or data types, and to specify values of these data types.
 This permits the protocol to be written without concern on how the
 implementation is constructed, or how the data type is represented
 during transfer.  The specification is simplified because it can be
 presented as "axioms" that will be proven by implementation.
 The formal specification of the syntax used should be referenced in
 the text of the standard.  Any extensions, subsets, alterations, or
 exceptions to that formal syntax should be defined within the
 standard.
 The STD 11/RFC 822 provides an example of this.  In RFC 822 (Section
 2 and Appendix D) the Backus-Naur Form (BNF) meta-language was
 extended to make its representation smaller and easier to understand.
 Another example is STD 16/RFC 1155 (Section 3.2) where a subset of
 the Abstract Syntax Notation One (ASN.1) is defined.
 The author of a standards track protocol needs to consider several
 things before they use a formal syntax notation.  Is the formal
 specification language being used parseable by an existing machine?
 If no parser exists, is there enough information provided in the
 specification to permit the building of a parser?  If not, it is
 likely the reader will not have enough information to decide what the
 notation means.  In addition, the author should remember machine
 parseable syntax is often unreadable by humans, and can make the
 specification excessive in length.  Therefore, syntax notations
 cannot take the place of a clearly written protocol description.

Scott Best Current Practice [Page 9] RFC 2360 Guide for Internet Standards Writers June 1998

2.13 IANA Considerations

 The common use of the Internet standard track protocols by the
 Internet community requires that unique values be assigned to
 parameter fields.  An IETF WG does not have the authority to assign
 these values for the protocol it developed.  The Internet Assigned
 Numbers Authority (IANA) is the central authority for the assignment
 of unique parameter values for Internet protocols.  The authors of a
 developing protocol need to coordinate with the IANA the rules and
 procedures to manage the number space.  This coordination needs to be
 completed prior to submitting the Internet Draft to the standards
 track.
 A document is in preparation that discusses issues related to
 identifier assignment policy and guidelines on specific text to task
 IANA with its administration.  Standard authors should obtain a copy
 of it when it is finalized as an RFC.
 For further information on parameter assignment and current
 assignments, authors can reference STD 2, RFC 1700, "Assigned
 Numbers" (http://www.iana.org).

2.14 Network Management Considerations

 When relevant, each standard needs to discuss how to manage the
 protocol being specified.  This management process should be
 compatible with the current IETF Standard management protocol.  In
 addition, a MIB must be defined within the standard or in a companion
 document.  The MIB must be compatible with current Structure of
 Management Information (SMI) and parseable using a tool such as
 SMICng.  Where management or a MIB is not necessary this section of
 the standard should explain the reason it is not relevant to the
 protocol.

2.15 Scalability Considerations

 The standard should establish the limitations on the scale of use,
 e.g., tens of millions of sessions, gigabits per second, etc., and
 establish limits on the resources used, e.g., round trip time,
 computing resources, etc.  This is important because it establishes
 the ability of the network to accommodate the number of users and the
 complexity of their relations.  The STD 53/RFC 1939 has an example of
 such a section.  If this is not applicable to the protocol, an
 explanation of why not should be included.

Scott Best Current Practice [Page 10] RFC 2360 Guide for Internet Standards Writers June 1998

2.16 Network Stability

 A standard should discuss the relationship between network topology
 and convergence behavior.  As part of this, any topology that would
 be troublesome for the protocol should be identified.  Additionally,
 the specification should address any possible destabilizing events,
 and means by which the protocol resists or recovers from them.  The
 purpose is to insure that the network will stabilize, in a timely
 fashion, after a change, and that a combination of errors or events
 will not plunge the network into chaos.  The STD 34/RFC 1058, as an
 example, has sections which discuss how that protocol handles the
 affects of changing topology.
 The obvious case this would apply to is a routing protocol.  However,
 an application protocol could also have dynamic behavior that would
 affect the network.  For example, a messaging protocol could suddenly
 dump a large number of messages onto the network.  Therefore, editors
 of an application protocol will have to consider possible impacts to
 network stability and convergence behavior.

2.17 Internationalization

 At one time the Internet had a geographic boundary and was English
 only.  The Internet now extends internationally.  Therefore, data is
 interchanged in a variety of languages and character sets.  In order
 to meet the requirements of an international Internet, a standard
 must conform to the policies stated in BCP 18/RFC 2277, "IETF Policy
 on Character Sets and Languages".

