Network Working Group                                   G. Scott, Editor
INTERNET DRAFT                        Defense Information Systems Agency
                                                           December 1997

                  Guide for Internet Standards Writers
                    <draft-ietf-stdguide-ops-04.txt>
                    <draft-ietf-stdguide-ops-05.txt>

Status of this Memo

  This document is an Internet Draft.  Internet Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
  and its working groups.  Note that other groups may also distribute
  working documents as Internet Drafts.

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  Rim).

  Distribution of this document is unlimited.

  This Internet Draft expires on 5 December 30 June 1997.

Abstract

  This document is a guide for Internet standard writers.  It defines
  those characteristics that make standards coherent, unambiguous, and
  easy to interpret.  Also, 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 1543, "Instructions ''Instructions to RFC Authors." Authors.''

  This version of the document is a draft.  It is intended to generate
  further discussion and addition by the STDGUIDE working group.
  Please send comments to stdguide@midnight.com.

CHANGES FROM PREVIOUS DRAFT

  Section 2.10

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

  References to avoid conflict with BCP 14/RFC 2119,
  "Key words for use in RFCs RFC 1583 were changed to Indicate Requirement Level." RFC 2178 which obsoleted it.

  Editorial changes and corrections were also made.

Table of Contents

1     Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
2     General Guidelines . . . . . . . . . . . . . . . . . . . . . .  3
2.1   Discussion of Security . . . . . . . . . . . . . . . . . . . .  3
2.2   Protocol Description . . . . . . . . . . . . . . . . . . . . .  5
2.3   Target Audience  . . . . . . . . . . . . . . . . . . . . . . . .  6
2.4   Level of Detail  . . . . . . . . . . . . . . . . . . . . . . . .  6
2.5   Protocol Versions  . . . . . . . . . . . . . . . . . . . . . . .  6
2.6   Decision History . . . . . . . . . . . . . . . . . . . . . . .  7
2.7   Response to Out of Specification Behavior  . . . . . . . . . . .  7
2.8   The Liberal/Conservative Rule  . . . . . . . . . . . . . . . . .  7
2.9   Handling of Protocol Options . . . . . . . . . . . . . . . . .  8
2.10  Indicating Requirement Levels  . . . . . . . . . . . . . . . . .  9
2.11  Notational Conventions . . . . . . . . . . . . . . . . . . . .  9 10
2.12  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 10
2.13  Network Management Considerations  . . . . . . . . . . . . . . . 10 11
2.14  Scalability Considerations . . . . . . . . . . . . . . . . . . 11
2.15  Network Stability  . . . . . . . . . . . . . . . . . . . . . . . 11
2.16  Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
3     Specific Guidelines  . . . . . . . . . . . . . . . . . . . . . . 12
3.1   Packet Diagrams  . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2   Summary Tables . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3   State Machine Descriptions . . . . . . . . . . . . . . . . . . 14
3.4   Character Sets . . . . . . . . . . . . . . . . . . . . . . . . 16
4     Document Checklist . . . . . . . . . . . . . . . . . . . . . . 16
5     Security Considerations  . . . . . . . . . . . . . . . . . . . . 17
6     References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7     Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . 18
8     Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 19

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 1543, "Instructions to RFC
  Authors," or its update.  Note that some guidelines may not apply in
  certain situations.

  The process for standardizing protocols and procedures is given in
  BCP 9/RFC 2026, "The Internet Standards Process -- Revision 3."  Some
  considerations for protocol design are given in RFC 1958,
  Architectural Principles of the Internet."
  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.
  Implementors 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.

2  General Guidelines

  It is important that multiple readers and implementors 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
  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.  For an implementor will find
  it easier to provide the security measures specified.  While users
  will understand the security measures, and so have a higher level of
  trust in the Internet.  Above all, new protocols and practices must
  not worsen overall Internet security.

  A significant threat to the Internet are those individuals who are
  motivated and capable of exploiting circumstances, events, or
  vulnerabilities of the system to cause harm.  Also, 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.

  This discussion of the threat model and other assumptions should
  appear early in the standard.  Doing so will establish a basis for
  the further discussion of security throughout the document.

