Describing Protocol Data Units with Augmented Packet Header Diagrams
draft-mcquistin-augmented-ascii-diagrams-08
Network Working Group S. McQuistin
Internet-Draft V. Band
Intended status: Experimental D. Jacob
Expires: 6 November 2021 C. S. Perkins
University of Glasgow
5 May 2021
Describing Protocol Data Units with Augmented Packet Header Diagrams
draft-mcquistin-augmented-ascii-diagrams-08
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 6 November 2021.
Copyright Notice
Copyright (c) 2021 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Limitations of Current Packet Format Diagrams . . . . . . 4
2.2. Formal languages in standards documents . . . . . . . . . 7
3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 7
4. Augmented Packet Header Diagrams . . . . . . . . . . . . . . 10
4.1. PDUs with Fixed and Variable-Width Fields . . . . . . . . 10
4.2. PDUs That Cross-Reference Previously Defined Fields . . . 13
4.3. PDUs with Non-Contiguous Fields . . . . . . . . . . . . . 16
4.4. PDUs with Constraints on Field Values . . . . . . . . . . 16
4.5. PDUs That Extend Sub-Structures . . . . . . . . . . . . . 18
4.6. Storing Data for Parsing . . . . . . . . . . . . . . . . 19
4.7. Connecting Structures with Functions . . . . . . . . . . 20
4.8. Specifying Enumerated Types . . . . . . . . . . . . . . . 21
4.9. Specifying Protocol Data Units . . . . . . . . . . . . . 22
4.10. Importing PDU Definitions from Other Documents . . . . . 22
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
9. Informative References . . . . . . . . . . . . . . . . . . . 24
Appendix A. ABNF specification . . . . . . . . . . . . . . . . . 26
A.1. Constraint Expressions . . . . . . . . . . . . . . . . . 26
A.2. Augmented packet diagrams . . . . . . . . . . . . . . . . 26
Appendix B. Tooling & source code . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
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.
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: 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 |
: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: TCP's header format (from [RFC793])
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 writing, 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.
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This draft describes early work. As consensus builds around the
particular syntax of the format described, a formal ABNF
specification (Appendix A) will be provided.
Example specifications of a number of IETF protocols described using
the Augmented Packet Header Diagram format are available. These
documents describe UDP [draft-mcquistin-augmented-udp-example], TCP
[draft-mcquistin-augmented-tcp-example], and QUIC
[draft-mcquistin-quic-augmented-diagrams]. Code that parses those
documents and automatically generates parser code for the described
protocols is described in Appendix B.
2. Background
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.
2.1. Limitations of Current Packet Format Diagrams
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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 2: QUIC's RESET_STREAM frame format (from [QUIC-TRANSPORT])
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:
Inconsistent syntax: 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
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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.
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 3, 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 cannot readily by understood by machine.
Poor linking between sub-structures: 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.
Lack of extension and evolution syntax: 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
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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).
Figure 3: DHCPv6's Relay Source Port Option (from [RFC8357])
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 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.
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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:
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.
Writing tools are diverse: Standards document writing 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
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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.
Expressiveness: 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 have grammars which 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 either restrict protocol designs to
those for which safe and secure parsers can be generated, or as a
minimum, ensure that protocol designers are aware of the
boundaries their designs cross, in terms of computability and
decidability [SASSAMAN].
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Minimise required change: 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.
4. Augmented Packet Header Diagrams
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 tools to write,
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.
4.1. PDUs with Fixed and Variable-Width Fields
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.
To ensure that there is no ambiguity, a PDU description can contain
only one field whose length is unspecified. The length of a single
field, where all other fields are of known (but perhaps variable)
length, can be inferred from the total size of the containing PDU.
A PDU description is introduced by the exact phrase "A/An _______ is
formatted as follows" within a paragraph. This is followed by the
PDU description itself, as a packet diagram within an <artwork>
element (itself optionally within a <figure> element) in the XML
representation, starting with a header line to show the bit width of
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the diagram. The description of the fields follows the diagram, as
an XML list (either <dl> or hanging <list>), after a paragraph that
begins with the text "where:".
PDU names must be unique, both within a document, and across all
documents that are linked together (i.e., using the structured
language defined in Section 4.10).
Each field is defined by a structured text definition and a prose
description. The structured text definition comprises 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. Field names cannot be the
same as a previously defined PDU name, and must be unique within a
given structure definition. The structured text definition is given
either in a <dt> tag (if using a <dl>) or as the "hangText" (if using
a hanging <list>) of a <t> element. The field's prose description is
given in the following <dd> element or within the same <t> element.
