Network Working Group S. McQuistin
Internet-Draft V. Band
Intended status: Informational C. Perkins
Expires: 9 January 2020 University of Glasgow
8 July 2019
Fully Specifying Protocol Parsing with Augmented ASCII Diagrams
draft-mcquistin-augmented-ascii-diagrams-00
Abstract
This document describes a machine-readable format for fully
specifying the process by which a protocol can be parsed. This
format combines a consistent ASCII packet diagram format with the use
of structured text, maintaining human readability while enabling
support for machine parsing. This document is itself an example of
how this format can be used.
Status of This Memo
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Copyright Notice
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Limitations of current ASCII packet diagrams usage . . . 3
2.2. Formal languages in standards documents . . . . . . . . . 6
3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 6
4. Augmented ASCII Packet Header Diagrams . . . . . . . . . . . 8
4.1. Fixed-width Field Format . . . . . . . . . . . . . . . . 8
4.2. Variable-width Field Format . . . . . . . . . . . . . . . 10
4.3. Cross-referencing and Sequences Format . . . . . . . . . 11
4.4. Optional Field Format . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. Informative References . . . . . . . . . . . . . . . . . . . 13
Appendix A. ABNF specification . . . . . . . . . . . . . . . . . 14
A.1. Constraint Expressions . . . . . . . . . . . . . . . . . 14
A.2. Augmented ASCII diagrams . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
ASCII packet header diagrams have become the de-facto format for
describing the syntax of binary protocols. In otherwise largely
textual documents, they allow for the visualisation of packet
formats, reducing human error, and aiding in the implementation of
parsers for the protocols that they specify. Given their widespread
use, and relatively structured form, ASCII packet header diagrams
provide a good base from which to develop a format that supports the
automatic generation of parser code from protocol standards
documents.
There are two broad issues with the existing ASCII packet header
diagrams that need to be addressed to enable machine-readability.
First, their use, while sufficiently consistent for human
readability, contains enough variation to make machine parsing
difficult: different documents tend to use subtly different formats
and conventions. Second, ASCII packet header diagrams alone do not
fully capture the parsing process for protocols, requiring
supplementary text. To support machine parsing, this supplementary
text must be consistently structured.
This document describes a consistent ASCII packet header format and
accompanying structured text constructs that allow for the parsing
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process of protocol headers to be fully specified. This provides
support for the automatic generation of parser code. Broad design
principles, that seek to maintain the primacy of human readability
and flexibility in authorship, are described, before the format
itself is given.
This document is itself an example of the approach that it describes,
with the ASCII packet diagrams and structured text format described
by example.
This draft describes early work. As consensus builds around the
particular syntax of the format described, both a formal ABNF
specification and code that parses it (and, as described above, this
document) will be provided.
2. Background
This section begins by considering how ASCII packet header diagrams
are used in existing documents. This exposes the limitations that
the current usage has in terms of machine-readability, guiding the
design of the format that this document proposes.
While this document focuses on the machine-readability of packet
header diagrams, this section also discusses the use of other
structured or formal languages within IETF documents. Considering
how and why these languages are used provides an instructive contrast
to the relatively incremental approach proposed here.
2.1. Limitations of current ASCII packet diagrams usage
ASCII packet header diagrams are commonplace in the IETF standards
documents for binary protocols. While there is no standard for how
these diagrams should be formatted, they have a broadly similar
structure, where the layout of a protocol data unit (PDU) or
structure is given in an ASCII diagram, and a description list of the
fields that it contains are given immediately below. An example of
this format is given in Figure 1.
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The RESET_STREAM frame is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application Error Code (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Final Size (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RESET_STREAM frames contain the following fields:
Stream ID: A variable-length integer encoding of the Stream ID of
the stream being terminated.
Application Protocol Error Code: A 16-bit application protocol
error code (see Section 20.1) which indicates why the stream is
being closed.
Final Size: A variable-length integer indicating the final size
of the stream by the RESET_STREAM sender, in unit of bytes.
Figure 1: QUIC's RESET_STREAM frame format (from [QUIC-TRANSPORT])
However, these diagrams, and their accompanying descriptions, are
formatted for human readers rather than for machine parsing. As a
result, while there is broad consistency in how ASCII packet diagrams
are formatted, there are a number of limitations that are prohibitive
to machine parsing:
Inconsistent syntax: There are two classes of consistency that are
required for parsability: internal consistency, within a document
or diagram, and external consistency, across all documents. Given
that ASCII packet diagrams are formatted for human readers, rather
than for machine parsing, there is sufficient variability in how
they are formatted that parsing is difficult.