  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 cryptographic algorithms are use, 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.

  Additionally, the effects the security measures have on the
  protocol's use and performance should be discussed.  Security
  measures will have an impact on the environment they are used in.
  Perhaps users will now be locked out of portions of the Internet
  previously open to them, or users will experience a degradation in
  the speed of service.  The user may decided to accept a greater risk
  in exchange for improved access or service.  But the user must be
  able to make an informed decision.  They need to understand the risks
  they are facing and the costs of reducing their risk.

  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
  summarized and make reference to the appropriate specification
  sections.  This will insure that the protocol's security measures are
  emphasized to implementor 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; ineffectiveness for a given network
  environment; or export control.  By listing the mechanisms they did
  not use and the reasons, editors can demonstrate that the protocol's
  WG gave security the necessary thought.  Also, 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.

  Currently, a RFC is being considered that would give

  A document giving further guidance on how
  to do a security analysis.  It will provide a listing of classes of
  attacks, and methods of analysis that are useful topics is in developing
  countermeasures to them.  Standards authors
  developmemnt.  Authors should obtain a current copy of this the completed RFC to assist
  help them in their preparation of 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.

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
  understanding the protocol, and where it fits in the Internet.  The
  Draft Standard RFC 1583 2178 was recommended to the STDGUIDE working guide
  as providing a good example of this in it's "Protocol Overview"
  section.

  The protocol's general description should 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,
  implementors, testers, help desk personnel, educators, end users, or
  others.  If there are multiple audiences being addressed in the
  document, what sections are 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.  Also, 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
  misinterpretations 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.

  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 Draft Standard RFC 1583 2178
  provides examples of this method.

2.5  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.

2.6  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 later, when the original parties have
  moved on.  Also, the knowledge of the "why" is as useful to an
  implementor 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
  implementors from taking nonstandard shortcuts.

2.7  Response to Out of Specification Behavior

  The STDGUIDE working group recommends that detail description of the
  actions taken in case of behavior that is deviant from or exceeds the
  specification be included.  This is an area where implementors 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
  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 critical
  resource or performance scaling limits are exceeded.  This is not
  necessary for every case.  It is necessary for cases where a risk of
  network degradation or operational failure exists.  In such cases, a
  consistent behavior between implementations is necessary.

2.8  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 will be expected to accept any packet from the
  network without failure or malfunction.  Therefore, the actions taken
  to achieve that result, need to be laid out in the protocol
  specification.  Standard authors should consider not just how to
  survive on the network, but achieve the highest level of cooperation
  possible to limit the amount of network disruption.  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 differing interpretations and mandate that they be
  either rejected completely or the nature of the attempt to recover
  some information from them.  For example, routing updates that
  contain 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.

2.9  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
  sahll
  shall show whether the option should be a mandatory part of the
  protocol or remains 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
  document.  Doing so would make it clear that the options are not
  integral to the implementation of the protocol, and would keep the
  main protocol specification clean.  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 intended to be
  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 may be occasions when mutually exclusive options appear within
  a 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
  problems the implementor or user may face.  The choice of one or the
  other options shall have no interoperability consequences between
  multiple implementations.

2.10  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.

2.11  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.  Also, 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.

2.12  IANA Considerations

  The common use of the Internet standard track protocols by the
  Internet community requires that the unique values be assigned to the
  parameter fields.  An IETF WG does not have the authority to assign
  these values for the protocol it is working on.  The Internet
  Assigned Numbers Authority (IANA) is the central coordinator for the
  assignment of unique parameter values for Internet protocols, and is
  responsible for establishing the procedures by which it does so.  The
  authors of a developing protocol that use a link, socket, port,
  protocol, etc., need to coordinate with the IANA the rules and
  procedures to be used to register constants and tags.  This
  coordination needs to be completed prior to submitting the internet
  draft to the standards track.  For further information on parameter
  assignment and current assignments, authors can reference STD 2/RFC
  1700, "Assigned Numbers."

2.13  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.  Also
  a MIB must be defined within the standard or in a companion document.
  The MIB must be compatible with current 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.

2.14  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.