2.18 Glossary

 Every standards track RFC should have a glossary, as words can have
 many meanings.  By defining any new words introduced, the author can
 avoid confusing or misleading the implementers.  The definition
 should appear on the word's first appearance within the text of the
 protocol specification, and in a separate glossary section.
 It is likely that definition of the protocol will rely on many words
 frequently used in IETF documents.  All authors must be knowledgeable
 of the common accepted definitions of these frequently used words.
 FYI 18/RFC 1983, "Internet Users' Glossary", provides definitions
 that are specific to the Internet.  Any deviation from these
 definitions by authors is strongly discouraged.  If circumstances
 require deviation, an author should state that he is altering the
 commonly accepted definition, and provide rationale as to the
 necessity of doing so.  The altered definition must be included in
 the Glossary section.

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 If the author uses the word as commonly defined, she does not have to
 include the definition in the glossary.  As a minimum, FYI 18/RFC
 1983 should be referenced as a source.

3 Specific Guidelines

 The following are guidelines on how to present specific technical
 information in standards.

3.1 Packet Diagrams

 Most link, network, and transport layer protocols have packet
 descriptions.  Packet diagrams included in the standard are very
 helpful to the reader.  The preferred form for packet diagrams is a
 sequence of long words in network byte order, with each word
 horizontal on the page and bit numbering at the top:
  0                   1                   2                   3
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Version| Prio. |                   Flow Label                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In cases where a packet is strongly byte-aligned rather than word-
 aligned (e.g., when byte-boundary variable-length fields are used),
 display packet diagrams in a byte-wide format.  The author can use
 different height boxes for short and long words, and broken boxes for
 variable-length fields:
                         0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        |    Length N   |
                        +-+-+-+-+-+-+-+-+
                        |               |
                        +    Address    +
                               ...
                        +   (N bytes)   +
                        |               |
                        +-+-+-+-+-+-+-+-+
                        |               |
                        +  2-byte field +
                        |               |
                        +-+-+-+-+-+-+-+-+

Scott Best Current Practice [Page 12] RFC 2360 Guide for Internet Standards Writers June 1998

3.2 Summary Tables

 The specifications of some protocols are particularly lengthy,
 sometimes covering a hundred pages or more.  In such cases, the
 inclusion of a summary table can reduce the risk of conformance
 failure by an implementation through oversight.  A summary table
 itemizes what in a protocol is mandatory, optional, or prohibited.
 Summary tables do not guarantee conformance, but serve to assist an
 implementer in checking that they have addressed all protocol
 features.
 The summary table will consist of, as a minimum, four (4) columns:
 Protocol Feature, Section Reference, Status, and
 References/Footnotes.  The author may add columns if they further
 explain or clarify the protocol.
 In the Protocol Feature column, list the protocol's characteristics,
 for example, a command word.  We recommend grouping series of related
 transactions under descriptive headers, for example, RECEPTION.
 Section reference directs the implementer to the section, paragraph,
 or page that describes the protocol feature in detail.
 Status indicates whether the feature is mandatory, optional, or
 prohibited.  The author can use either a separate column for each
 possibility, or a single column with appropriate codes.  These codes
 need to be defined at the start of the summary table to avoid
 confusion.  Possible status codes:
     M    - must or mandatory
     MN   - must not
     O    - optional
     S    - should
     SN   - should not
     X    - prohibited
 In the References/Footnotes column authors can point to other RFCs
 that are necessary to consider in implementing this protocol feature,
 or any footnotes necessary to explain the implementation further.
 The STD 3/RFC 1122/RFC 1123 provides examples of summary tables.

3.3 State Machine Descriptions

 A convenient method of presenting a protocol's behavior is as a
 state-machine model.  That is, a protocol can be described by a
 series of states resulting from a command, operation, or transaction.
 State-machine models define the variables and constants that