The format of the "Relay Source Port Option" is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_RELAY_PORT | Option-Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Source Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Where:
Option-Code: OPTION_RELAY_PORT. 16-bit value, 135.
Option-Len: 16-bit value to be set to 2.
Downstream Source Port: 16-bit value. To be set by the IPv6
relay either to the downstream relay agent's UDP source port
used for the UDP packet, or to zero if only the local relay
agent uses the non-DHCP UDP port (not 547).
Figure 2: DHCPv6's Relay Source Port Option (from [RFC8357])
Figure 1 gives an example of internal inconsistency. Here, the
ASCII diagram shows a field labelled "Application Error Code",
while the accompanying description lists the field as "Application
Protocol Error Code". The use of an abbreviated name is suitable
for human readers, but makes parsing the structure difficult for
machines. Figure 2 gives a further example, where the description
lists a field "Option-Code" that does not appear in the ASCII
diagram. In addition, the description list describes each field
as being 16 bits in length, while the diagram shows the
OPTION_RELAY_PORT as 13 bits, and Option-Len as 19 bits. Another
example of this -- where the diagram and accompanying text
disagree -- is in [RFC6958], where the packet header diagram
showing the structure of the Burst/Gap Loss Metrics Report Block
shows the Number of Bursts field as being 12 bits wide but the
corresponding text describes it as 16 bits.
Comparing Figure 1 with Figure 2 exposes external inconsistency
across documents. While the ASCII diagrams themselves are broadly
similar, the text surrounding the diagrams is formatted
differently. If machine parsing is to be made possible, then this
text must be structured consistently.
Ambiguous constraints: The constraints that are enforced on a
particular field are often described ambiguously, or in a way that
cannot be parsed easily. In Figure 2, each of the three fields in
the structure is constrained. The first two fields ("Option-Code"
and "Option-Len") are to be set to constant values (note the
inconsistency in how these constraints are expressed in the
description). However, the third field ("Downstream Source Port")
can take a value from a constrained set. This constraint is
expressed in prose that can easily be parsed by humans, but not by
machines.
Poor linking between sub-structures: Protocol data units and other
structures are often comprised of sub-structures that are defined
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elsewhere, either in the same document, or within another
document. Chaining these structures together is essential for
machine parsing: the parsing process for a protocol data unit is
only fully expressed if all elements can be parsed.
Figure 1 highlights the difficulty that machine parsers have in
chaining structures together. Two fields ("Stream ID" and "Final
Size") are described as being encoded as variable-length integers;
this is a structure described elsewhere in the same document.
Structured text is required both alongside the definition of the
containing structure and with the definition of the sub-structure,
to allow a parser to link the two together.
2.2. Formal languages in standards documents
A small proportion of IETF standards documents contain structured and
formal languages, including ABNF [RFC5234], ASN.1 [ASN1], C, CBOR
[RFC7049], JSON, the TLS presentation language [RFC8446], YANG models
[RFC7950], and XML. While this broad range of languages may be
problematic for the development of tooling to parse specifications,
these, and other, languages serve a range of different use cases.
ABNF, for example, is typically used to specify text protocols, while
ASN.1 is used to specify data structure serialisation. This document
specifies a structured language for specifying the parsing of binary
protocol data units.
3. Design Principles
The use of structures that are designed to support machine
readability may potentially interfere with the existing ways in which
protocol specifications are used and authored. To the extent that
these existing uses are more important than machine readability, such
interference must be minimised.
In this section, the broad design principles that underpin the format
described by this document are given. However, these principles
apply more generally to any approach that introduces structured and
formal languages into standards documents.
It should be noted that these are design principles: they expose the
trade-offs that are inherent within any given approach. Violating
these principles is sometimes necessary and beneficial, and this
document sets out the potential consequences of doing so.
The central tenet that underpins these design principles is a
recognition that the standardisation process is not broken, and so
does not need to be fixed. Failure to recognise this will likely
lead to approaches that are incompatible with the standards process,
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or that will see limited adoption. However, the standards process
can be improved with appropriate approaches, as guided by the
following broad design principles:
Most readers are human: Primarily, standards documents should be
written for people, who require text and diagrams that they can
understand. Structures that cannot be easily parsed by people
should be avoided, and if included, should be clearly delineated
from human-readable content.
Any approach that shifts this balance -- that is, that primarily
targets machine readers -- is likely to be disruptive to the
standardisation process, which relies upon discussion centered
around documents written in prose.
Authorship tools are diverse: Authorship is a distributed process
that involves a diverse set of tools and workflows. The
introduction of machine-readable structures into specifications
should not require that specific tools are used to produce
standards documents, to ensure that disruption to existing
workflows is minimised. This does not preclude the development of
optional, supplementary tools that aid in the authoring machine-
readable structures.
