Binary Application Record Encoding (BARE)
draft-devault-bare-00
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| Author | Drew DeVault | ||
| Last updated | 2020-10-19 | ||
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draft-devault-bare-00
Internet Engineering Task Force D. DeVault
Internet-Draft SourceHut
Intended status: Informational 19 October 2020
Expires: 22 April 2021
Binary Application Record Encoding (BARE)
draft-devault-bare-00
Abstract
The Binary Application Record Encoding (BARE) is a data format used
to represent application records for storage or transmission between
programs. BARE messages are concise and have a well-defined schema,
and implementations may be simple and broadly compatible. A schema
language is also provided to express message schemas out-of-band.
Comments
Comments are solicited and should be addressed to the mailing list at
~sircmpwn/public-inbox@lists.sr.ht and/or the author(s).
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 April 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
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
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of the BARE Message Encoding . . . . . . . . . 3
2.1. Primitive Types . . . . . . . . . . . . . . . . . . . . . 3
2.2. Aggregate Types . . . . . . . . . . . . . . . . . . . . . 5
2.3. User-Defined Types . . . . . . . . . . . . . . . . . . . 6
2.4. Invariants . . . . . . . . . . . . . . . . . . . . . . . 6
3. BARE Schema Language Specification . . . . . . . . . . . . . 7
3.1. Lexical Analysis . . . . . . . . . . . . . . . . . . . . 7
3.2. ABNF Grammar . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Semantic Elements . . . . . . . . . . . . . . . . . . . . 8
4. Application Considerations . . . . . . . . . . . . . . . . . 9
5. Future Considerations . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Example message schema . . . . . . . . . . . . . . . 11
Appendix B. Example Messages . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The purpose of the BARE message encoding, like hundreds of others, is
to encode application messages. The goals of such encodings vary
(leading to their proliferation); BARE's goals are the following:
* Concise messages
* A well-defined message schema
* Broad compatibility with programming environments
* Simplicity of implementation
This document specifies the BARE message encoding, as well as a
schema language which may be used to describe the layout of a BARE
message. The schema of a message must be agreed upon in advance by
each party exchanging a BARE message; message structure is not
encoded into the representation. The schema language is useful for
this purpose, but not required.
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1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Specification of the BARE Message Encoding
A BARE message is a single value of a pre-defined type, which may be
of an aggregate type enclosing multiple values. Unless otherwise
specified there is no additional container or structure around the
value; it is encoded plainly.
A BARE message does not necessarily have a fixed length, but the
schema author may make a deliberate choice to constrain themselves to
types of well-defined lengths if this is desired.
The names for each type are provided to establish a vocabulary for
describing a BARE message schema out-of-band, by parties who plan to
exchange BARE messages. The type names used here are provided for
this informative purpose, but are more rigourously specified by the
schema language specification in Section 3.
2.1. Primitive Types
Primitive types represent exactly one value.
uint
An unsigned integer with a variable-length encoding. Each
octet of the encoded value has the most-significant bit set,
except for the last octet. The remaining bits are the
integer value in 7-bit groups, least-significant first.
The maximum precision of such a number is 64-bits. The
maximum length of an encoded uint is therefore 10 octets.
int
A signed integer with a variable-length encoding. Signed
integers are represented as uint using a "zig-zag" encoding:
positive values x are written as 2x + 0, negative values are
written as 2(^x) + 1. In other words, negative numbers are
complemented and whether to complement is encoded in bit 0.
The maximum precision of such a number is 64-bits. The
maximum length of an encoded int is therefore 10 octets.
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u8, u16, u32, u64
Unsigned integers of a fixed precision, respectively 8, 16,
32, and 64 bits. They are encoded in little-endian (least
significant octet first).
i8, i16, i32, i64
Signed integers of a fixed precision, respectively 8, 16, 32,
and 64 bits. They are encoded in little-endian (least
significant octet first), with two's compliment notation.
f32, f64
Floating-point numbers represented with the IEEE 754
[IEEE.754.1985] binary32 and binary64 floating point number
formats.
bool
A boolean value, either true or false, encoded as a u8 type
with a value of one or zero, respectively representing true
or false.