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 establish a state, the events that cause state transitions and the
 actions that result from those transitions.  Through these models, an
 understanding of the protocol's dynamic operation as sequence of
 state transitions that occur for any given event is possible.  State
 transitions can be detailed by diagrams, tables, or time lines.
 Note that state-machine models are never to take the place of
 detailed text description of the specification.  They are adjuncts to
 the text.  The protocol specification shall always take precedence in
 the case of a conflict.
 When using a state transition diagram, show each possible protocol
 state as a box connected by state transition arcs.  The author should
 label each arc with the event that causes the transition, and, in
 parentheses, any actions taken during the transition.  The STD 5/RFC
 1112 provides an example of such a diagram.  As ASCII text is the
 preferred storage format for RFCs, only simple diagrams are possible.
 Tables can summarize more complex or extensive state transitions.
 In a state transition table, the different events are listed
 vertically and the different states are listed horizontally.  The
 form, action/new state, represents state transitions and actions.
 Commas separate multiple actions, and succeeding lines are used as
 required.  The authors should present multiple actions in the order
 they must be executed, if relevant.  Letters that follow the state
 indicate an explanatory footnote.  The dash ('-') indicates an
 illegal transition.  The STD 51/RFC 1661 provides an example of such
 a state transition table.  The initial columns and rows of that table
 follow as an example:
         | State
         |    0         1         2         3         4         5
   Events| Initial   Starting  Closed    Stopped   Closing   Stopping
   ------+-----------------------------------------------------------
    Up   |    2     irc,scr/6     -         -         -         -
    Down |    -         -         0       tls/1       0         1
    Open |  tls/1       1     irc,scr/6     3r        5r        5r
    Close|    0       tlf/0       2         2         4         4
         |
     TO+ |    -         -         -         -       str/4     str/5
     TO- |    -         -         -         -       tlf/2     tlf/3
 The STD 18/RFC 904 also presents state transitions in table format.
 However, it lists transitions in the form n/a, where n is the next
 state and a represents the action.  The method in RFC 1661 is
 preferred as new state logically follows action.  In addition, RFC
 904's Appendix C models transitions as the Cartesian product of two
 state machines.  This is a more complex representation that may be

Scott Best Current Practice [Page 14] RFC 2360 Guide for Internet Standards Writers June 1998

 difficult to comprehend for those readers that are unfamiliar with
 the format.  We recommend that authors present tables as defined in
 the previous paragraph.
 A final method of representing state changes is by a time line.  The
 two sides of the time line represent the machines involved in the
 exchange.  The author lists the states the machines enter as time
 progresses (downward) along the outside of time line.  Within the
 time line, show the actions that cause the state transitions.  An
 example:
          client                                     server
             |                                          |
             |                                          |   LISTEN
 SYN_SENT    |-----------------------                   |
             |                       \ syn j            |
             |                        ----------------->|   SYN_RCVD
             |                                          |
             |                        ------------------|
             |        syn k, ack j+1 /                  |
 ESTABLISHED |<----------------------                   |
             |                                          |

4 Document Checklist

 The following is a checklist based on the above guidelines that can
 be applied to a document:
 o Does it identify the security risks?  Are countermeasures for each
   potential attack provided?  Are the effects of the security
   measures on the operating environment detailed?
 o Does it explain the purpose of the protocol or procedure?  Are the
   intended functions and services addressed?  Does it describe how it
   relates to existing protocols?
 o Does it consider scaling and stability issues?
 o Have procedures for assigning numbers been coordinated with IANA?
 o Does it discuss how to manage the protocol being specified?  Is a
   MIB defined?
 o Is a target audience defined?
 o Does it reference or explain the algorithms used in the protocol?
 o Does it give packet diagrams in recommended form, if applicable?
 o Is there a change log?
 o Does it describe differences from previous versions, if
   applicable?
 o Does it separate explanatory portions of the document from
   requirements?
 o Does it give examples of protocol operation?

Scott Best Current Practice [Page 15] RFC 2360 Guide for Internet Standards Writers June 1998

 o Does it specify behavior in the face of incorrect operation by
   other implementations?
 o Does it delineate which packets should be accepted for processing
   and which should be ignored?
 o If multiple descriptions of a requirement are given, does it
   identify one as binding?
 o How many optional features does it specify?  Does it separate them
   into option classes?
 o Have all combinations of options or option classes been examined
   for incompatibility?
 o Does it explain the rationale and use of options?
 o Have all mandatory and optional requirements be identified and
   documented by the accepted key words that define Internet
   requirement levels?
 o Does it conform to the current internationalization policies of
   the IETF?
 o Are the recommended meanings for common Internet terms used?
 o If not, are new or altered definitions for terms given in a
   glossary?

5 Security Considerations

 This document does not define a protocol or procedure that could be
 subject to an attack.  It establishes guidelines for the information
 that should be included in RFCs that are to be submitted to the
 standards track.  In the area of security, IETF standards authors are
 called on to define clearly the threats faced by the protocol and the
 way the protocol does or does not provide security assurances to the
 user.

6 References

 [RFC 791]   Postel, J., "Internet Protocol (IP)", STD 5, RFC 791
             September 1981.
 [RFC 904]   Mills, D., "Exterior Gateway Protocol formal
             specification", RFC 904, April 1984.
 [RFC 1058]  Hedrick, C., "Routing Information Protocol", STD 34,
             RFC 1058, June 1988.
 [RFC 1112]  Deering, S., "Host extensions for IP multicasting",
             STD 5, RFC 1112, August 1989.
 [RFC 1122]  Braden, R., "Requirements for Internet Hosts --
             Communication Layers", STD 3, RFC 1122, October 1989.

