| Internet-Draft | Augmented Packet Diagrams | February 2020 |
| McQuistin, et al. | Expires 7 August 2020 | [Page] |
This document describes a machine-readable format for specifying the syntax of protocol data units within a protocol specification. This format is comprised of a consistently formatted packet header diagram, followed by structured explanatory text. It is designed to maintain human readability while enabling support for automated parser generation from the specification document. This document is itself an example of how the format can be used.¶
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Packet header diagrams have become a widely used 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.¶
Figure 1 gives an example of how packet header diagrams are used to define binary protocol formats. The format has an obvious structure: the diagram clearly delineates each field, showing its width and its position within the header. This type of diagram is designed for human readers, but is consistent enough that it should be possible to develop a tool that generates a parser for the packet format from the diagram.¶
: 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
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Source Port | Destination Port |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Sequence Number |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Acknowledgment Number |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Data | |U|A|P|R|S|F| |
: | Offset| Reserved |R|C|S|S|Y|I| Window |
: | | |G|K|H|T|N|N| |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Checksum | Urgent Pointer |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | Options | Padding |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | data |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unfortunately, the format of such packet diagrams varies both within and between documents. This variation makes it difficult to build tools to generate parsers from the specifications. Better tooling could be developed if protocol specifications adopted a consistent format for their packet descriptions. Indeed, this underpins the format described by this draft: we want to retain the benefits that packet header diagrams provide, while identifying the benefits of adopting a consistent format.¶
This document describes a consistent packet header diagram format and accompanying structured text constructs that allow for the parsing 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 packet header diagrams and structured text format described by example. Examples that do not form part of the protocol description language are marked by a colon at the beginning of each line; this prevents them from being parsed by the accompanying tooling.¶
This draft describes early work. As consensus builds around the particular syntax of the format described, both a formal ABNF specification (Appendix A) and code (Appendix B) that parses it (and, as described above, this document) will be provided.¶
This section begins by considering how 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 format 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.¶
: 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.
Packet header diagrams are frequently used in IETF standards to describe the format of 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 shown in diagrammatic form, followed by a description list of the fields that it contains. An example of this format, taken from the QUIC specification, is given in Figure 2.¶
These packet header diagrams, and the accompanying descriptions, are formatted for human readers rather than for automated processing. As a result, while there is rough consistency in how packet header diagrams are formatted, there are a number of limitations that make them difficult to work with programmatically:¶
There are two classes of consistency that are needed to support automated processing of specifications: internal consistency within a diagram or document, and external consistency across all documents.¶
Figure 2 gives an example of internal inconsistency. Here, the packet 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 3 gives a further example, where the description includes an "Option-Code" field that does not appear in the packet diagram; and where the description states that each field is 16 bits in length, but the diagram shows the OPTION_RELAY_PORT as 13 bits, and Option-Len as 19 bits. Another example is [RFC6958], where the packet format 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 2 with Figure 3 exposes external inconsistency across documents. While the packet format diagrams are broadly similar, the surrounding text is formatted differently. If machine parsing is to be made possible, then this text must be structured consistently.¶
Protocol data units and other structures are often comprised of sub-structures that are defined 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 2 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.¶
Protocols are often specified across multiple documents, either because the protocol explicitly includes extension points (e.g., profiles and payload format specifications in RTP [RFC3550]) or because definition of a protocol data unit has changed and evolved over time. As a result, it is essential that syntax be provided to allow for a complete definition of a protocol's parsing process to be constructed across multiple documents.¶
: 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 |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:
: 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).