If a value other than one or zero is found in the u8
representation of the bool, the message is considered
invalid, and the decoder SHOULD raise an error if it
encounters such a value.
enum
An unsigned integer value from a set of possible values
agreed upon in advance, encoded with the uint type.
An enum whose uint value is not a member of the values agreed
upon in advance is considered invalid, and the decoder SHOULD
raise an error if it encounters such a value.
Note that this makes the enum type unsuitable for
representing a several enum values which have been combined
with a bitwise OR operation.
string
A string of text. The length of the text in octets is
encoded first as a uint, followed by the text data
represented with the UTF-8 encoding [RFC3629].
If the data is found to contain invalid UTF-8 sequences, it
is considered invalid, and the decoder SHOULD raise an error
if it encounters such a value.
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data<length>
Arbitrary data with a fixed "length" in octets, e.g.
data<16>. The data is encoded literally in the message, and
MUST NOT be greater than 18,446,744,073,709,551,615 octets in
length (the maximum value of a u64).
data
Arbitrary data of a variable length in octets. The length is
encoded first as a uint, followed by the data itself encoded
literally.
void
A type with zero length. It is not encoded into BARE
messages.
2.2. Aggregate Types
Aggregate types may store zero or more primitive or aggregate values.
optional<type>
A value of "type" which may or may not be present, e.g.
optional<u32>. Represented as either a u8 with a value of
zero, indicating that the optional value is unset; or a u8
with a value of one, followed by the encoded data of the
optional type.
An optional value whose initial u8 is set to a number other
than zero or one is considered invalid, and the decoder
SHOULD raise an error if it encounters such a value.
[length]type
A list of "length" values of "type", e.g. [10]uint. The
length is not encoded into the message. The encoded values
of each member of the list are concatenated to form the
encoded list.
[]type
A variable-length list of values of "type", e.g. []string.
The length of the list (in values) is encoded as a uint,
followed by the encoded values of each member of the list
concatenated.
map[type A]type B
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An associative list of values of type B keyed by values of
type A, e.g. map[u32]string. The encoded representation of a
map begins with the number of key/value pairs as a uint,
followed by the encoded key/value pairs concatenated. Each
key/value pair is encoded as the encoded key concatenated
with the encoded value.
A message with repeated keys is considered invalid, and the
decoder SHOULD raise an error if it encounters such a value.
(type | type | ...)
A tagged union whose value may be one of any type from a set
of types, e.g. (int | uint | string). Each type in the set
is assigned a numeric identifier. The value is encoded as
the selected type's identifier represented with the uint
encoding, followed by the encoded value of that type.
A union with a tag value that does not have a corresponding
type assigned is considered invalid, and the decoder SHOULD
raise an error if it encounters such a value.
struct
A set of values of arbitrary types, concatenated in an order
agreed upon in advance. Each value is referred to as a
"field", and field has a name and type.
2.3. User-Defined Types
A user-defined type gives a name to another type. This creates a
distinct type whose representation is equivalent to the named type.
An arbitrary number of user-defined types may be used for the same
underlying type; each is distinct from the other.
2.4. Invariants
The following invariants are specified:
* Any type which is ultimately a void type (either directly or via a
user-defined type) MUST NOT be used as an optional type, struct
member, list member, map key, or map value. Void types may only
be used as members of the set of types in a tagged union.
* The lengths of fixed-length arrays and data types MUST be at least
one.
* Structs MUST have at least one field.
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* Unions MUST have at least one type, and each type MUST NOT be
repeated.
* Map keys MUST be of a primitive type which is not data or
data<length>.
* Each named value of an enum type MUST have a unique value.
3. BARE Schema Language Specification
The use of the schema language is optional. Implementations SHOULD
support decoding arbitrary BARE messages without a schema document,
by defining the schema in a manner which utilizes more native tools
available from the programming environment.