Prose descriptions may include structured text (e.g., as defined in
Section 4.6).
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:
Version (V): 4 bits. This is a fixed-width field, whose full label
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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.
Internet Header Length (IHL): 4 bits. This is a shorter field, whose
full label is too large to be shown in the diagram. A short label
(IHL) is used in the diagram, and this short label is provided, in
brackets, after the full label in the description list.
Differentiated Services Code Point (DSCP): 6 bits. This is a fixed-
width field, as previously discussed.
Explicit Congestion Notification (ECN): 2 bits. This is a fixed-
width field, as previously discussed.
Total Length (TL): 2 bytes. This is a fixed-width field, as
previously discussed. Where fields are an integral number of
bytes in size, the field length can be given in bytes rather than
in bits.
Identification: 2 bytes. This is a fixed-width field, as previously
discussed.
Flags: 3 bits. This is a fixed-width field, as previously discussed.
Fragment Offset: 13 bits. This is a fixed-width field, as previously
discussed.
Time to Live (TTL): 1 byte. This is a fixed-width field, as
previously discussed.
Protocol: 1 byte. This is a fixed-width field, as previously
discussed.
Header Checksum: 2 bytes. This is a fixed-width field, as previously
discussed.
Source Address: 32 bits. This is a fixed-width field, as previously
discussed.
Destination Address: 32 bits. This is a fixed-width field, as
previously discussed.
Options: (IHL-5)*32 bits. This is a variable-length field, whose
length is defined by the value of the field with short label IHL
(Internet Header Length). Constraint expressions can be used in
place of constant values: the grammar for the expression language
is defined in Appendix A.1. Constraints can include a previously
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defined field's short or full label, where one has been defined.
Short variable-length fields are indicated by "..." instead of a
pipe at the end of the row.
Payload: TL - ((IHL*32)/8) bytes. This is a multi-row variable-
length field, constrained by the values of fields TL and IHL.
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.
4.2. PDUs That Cross-Reference Previously Defined Fields
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:
SSRC: 32 bits. This is a fixed-width field, as described previously.
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:
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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:
Version (V): 2 bits. This is a fixed-width field, as described
previously.
Padding (P): 1 bit. This is a fixed-width field, as described
previously.
Extension (X): 1 bit. This is a fixed-width field, as described
previously.
CSRC count (CC): 4 bits. This is a fixed-width field, as described
previously.
Marker (M): 1 bit. This is a fixed-width field, as described
previously.
Payload Type (PT): 7 bits. This is a fixed-width field, as described
previously.
Sequence Number (PT): 16 bits. This is a fixed-width field, as
described previously.
Timestamp (PT): 32 bits. This is a fixed-width field, as described
previously.
Synchronization Source identifier: 1 Source Identifier. This is a
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field whose structure is a previously defined PDU format (Source
Identifier). To indicate this, the width of the field is
expressed in terms of cross-referenced structure. When used in
constraint expressions, PDU names refer to the length of that PDU
structure.
Contributing Source identifiers: CC Source Identifier. 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. Where the length is
unknown, the type of each element of the sequence must be given in
square brackets.
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.
Header Extension: 32 bits; present only when X == 1. 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 or "hangText"
attribute).
[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.]
Payload. The length of the Payload is not specified, and hence needs
to be inferred from the total length of the packet and the lengths
of the known fields. There can only be one field of unspecified
size in a PDU. Fields where the length is not specified may also
denote this with the phrase "variable length" in place of the
length definition.
Padding: PC bytes; present only when (P == 1) && (PC > 0). This is a
variable size field, with size dependent on a later field in the
packet. Fields can only depend on the value of a later field if
they follow a field with unspecified size.
Padding Count (PC): 1 byte; present only when P == 1. This is a
fixed-width field, as previously discussed.
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4.3. PDUs with Non-Contiguous Fields
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:
Method (M): 12 bits (split field). This field is comprised of
multiple sub-fields (M0 through MB) as shown in the diagram. That
these sub-fields should be concatenated, after parsing, into a
single field is indicated by their being labelled using the 'M'
short field name followed by a single hexadecimal digit, with the
least significant bit labelled with 0, and subsequent bits
labelled in sequence.
Class (C): 2 bits (split field). This field follows the same format
as M described above.
4.4. PDUs with Constraints on Field Values
A PDU may be defined not only by the layout and type of its fields,
but also by the value of those fields. For example, field values may
be constrained to be of a known exact value or to be within a range.
