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


   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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   This Internet-Draft will expire on 9 January 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text

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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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

   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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         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

   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-

   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-

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    |


   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

   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

   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

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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         :
      :                                                               |


   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

4.3.  Cross-referencing and Sequences Format

   A Cross-referencing and Sequences Format packet is formatted as

       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]                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +                                               +-+-+-+-+-+-+-+-+
      |                                               |


   Field8 (F): 8 bits  This is a fixed-width field, as described

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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                         |


   Field8 (F): 8 bits      This is a fixed-width field, as described

   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

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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

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), 23 April 2019,

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008,

   [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,

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, October 2013,

   [RFC7950]  Bjorklund, M., "The YANG 1.1 Data Modeling Language",

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              RFC 7950, August 2016,

   [RFC8357]  Deering, S. and R. Hinden, "Generalized UDP Source Port
              for DHCP Relay", RFC 8357, March 2018,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, August 2018,

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
   G12 8QQ

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   United Kingdom


   Vivian Band
   University of Glasgow
   School of Computing Science
   G12 8QQ
   United Kingdom


   Colin Perkins
   University of Glasgow
   School of Computing Science
   G12 8QQ
   United Kingdom


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