2.15  Network Stability

  A standard should discuss the relationship between network topology
  and convergence behavior.  As part of this, any topology which would
  be troublesome for the protocol should be identified.  Additionally,
  the specification should address any possible destablizing events,
  and how 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.

2.16  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 implementer.  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.

  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.  The STDGUIDE working group recommends that packet
  diagrams be included in the standard, as they 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 +
                           |               |
                           +-+-+-+-+-+-+-+-+

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
  implementor 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 describe the feature, for example, a
  command word.  We recommend grouping series of related transactions
  under descriptive headers, for example, RECEPTION.

  Section reference directs the implementor 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 either use 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
       M - 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
  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, read events vertically and states
  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 are below 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.  Also, this RFC's
  Appendix C models transitions as the Cartesian product of two state
  machines.  This is a more complex representation that may be
  difficult to comprehend for those readers that are unfamiliar with
  the format.  The working group recommends 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 |<----------------------                   |
               |                                          |

3.4  Character Sets

  At one time the Internet had a geographic boundary and was English
  only.  Since the Internet now extends internationally, application
  protocols must assume that the contents of any text string may be in
  a language other than English.  Therefore, new or updated protocols
  which transmit text must use ISO 10646 as the default Coded Character
  Set, and RFC 2044, "UTF-8, a transformation format of Unicode and ISO
  10646" as the default Character Encoding Scheme.  An exception is the
  use of US-ASCII for a protocol's controlling commands and replies.
  Protocols that have a backwards compatibility requirement should use
  the default of the existing protocol.  This is in keeping with the
  recommendations of RFC 2130, "The Report of the IAB Character Set
  Workshop held 29 February - 1 March 1996."

4  Document Checklist

  The following is a checklist based on these 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 Are procedures for assigning numbers provided as guidance for 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 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?
  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 use the recommended Internet meanings for any terms use to
     specify the protocol?
  o 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 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   "Internet Protocol (IP)," J. Postel, September 1981.

  RFC 904   "Exterior Gateway Protocol formal specification," D. Mills,
  RFC 1112  "Host extensions for IP multicasting," S. Deering,
            August 1989

  RFC 1122  "Requirements for Internet Hosts -- Communication Layers,"
            R.  Braden, October 1989
  RFC 1123  "Requirements for Internet hosts -- Application and
            Support," R.  Braden, October 1989
  RFC 1700  "Assigned Numbers," J. Reynolds, J. Postel, October 1994

  RFC 1983  "Internet Users' Glossary," G. Malkin, August 1996

  RFC 1939  "Post Office Protocol - Version 3," J. Meyers, M. Rose,
  RFC 2026  "The Internet Standards Process -- Revision 3," S. Bradner,
            October 1996

  RFC 2044  "UTF-8, a transformation format of Unicode and ISO 10646,"
            F. Yergeau, October 1996

  RFC 2119  "Key words for use in RFCs to Indicate Requirement Level,"
  RFC 2130  "The Report of the IAB Character Set Workshop held 29
            February - 1 March 1996," C. Weider, C. Preston,
            K. Simonsen, H. Alvestrand, R. Atkinson, M. Crispin,

7  Acknowledgments

  Peter Desnoyers and Art Mellor began the work on this document.  The
  area directors that oversaw the STDGUIDE WG's efforts were
  Scott Bradner, Mike O'Dell, and John Curran.  Others that contributed
  to this document were:

     Bernard Aboba
     Harald T. Alvestrand
     Fred Baker
     Brian Carpenter
     Robert Elz
     Dirk Fieldhouse
     Dale Francisco
     Gary Malkin
     Neal McBurnett
     Craig Partridge
     Henning Schulzrinne
     Kurt Starsinic
     James Watt

8  Editor's Address

  Gregor D. Scott
  Director, Defense Information Systems Agency
  ATTN: JIEO-JEBBD JIEO-JEBBC
  Ft. Monmouth, NJ  07703-5613
  USA

  Phone:    (908)    (732) 427-6856
  Fax:      (908)      (732) 532-0853
  EMail:    scottg@ftm.disa.mil

  This Internet Draft expires on 30 June 1997.