More generally, our format enables a boolean expression to be
attached to a field, which must be true for the PDU to be parsed
successfully.
This format is illustrated using the QUIC Long Header Packet format
[QUIC-TRANSPORT]. A Long Header is formatted as follows:
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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
+-+-+-+-+-+-+-+-+
|1|1| T | R | P |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (DCID) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Connection ID (SCID) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Header Form (HF): 1 bit; HF == 1. This is a fixed-width field,
constrained to be a of an known, exact value. At most one field
value constraint may be given, and if provided, it must be given
as a boolean expression, separated by a semi-colon in the field
definition name (i.e., the text contained within the <dt> tag or
"hangText" attribute). If present, a value constraint must follow
the name, short name, and length of the field, but appear before
any presence constraint, if applicable. The order of the field
must be the same in both the diagram and description list.
Fixed Bit (FB): 1 bit; FB == 1. This is a fixed-width field, with a
value constraint, as previously described.
Long Packet Type (T): 2 bits. This is a fixed-width field as
previously described.
Reserved Bits (R): 2 bits. This is a fixed-width field as previously
described.
Packet Number Length (P): 2 bits. This is a fixed-width field as
previously described.
Version: 32 bits. This is a fixed-width field as previously
described.
DCID Len (DLen): 1 byte; DLen <= 20. This is a fixed-width field,
with a value constraint, as previously described. Note that the
constraint language is not limited to equality; it is defined
fully in Appendix A.1.
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Destination Connection ID: DLen bytes. This is a variable-width
field as previously described.
SCID Len (SLen): 1 byte; SLen <= 20. This is a fixed-width field,
with a value constraint, as previously described.
Source Connection ID: SLen bytes. This is a variable-width field as
previously described.
4.5. PDUs That Extend Sub-Structures
A PDU may not only use or reference existing sub-structures, but they
may extend them, adding new fields, or enforcing different or
additional constraints.
Where a sub-structure is extended, the diagram may show the sub-
structure as a block, labelled with the sub-structure name. It may
also be desirable to show the sub-structure diagram in full; in this
case, the fields must be given in the same order and be of the same
length. New field constraints can be shown. Similarly, in the
description list, those fields inherited without change (i.e., with
no change to their constraints) do not need to be repeated. Those
with different or additional constraints must be described, and the
order of the fields in the description list must match that of the
sub-structure and the containing structure.
This format is illustrated using the QUIC Retry Packet format
[QUIC-TRANSPORT]. A Retry 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: Long Header :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retry Token ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Retry Integrity Tag +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
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Long Header (LH): 1 Long Header; LH.T == 3. This field is a
previously defined sub-structure. Its constraints can access
fields in that sub-structure. In this example, the T field of the
Long Header must be equal to 3.
Retry Token. This is a variable-length field as previously defined.
Retry Integrity Tag: 128 bits. This is a fixed-width field as
previously defined.
As shown, the Long Header packet sub-structure is included. The
Retry Packet enforces a new value constraint on the Long Packet Type
(T) field.
4.6. Storing Data for Parsing
The parsing process may require data from previously parsed
structures. This means that data needs to be stored persistently
throughout the process. This data needs to be identified.
That the value of a particular field be stored upon parsing is
indicated by the exact phrase "On receipt, the value of <field name>
is stored as <stored name>." being present at the end of the
description of a field (i.e., at the end of the <dd> or <t> element.)
An Initial 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: Long Header :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Long Header (LH): 1 Long Header; LH.T == 0. This is field is a sub-
structure, with a constraint, as previously defined. On receipt,
the value of LH.DCID is stored as Initial DCID.
In this example, the value of the DCID field of the Long Header sub-
structure is stored as Initial DCID.
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4.7. Connecting Structures with Functions
The parsing or serialisation of some binary formats cannot be fully
described without the use of functions. These functions take
arguments (values from another structure), perform some computation,
and generate a new structure.
Given the goal of fully capturing the parsing or serialisation of
binary protocols, it is necessary to include the signature of these
helper functions.
Function signatures are described in <artwork> elements. They are
constructed as the word "func", followed by a space, then the name of
the function. This is immediately followed by a set of brackets
containing a comma separated list of the function's parameters,
formatted as "<parameter name>: <parameter type>". This is followed
by "->" and the return type of the function, followed by a colon.
The body of the function is not captured, owing to the complexity of
both capturing and translating arbitrary code. As a result, it can
be described in whichever format is most suitable for the document
and its readership.