However, it may be useful to have a schema document for use with code
generation, documentation, or interoperability. A domain-specific
language is provided for this purpose.
3.1. Lexical Analysis
During lexical analysis, "#" is used for comments; if encountered,
the "#" character and any subsequent characters are discarded until a
line feed (%x0A) is found.
3.2. ABNF Grammar
The syntax of the schema language is provided here in Augmented
Backus-Naur form [RFC5234]. However, this grammar differs from
[RFC5234] in that strings are case-sensitive (e.g. "type" does not
match TypE).
schema = [WS] user-types [WS]
user-type = "type" WS user-type-name WS non-enum-type
user-type =/ "enum" WS user-type-name WS enum-type
user-types = user-type / (user-types WS user-type)
type = non-enum-type / enum-type
non-enum-type = primitive-type / aggregate-type / user-type-name
user-type-name = UPPER *(ALPHA / DIGIT) ; First letter is uppercase
primitive-type = "int" / "i8" / "i16" / "i32" / "i64"
primitive-type =/ "uint" / "u8" / "u16" / "u32" / "u64"
primitive-type =/ "f32" / "f64"
primitive-type =/ "bool"
primitive-type =/ "string"
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primitive-type =/ "data" / ("data<" integer ">")
primitive-type =/ "void"
enum-type = "{" [WS] enum-values [WS] "}"
enum-values = enum-value / (enum-values WS enum-value)
enum-value = enum-value-name
enum-value =/ (enum-value-name [WS] "=" [WS] integer)
enum-value-name = UPPER *(UPPER / DIGIT / "_")
aggregate-type = optional-type
aggregate-type =/ array-type
aggregate-type =/ map-type
aggregate-type =/ union-type
aggregate-type =/ struct-type
optional-type = "optional<" type ">"
array-type = "[" [integer] "]" type
integer = 1*DIGIT
map-type = "map[" type "]" type
union-type = "(" union-members ")"
union-members = union-member
union-members =/ (union-members [WS] "|" [WS] union-member)
union-member = type [[WS] "=" [WS] integer]
struct-type = "{" [WS] fields [WS] "}"
fields = field / (fields WS field)
field = 1*ALPHA [WS] ":" [WS] type
UPPER = %x41-5A ; uppercase ASCII letters
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
WS = 1*(%x0A / %x09 / " ") ; whitespace
See Appendix A for an example schema written in this language.
3.3. Semantic Elements
The names of fields and user-defined types are informational: they
are not represented in BARE messages. They may be used by code
generation tools to inform the generation of field and type names in
the native programming environment.
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Enum values are also informational. Values without an integer token
are assigned automatically in the order that they appear, starting
from zero and incrementing for each subsequent unassigned value. If
a value is explicitly specified, automatic assignment continues from
that value plus one for subsequent enum values.
Union type members are assigned a tag in the order that they appear,
starting from zero and incrementing for each subsequent type. If a
tag value is explicitly specified, automatic assignment continues
from that value plus one for subsequent values.
4. Application Considerations
Message authors who wish to design a schema which is backwards- and
forwards-compatible with future messages are encouraged to use union
types for this purpose. New types may be appended to the members of
a union type while retaining backwards compatibility with older
message types. The choice to do this must be made from the first
message version-- moving a struct into a union _does not_ produce a
backwards-compatible message.
The following schema provides an example:
type Message (MessageV1 | MessageV2 | MessageV3)
type MessageV1 ...
type MessageV2 ...
type MessageV3 ...
An updated schema which adds a MessageV4 type would still be able to
decode versions 1, 2, and 3.
If a message version is later deprecated, it may be removed in a
manner compatible with future versions 2 and 3 if the initial tag is
specified explicitly.
type Message (MessageV2 = 1 | MessageV3)
5. Future Considerations
To ensure message compatibility between implementations and
backwards- and forwards-compatibility of messages, constraints on
vendor extensions are required. This specification is final, and new
types or extensions will not be added in the future. Implementors
MUST NOT define extensions to this specification.
