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
RFC2223, "InstrUCtions to RFCAuthors".
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
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 RFC2223, "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/RFC2026, "The Internet
Standards Process -- Revision 3". In addition, some considerations
for protocol design are given in RFC1958, "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
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.
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 RFCto
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/RFC1350, STD 51/RFC1662, and STD 53/RFC1939. RFC2316, "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
understanding the protocol, and where it fits in the Internet. The
STD 54/RFC2328 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.
One approach is to divide the standard into sections: one describing
the protocol concisely, while another section consists of explanatory
text. The STD 3/RFC1122/RFC1123 and STD 54/RFC2328 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
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/RFC791, 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
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
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/RFC2119, "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/RFC1122/RFC1123.
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/RFC822 provides an example of this. In RFC822 (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/RFC1155 (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.
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, RFC1700, "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/RFC1939 has an example of
such a section. If this is not applicable to the protocol, an
explanation of why not should be included.
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/RFC1058, 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/RFC2277, "IETF Policy
on Character Sets and Languages".
2.18 Glossary
Every standards track RFCshould 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/RFC1983, "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. 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 +

+-+-+-+-+-+-+-+-+
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/RFC1122/RFC1123 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, 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/RFC1661 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/RFC904 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 RFC1661 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
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?
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
[RFC791] Postel, J., "Internet Protocol (IP)", STD 5, RFC791
September 1981.
[RFC904] Mills, D., "Exterior Gateway Protocol formal
specification", RFC904, April 1984.
[RFC1058] Hedrick, C., "Routing Information Protocol", STD 34,
RFC1058, June 1988.
[RFC1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC1112, August 1989.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC1122, October 1989.
[RFC1123] Braden, R., "Requirements for Internet hosts --
Application and Support", STD 3, RFC1123, October 1989.
[RFC1311] Postel, J., "Introduction to the STD Notes", RFC1311,
March 1992.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
RFC1350, July 1992.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC1661, July 1994.
[RFC1662] Simpson, W., "PPP in HLDC-like Framing", STD 51, RFC1662,
July 1994.
[RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC1700, October 1994. (http://www.iana.org)
[RFC1939] Meyers, J., and M. Rose, "Post Office Protocol - Version
3", STD 53, RFC1939, May 1996.
[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC1958, June 1996.
[RFC1983] Malkin, G., "Internet Users" Glossary", FYI 18, RFC1983,
August 1996.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.
[RFC2223] Postel, J. and J. Reynolds, "Instructions to RFCAuthors",
RFC2223, October 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Language", RFC2277, January 1998.
[RFC2316] Bellovin, S., "Report of the IAB Security Architecture
Workshop", RFC2316, April 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
9 Appendix
CHANGES FROM DRAFT -06
The following changes were made following IESG review:
References to RFC1543 were changed to RFC2223 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 RFC2315 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 RFCnumber 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 RFC2044 were changed to RFC2279 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.
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
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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
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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
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