The immediate impact of requiring specific tooling is that
adoption is likely to be limited. A long-term impact might be
that authors whose workflows are incompatible might be alienated
from the process.
Canonical specifications: As far as possible, machine-readable
structures should not replicate the human readable specification
of the protocol within the same document. Such structures should
form part of a canonical specification of the protocol. Adding
supplementary machine-readable structures, in parallel to the
existing human readable text, is undesirable because it could
create the potential for inconsistency.
As an example, program code that describes how a protocol data
unit can be parsed might be provided as an appendix within a
standards document. This code would provide a specification of
the protocol that is separate to the prose description in the main
body of the document. This has the undesirable effect of
introducing the potential for the program code to specify
behaviour that the prose-based specification does not, and vice-
versa.
Expressiveness: Any approach should be expressive enough to capture
the syntax and parsing process for the majority of binary
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protocols. If a given language is not sufficiently expressive,
then adoption is likely to be limited. At the limits of what can
be expressed by the language, authors are likely to revert to
defining the protocol in prose: this undermines the broad goal of
using structured and formal languages. Equally, though,
understandable specifications and ease of use are critical for
adoption. A tool that is simple to use and addresses the most
common use cases might be preferred to a complex tool that
addresses all use cases.
Minimise required change: Any approach should require as few changes
as possible to the way documents are formatted, authored, and
published. Forcing adoption of a particular structured or formal
language is incompatible with the IETF's standardisation process:
there are very few components of standards documents that are non-
optional.
4. Augmented ASCII Packet Header Diagrams
The design principles described in Section 3 can largely be met by
the existing uses of ASCII packet header diagrams. These diagrams
aid human readability, do not require new or specialised authorship
tools, do not split the specification into multiple parts, can
express most binary protocol features, and require no changes to the
existing publication processes.
However, as discussed in Section 2.1 there are limitations to how
ASCII diagrams are used that must be addressed if they are to be
parsed by machine. In this section, an augmented ASCII packet header
diagram format is described.
The concept is first illustrated by example. This is appropriate,
given the visual nature of the language. In future drafts, these
examples will be parsable using provided tools, and a formal
specification of the augmented ASCII packet diagrams will be given in
Appendix A.
In the augmented ASCII packet diagrams, each protocol data unit is
described in its own section of the document. This enables cross-
referencing between data units using section numbering. In this
specification-by-example, each element of the format will be
described as part of a separate PDU.
4.1. Fixed-width Field Format
The simplest PDU is one that contains only a set of fixed-width
fields in a known order, with no optional fields or variation in the
packet format.
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A Fixed-width Field Format packet is formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F2 | Field30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Field64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Field48 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field8 |
+-+-+-+-+-+-+-+-+
where:
Field2 (F2): 2 bits This is a short field, and the diagram cannot
show its full label. A short label (F2) is used
in the diagram, and this short label is
provided, in brackets, after the full label in
the description list. The field's width -- 2
bits -- is given in the label of the description
list, separated from the field's label by a
colon.
Field30: 30 bits This is a longer field whose full label can be
shown in the diagram.
Field64: 8 bytes This is a field that spans multiple rows. Where
fields are an integral number of bytes in size,
and start and end on a byte boundary, the field
length can be given in bytes rather than in
bits.
Field48: 48 bits This is another multi-row field. As
illustrated, fields are not required to end of a
32-bit boundary.
Field8: 1 byte This field has been drawn on the next line,
where it would have fit on the previous line.
Where possible, the formatting of the diagram
should be flexible to meet the needs of human
readers.
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4.2. Variable-width Field Format
Some packet formats include variable-width fields, where the size of
a field is either derived from the value of some previous field, or
is unspecified and inferred from the total size of the packet and the
size of the other fields. A packet can contain only one unspecified
length field, to ensure there is no ambiguity.
A Variable-width Field Format packet is formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FieldVar - single row ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: FieldVar - multi-row :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: FieldVar - multi-row, unspecified length :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Field8 (F): 8 bits This is a fixed-width field, as described
previously. As shown, while this field has a short label (F),
this does not need to be used in the diagram.
FieldVar - single row: 2^F bits This is a variable-length field,
whose length is defined by the value of the field with short label
F (Field8). Constraint expressions can be used in place of
constant values: the grammar for the expression language is
defined in Section a.1. Where fields labels are used in a
constraint, the field being referred to must have been defined
before its label is used. Short variable-length fields are
indicated by "..." instead of a pipe at the end of the row.