Scott Best Current Practice [Page 16] RFC 2360 Guide for Internet Standards Writers June 1998

 [RFC 1123]  Braden, R., "Requirements for Internet hosts --
             Application and Support", STD 3, RFC 1123, October 1989.
 [RFC 1311]  Postel, J., "Introduction to the STD Notes", RFC 1311,
             March 1992.
 [RFC 1350]  Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
             RFC 1350, July 1992.
 [RFC 1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
             RFC 1661, July 1994.
 [RFC 1662]  Simpson, W., "PPP in HLDC-like Framing", STD 51, RFC 1662,
             July 1994.
 [RFC 1700]  Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
             RFC 1700, October 1994.  (http://www.iana.org)
 [RFC 1939]  Meyers, J., and M. Rose, "Post Office Protocol - Version
             3", STD 53, RFC 1939, May 1996.
 [RFC 1958]  Carpenter, B., "Architectural Principles of the Internet",
             RFC 1958, June 1996.
 [RFC 1983]  Malkin, G., "Internet Users' Glossary", FYI 18, RFC 1983,
             August 1996.
 [RFC 2026]  Bradner, S., "The Internet Standards Process -- Revision 3",
             RFC 2026, October 1996.
 [RFC 2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Level", BCP 14, RFC 2119, March 1997.
 [RFC 2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [RFC 2223]  Postel, J. and J. Reynolds, "Instructions to RFC Authors",
             RFC 2223, October 1997.
 [RFC 2277]  Alvestrand, H., "IETF Policy on Character Sets and
             Language", RFC 2277, January 1998.
 [RFC 2316]  Bellovin, S., "Report of the IAB Security Architecture
             Workshop", RFC 2316, April 1998.

Scott Best Current Practice [Page 17] RFC 2360 Guide for Internet Standards Writers June 1998

7 Acknowledgments

 Peter Desnoyers and Art Mellor began the work on this document.
 Others that contributed were:
   Bernard Aboba
   Harald T. Alvestrand
   Fred Baker
   Scott Bradner
   Brian Carpenter
   Robert Elz
   Dirk Fieldhouse
   Dale Francisco
   Gary Malkin
   Neal McBurnett
   Thomas Narten
   Craig Partridge
   Vern Paxson
   Mike O'Dell
   Henning Schulzrinne
   Kurt Starsinic
   James Watt

8 Editor's Address

 Gregor D. Scott
 Director, Defense Information Systems Agency
 ATTN: JIEO-JEBBC
 Ft. Monmouth, NJ  07703-5613
 USA
 Phone:    (732) 427-6856
 Fax:      (732) 532-0853
 EMail:    scottg@ftm.disa.mil

Scott Best Current Practice [Page 18] RFC 2360 Guide for Internet Standards Writers June 1998

9 Appendix

CHANGES FROM DRAFT -06

 The following changes were made following IESG review:
 References to RFC 1543 were changed to RFC 2223 that obsoleted it.
 In section 2.1, "export control" was dropped as a valid reason for
 not selecting a security mechanism.  In addition, ambiguous or
 conflicting sentences were removed.
 In section 2.1 reference made to RFC 2315 as an additional source of
 information.
 Section 2.5 was changed to highlight the Change Log's purpose as
 assistance to implementers.
 The IANA Considerations section (2.13) was rewritten to highlight
 that the IANA guidelines document is work in progress but should be
 used when it becomes available.
 Section 3.4 Character Sets was deleted and replaced by section 2.17
 Internationalization.
 Spelling and grammar corrections were made.

CHANGES FROM DRAFT -05

 A sentence pointing to a pending document that further addresses IANA
 considerations was added to section 2.13.  The current draft of that
 document is draft-iesg-iana-considerations-02.txt.  A clause stating
 that the IANA established the assignment policies was removed since it
 appeared to conflict with the intent of the referenced ID.
 Placeholders for the BCP and RFC number have been added to the text
 and reference section.
 A new section (2.5) requiring change logs as documents progress along
 the standards track was added.
 References to RFC 2044 were changed to RFC 2279 that obsoleted it.
 Spelling and grammar corrections were made.

CHANGES FROM DRAFT -04

 A paragraph pointing to a pending document that further addresses
 security was updated.

Scott Best Current Practice [Page 19] RFC 2360 Guide for Internet Standards Writers June 1998

10 Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Scott Best Current Practice [Page 20]

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