Those values that are stored persistently, as defined in Section 4.6,
are accessible by functions.
As an example, the "apply_protection" function is defined as:
func apply_protection(to: Unprotected Packet)
-> Protected Packet:
apply packet protection to payload
apply header protection to first_byte and packet_number
construct appropriate Protected Packet based on first_byte
return Protected Packet
In this example, 'Unprotected Packet' and 'Protected Packet' are
existing types.
To indicate that a PDU is created from another by way of a function,
the sentence "A/An <PDU name A> is parsed from a <PDU name B> using
the <function name> function" is used. This indicates that a PDU A
is generated by passing PDU B into the named function. The function
must take a single parameter, of the same type as PDU B, and return a
PDU B.
To indicate that a PDU can be serialised to another by way of a
function, the sentence "A/An <PDU name A> is serialised to a <PDU
name B> using the <function name> function" is used. This indicates
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that a PDU B is generated by passing PDU A into the named function.
The function must take a single parameter, of the same type as PDU A,
and return a PDU B.
4.8. Specifying Enumerated Types
In addition to the use of the sub-structures, it is desirable to be
able to define a type that may take the value of one of a set of
alternative structures.
The alternative structures that comprise an enumerated type are
identified using the exact phrase "The <enumerated type name> is one
of: <list of structure names>" where the list of structure names is a
comma separated list (with the last element, if there is more than
one element, preceded by 'or'), each optionally preceded by "a" or
"an". The structure names must be defined within the document or a
linked document.
Where an enumerated type has only two variants, an alternative phrase
can be used: "The <enumerated type name> is either a <variant 1 name>
or <variant 2 name>". The names of the variants must be defined
within the document or a linked document.
A PING Frame 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Frame Type (FT): 1 Variable Length Integer Encoding; FT.T == 1. Fram
e type, set to 1 for PING frames.
A HANDSHAKE_DONE Frame 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Frame Type (FT): 1 Variable Length Integer Encoding; FT.T == 30. Fra
me type, set to 30 for HANDSHAKE_DONE frames.
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A Frame is either a PING Frame or a HANDSHAKE_DONE Frame.
4.9. Specifying Protocol Data Units
A document will set out different structures that are not, on their
own, protocol data units. To capture the parsing or serialisation of
a protocol, it is necessary to be able to identify or construct those
packets that are valid PDUs. As a result, it is necessary for the
document to identify those structures that are PDUs.
The PDUs that comprise a protocol are identified using the exact
phrase "This document describes the <protocol name> protocol. The
<protocol name> protocol uses <list of PDU names>" where the list of
PDU names is a comma separated list (with the last element, if there
is more than one element, preceded by 'and'), each optionally
preceded by "a" or "an". The PDU names must be structure names
defined in the document or a linked document. The PDU names are
pluralised in the list. A document must contain exactly one instance
of this phrase.
This document describes the Example protocol. The Example protocol
uses Long Headers, STUN Message Types, IPv4 Headers, and RTP Data
Packets.
4.10. Importing PDU Definitions from Other Documents
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.
5. Open Issues
* Need a simple syntax for defining a list of identical objects, and
a way of referring to the size of the enclosing packet. The
format cannot currently represent RFC 6716 section 3.2.3, and
should be able to (the underlying type system can do so).
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* Need some discussion about the checks that the tooling might
perform, and the implications of those checks. For example, the
tooling checks for consistency between the diagram and the
description list of fields, ensuring that fields match by name and
width. -01 of this draft had a field that mismatched because of
case: is this something that the tooling should identify? More
broadly, what is the trade-off between the rigour that the tooling
can enforce, and the flexibility desired/needed by authors?
* Need to describe the rules governing the import of PDU definitions
from other documents.
6. IANA Considerations
This document contains no actions for IANA.
7. 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.
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.
8. Acknowledgements
The authors would like to thank Marc Petit-Huguenin for extensive
feedback on the draft, including work on formalising the constraint
syntax as given in Appendix A.1.
Wesley Eddy provided valuable feedback on the description format
through adopting it in [draft-ietf-tcpm-rfc793bis].
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.
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9. Informative References
[RFC8357] Deering, S. and R. Hinden, "Generalized UDP Source Port
for DHCP Relay", RFC 8357, March 2018,
<https://www.rfc-editor.org/info/rfc8357>.
[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-27, 21 February 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-27.txt>.