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To support the encoding of novel data structures, the implementor
SHOULD make use of user-defined types in combination with the data or
data<length> types.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
Message parsers are common vectors for security vulnerabilities.
BARE addresses this by making the message format as simple as
possible. However, the parser MUST be prepared to handle a number of
error cases when decoding untrusted messages, such as a union type
with an invalid tag, or an enum with an invalid value. Such errors
may also arise by mistake, for example when attempting to decode a
message with the wrong schema.
Support for data types of an arbitrary, message-defined length
(lists, maps, strings, etc) is commonly exploited to cause the
implementation to exhaust its resources while decoding a message.
However, legitimate use-cases for extremely large data types
(possibly larger than the system has the resources to store all at
once) do exist. The decoder MUST manage its resources accordingly,
and SHOULD provide the application a means of providing their own
decoder implementation for values which are expected to be large.
There is only one valid interpretation of a BARE message for a given
schema, and different decoders and encoders should be expected to
provide that interpretation. If an implementation has limitations
imposed from the programming environment (such as limits on numeric
precision), the implementor MUST document these limitations, and
prevent conflicting interpretations from causing undesired behavior.
8. Normative References
[IEEE.754.1985]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754, August 1985.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
Appendix A. Example message schema
The following is an example of a schema written in the BARE schema
language.
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type PublicKey data<128>
type Time string # ISO 8601
enum Department {
ACCOUNTING
ADMINISTRATION
CUSTOMER_SERVICE
DEVELOPMENT
# Reserved for the CEO
JSMITH = 99
}
type Customer {
name: string
email: string
address: Address
orders: []{
orderId: i64
quantity: i32
}
metadata: map[string]data
}
type Employee {
name: string
email: string
address: Address
department: Department
hireDate: Time
publicKey: optional<PublicKey>
metadata: map[string]data
}
type TerminatedEmployee void
type Person (Customer | Employee | TerminatedEmployee)
type Address {
address: [4]string
city: string
state: string
country: string
}
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Appendix B. Example Messages
Some basic example messages in hexadecimal are provided for the
schema specified in Appendix A.
A "Person" value of type "Customer" with the following values:
name James Smith
email jsmith@example.org
address 123 Main Street; Philadelphia; PA; United States
orders (1) orderId: 4242424242; quantity: 5
metadata (unset)
Encoded BARE message:
00 0b 4a 61 6d 65 73 20 53 6d 69 74 68 12 6a 73
6d 69 74 68 40 65 78 61 6d 70 6c 65 2e 6f 72 67
0b 31 32 33 20 4d 61 69 6e 20 53 74 00 00 00 0c
50 68 69 6c 61 64 65 6c 70 68 69 61 02 50 41 0d
55 6e 69 74 65 64 20 53 74 61 74 65 73 01 b2 41
de fc 00 00 00 00 05 00 00 00 00
A "Person" value of type "Employee" with the following values:
name Tiffany Doe
email tiffanyd@acme.corp
address 123 Main Street; Philadelphia; PA; United States
department ADMINISTRATION
hireDate 2020-06-21T21:18:05Z
publicKey (unset)
metadata (unset)
Encoded BARE message:
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01 0b 54 69 66 66 61 6e 79 20 44 6f 65 12 74 69
66 66 61 6e 79 64 40 61 63 6d 65 2e 63 6f 72 70
0b 31 32 33 20 4d 61 69 6e 20 53 74 00 00 00 0c
50 68 69 6c 61 64 65 6c 70 68 69 61 02 50 41 0d
55 6e 69 74 65 64 20 53 74 61 74 65 73 01 19 32
30 32 30 2d 30 36 2d 32 31 54 32 31 3a 31 38 3a
30 35 2b 30 30 3a 30 30 00 00
A "Person" value of type "TerminatedEmployee":
Encoded BARE message:
02
Author's Address
Drew DeVault
SourceHut
454 E. Girard Ave #2R
Philadelphia, PA 19125
United States of America
Phone: +1 719 213 5473
Email: sir@cmpwn.com
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