FieldVar - multi-row: 2^F bits This is a multi-row variable-length
field, again constrained by the value of field F. Instead of the
"..." notation, ":" is used to indicate that the field is
variable-length. The use of ":" instead of "..." indicates the
field is likely to be a longer, multi-row field. However,
semantically, there is no difference: these different notations
are for the benefit of human readers.
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FieldVar - multi-row, unspecified length This is a variable-width
field whose length is implied by the lengths of the other fields.
At parsing time, the length of the PDU is known, and this can be
used to determine the length of fields whose length is undefined.
Each PDU can only leave a single field's length undefined: all
other fields must be fixed-length, or have their widths
constrained.
4.3. Cross-referencing and Sequences Format
A Cross-referencing and Sequences Format packet is formatted as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ FieldFixedXRef +
| |
+ +
| |
+ +-+-+-+-+-+-+-+-+
| | :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
: :
: FieldVarXref :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ [SeqFieldFixedXRef] +
| |
+ +
| |
+ +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Field8 (F): 8 bits This is a fixed-width field, as described
previously.
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FieldFixedXRef: 1 Fixed-width Field Format This is a field whose
structure is a previously defined PDU format. To indicate this,
the width of the field is given in units of the cross-referenced
structure (here, Fixed-width Field Format).
FieldVarXref: 1 Variable-width Field Format This field references a
variable-width structure. It can be drawn to any width as
appropriate, but must use a variable-width notation. Where
multiple variable-width field format structures are referenced,
the requirement that only one field's length can be unspecified
applies to the enclosing structure.
SeqFieldFixedXRef: 2 Fixed-width Field Format Where a field is
comprised of a sequence of previously defined structures, square
brackets can be used to indicate this in the diagram. The length
of the sequence can be defined using the constraint expression
grammar as described earlier.
4.4. Optional Field Format
An Optional Field Format packet is formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionalField |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Field8 (F): 8 bits This is a fixed-width field, as described
previously.
OptionalField: 4 bytes Present only when F > 3. This is a field
whose presence is predicated on an expression
given in the constraint expression grammar
described earlier. Optional fields can be of
any previously defined format (e.g., fixed-
or variable-width). Optional fields are
indicated by the presence of a "Present only
when [expr]." as the first line in their
description
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5. IANA Considerations
This document contains no actions for IANA.
6. Security Considerations
Poorly implemented parsers are a frequent source of security
vulnerabilities in protocol implementations. Structuring the
description of a protocol data unit so that a parser can be
automatically derived from the specification can reduce the
likelihood of vulnerable implementations.
7. Acknowledgements
The authors would like to thank David Southgate for preparing a
prototype implementation of some of the ideas described here.
This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
8. Informative References
[ASN1] ITU-T, "ITU-T Recommendation X.680, X.681, X.682, and
X.683", ITU-T Recommendation X.680, X.681, X.682, and
X.683.
[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), 23 April 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-20.txt>.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6958] Clark, A., Zhang, S., Zhao, J., and Q. Wu, "RTP Control
Protocol (RTCP) Extended Report (XR) Block for Burst/Gap
Loss Metric Reporting", RFC 6958, May 2013,
<https://www.rfc-editor.org/info/rfc6958>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013,
<https://www.rfc-editor.org/info/rfc7049>.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language",
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RFC 7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8357] Deering, S. and R. Hinden, "Generalized UDP Source Port
for DHCP Relay", RFC 8357, March 2018,
<https://www.rfc-editor.org/info/rfc8357>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Appendix A. ABNF specification
A.1. Constraint Expressions
cond-expr = eq-expr "?" cond-expr ":" eq-expr
eq-expr = bool-expr eq-op bool-expr
bool-expr = ord-expr bool-op ord-expr
ord-expr = add-expr ord-op add-expr
add-expr = mul-expr add-op mul-expr
mul-expr = expr mul-op expr
expr = *DIGIT / field-name
field-name = *ALPHA
mul-op = "*" / "/" / "%"
add-op = "+" / "-"
ord-op = "<=" / "<" / ">=" / ">"
bool-op = "&&" / "||" / "!"
eq-op = "==" / "!="
A.2. Augmented ASCII diagrams
Future revisions of this draft will include an ABNF specification for
the augmented ASCII diagram format described in Section 4. Such a
specification is omitted from this draft given that the format is
likely to change as its syntax is developed. Given the visual nature
of the format, it is more appropriate for discussion to focus on the
examples given in Section 4.
Authors' Addresses
Stephen McQuistin
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
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United Kingdom
Email: sm@smcquistin.uk
Vivian Band
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: vivianband0@gmail.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: csp@csperkins.org
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