[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>.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language",
RFC 7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[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.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013,
<https://www.rfc-editor.org/info/rfc7049>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003,
<https://www.rfc-editor.org/info/rfc3550>.
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[draft-ietf-tram-stunbis-21]
Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", Work in Progress, Internet-
Draft, draft-ietf-tram-stunbis-21, 21 March 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-tram-
stunbis-21.txt>.
[RFC791] Postel, J., "Internet Protocol", RFC 791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC793] Postel, J., "Transmission Control Protocol", RFC 793,
September 1981, <https://www.rfc-editor.org/info/rfc793>.
[LANGSEC] LANGSEC, "LANGSEC: Language-theoretic Security",
<http://langsec.org>.
[SASSAMAN] Sassaman, L., Patterson, M. L., Bratus, S., and A.
Shubina, "The Halting Problems of Network Stack
Insecurity", ;login: -- December 2011, Volume 36, Number
6, <https://www.usenix.org/publications/login/december-
2011-volume-36-number-6/halting-problems-network-stack-
insecurity>.
[draft-mcquistin-augmented-udp-example]
McQuistin, S., Band, V., Jacob, D., and C. S. Perkins,
"Describing UDP with Augmented Packet Header Diagrams",
Work in Progress, Internet-Draft, draft-mcquistin-
augmented-udp-example-01, 5 May 2021,
<http://www.ietf.org/internet-drafts/draft-mcquistin-
augmented-udp-01.txt>.
[draft-mcquistin-augmented-tcp-example]
McQuistin, S., Band, V., Jacob, D., and C. S. Perkins,
"Describing TCP with Augmented Packet Header Diagrams",
Work in Progress, Internet-Draft, draft-mcquistin-
augmented-tcp-example-01, 5 May 2021,
<http://www.ietf.org/internet-drafts/draft-mcquistin-
augmented-tcp-example-01.txt>.
[draft-mcquistin-quic-augmented-diagrams]
McQuistin, S., Band, V., Jacob, D., and C. S. Perkins,
"Describing QUIC's Protocol Data Units with Augmented
Packet Header Diagrams", Work in Progress, Internet-Draft,
draft-mcquistin-quic-augmented-diagrams-04, 5 May 2021,
<http://www.ietf.org/internet-drafts/draft-mcquistin-quic-
augmented-diagrams-04.txt>.
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[draft-ietf-tcpm-rfc793bis]
Eddy, W., "Transmission Control Protocol (TCP)
Specification", Work in Progress, Internet-Draft, draft-
ietf-tcpm-rfc793bis-21, 3 May 2021, <http://www.ietf.org/
internet-drafts/draft-ietf-tcpm-rfc793bis-21.txt>.
Appendix A. ABNF specification
A.1. Constraint Expressions
constant = %x31-39 *(%x30-39) ; natural numbers without leading 0s
short-name = ALPHA *(ALPHA / DIGIT / "-" / "_")
name = short-name *(" " short-name)
sp = [" "] ; optional space in expression
bool-expr = "(" sp bool-expr sp ")" /
"!" sp bool-expr /
bool-expr sp bool-op sp bool-expr /
bool-expr sp "?" sp expr sp ":" sp expr /
expr sp cmp-op sp expr
bool-op = "&&" / "||"
cmp-op = "==" / "!=" / "<" / "<=" / ">" / ">="
expr = "(" sp expr sp ")" /
expr sp op sp expr /
bool-expr "?" expr ":" expr /
name / short-name "." short-name /
constant
op = "+" / "-" / "*" / "/" / "%" / "^"
length = expr sp unit / "[" sp name sp "]" / "variable length"
unit = %s"bit" / %s"bits" / %s"byte" / %s"bytes" / name
A.2. Augmented packet diagrams
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.
Appendix B. Tooling & source code
The source for this draft is available from https://github.com/
glasgow-ipl/draft-mcquistin-augmented-ascii-diagrams.
The source code for tooling that can be used to parse this document
is available from https://github.com/glasgow-ipl/ips-protodesc-code.
This tooling supports the automatic generation of Rust parser code
from protocol descriptions written in the Augmented Packet Header
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Diagram format. It also provides test harnesses that demonstrate
that example descriptions of UDP
[draft-mcquistin-augmented-udp-example] and TCP
[draft-mcquistin-augmented-udp-example] function as expected.
Authors' Addresses
Stephen McQuistin
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: sm@smcquistin.uk
Vivian Band
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: vivianband0@gmail.com
Dejice Jacob
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: d.jacob.1@research.gla.ac.uk
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: csp@csperkins.org
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