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.¶
The use of structures that are designed to support machine readability might 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, or that will see limited adoption. However, the standards process can be improved with appropriate approaches, as guided by the following broad design principles:¶
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 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.¶
As far as possible, machine-readable structures should not replicate the human readable specification of the protocol within the same document. Machine-readable 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 creates 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.¶
Any approach should be expressive enough to capture the syntax and parsing process for the majority of binary 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.¶
It may be desirable to restrict expressiveness, however, to guarantee intrinsic safety, security, and computability properties of both the generated parser code for the protocol, and the parser of the description language itself. In much the same way as the language-theoretic security ([LANGSEC]) community advocates for programming language design to be informed by the desired properties of the parsers for those languages, protocol designers should be aware of the implications of their design choices. The expressiveness of the protocol description languages that they use to define their protocols can force such awareness.¶
Broadly, those languages that are more expressive tend to have parsers that are more complex and less safe. As a result, while considering the other goals described in this document, protocol description languages should attempt to be minimally expressive, and restrict protocol designs to those for which safe and secure parsers can be generated.¶
Any approach should require as few changes as possible to the way that 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.¶
The design principles described in Section 3 can largely be met by the existing uses of 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 existing publication processes.¶
However, as discussed in Section 2.1 there are limitations to how packet header diagrams are used that must be addressed if they are to be parsed by machine. In this section, an augmented 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 packet diagrams will be given in Appendix A.¶
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.¶
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 PDU description is introduced by the exact phrase "A/An _______ is formatted as follows:" at the end of a paragraph. This is followed by the PDU description itself, as a packet diagram within an <artwork> element in the XML representation, starting with a header line to show the bit width of the diagram. The description of the fields follows the diagram, as an XML <dl> list, after a paragraph containing the text "where:".¶
Each field of the description starts with a <dt> tag comprising the field name and an optional short name in parenthesis. These are followed by a colon, the field length, an optional presence expression (described in Section 4.2), and a terminating period. The following <dd> tag contains a prose description of the field. Field names cannot be the same as a previously defined PDU name.¶
For example, this can be illustrated using the IPv4 Header Format [RFC791]. An IPv4 Header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL | DSCP |ECN| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: Payload :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
where:¶
This is a fixed-width field, whose full label is shown in the diagram. The field's width -- 4 bits -- is given in the label of the description list, separated from the field's label by a colon.¶
Binary formats often reference sub-structures that have been defined earlier in the specification. For example, in RTP [RFC3550], the Contributing Source Identifiers in an RTP Data Packet are defined as comprising a list of Source Identifier elements. A Source Identifier 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
where:¶
The following example shows how a Source Identifier can be referenced in the description of an RTP Data Packet. It also shows how the presence of some fields in a format may be dependent on the values of an earlier field.¶
An RTP Data 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V |P|X| CC |M| PT | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Source identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Contributing Source identifiers] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Extension |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload :
: :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding | Padding Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
where:¶
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.¶
In this example, both a PDU name (Source Identifier) and a field name (CC) are used in the constraint expression. The PDU name refers to the length of the PDU, while the field name refers to the value of the field. This is possible because field names cannot be the same as previously defined PDU names.¶
This is a field whose presence is predicated on an expression given using 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 "; present only when [expr]." at the end of the definition term (i.e., the text contained within the <dt> tag).¶
[Note that this example deviates from the format as described in [RFC3550]. As specified in that document, the Header Extension would be a cross-referenced structure. This is not shown here for brevity.]¶
In some binary formats, fields are striped across multiple non-contiguous bits. This is often to allow for backwards compatibility with previous definitions of the same fields in earlier documents: striping in this way allows for careful use of the possible range of values.¶
This format is illustrated using the STUN Message Type [draft-ietf-tram-stunbis-21]. A STUN Message Type is formatted as follows:¶
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|M|M|M|M|C|M|M|M|C|M|M|M|M|
|B|A|9|8|7|1|6|5|4|0|3|2|1|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
where:¶
Protocols are often specified across multiple documents, either because the specification of a protocol's data units has changed over time, or because of explicit extension points contained in the protocol's original specification. To allow a document to make use of a previous PDU definition, it is possible to import PDU definitions (written in the format described in this document) from other documents.¶
A PDU definition is imported using the exact phrase "A/An ________ is formatted as described in <document identifier>". The document identifier must refer, unambiguously, to an existing document. An Internet-Draft is identified by its name. RFCs are identified by "RFC" followed by their number.¶
This document contains no actions for IANA.¶
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.¶
As described in Section 3, the expressiveness of a protocol description language has implications for the safety, security, and computability properties of the parser for the protocol description language itself, and on the generated parser code for the protocols described using it. The language-theoretic security ([LANGSEC]) community explores the security implications of programming language design; the principles developed in that community should guide the development of protocol description languages.¶
The authors would like to thank David Southgate for preparing a prototype implementation of some of the ideas described here.¶
The authors would like to thank Marc Petit-Huguenin for feedback on the draft.¶
This work has received funding from the UK Engineering and Physical Sciences Research Council under grant EP/R04144X/1.¶
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-ws / "(" expr ")"
field-name = *ALPHA
field-name-ws = *(field-name " ")
mul-op = "*" / "/" / "%"
add-op = "+" / "-"
ord-op = "<=" / "<" / ">=" / ">"
bool-op = "&&" / "||" / "!"
eq-op = "==" / "!="
¶
Future revisions of this draft will include an ABNF specification for the augmented packet 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.¶
The source code for tooling that can be used to parse this document is available from https://github.com/lumisota/improving-protocol-standards.¶