CTF2‑SPEC‑2.0: Common Trace Format version 2

This document is the official specification of the Common Trace Format (CTF) version 2 (CTF2‑SPEC‑2.0), as published by the DiaMon Workgroup.

RFC 2119

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, MAY, and OPTIONAL in this document, when emphasized, are to be interpreted as described in RFC 2119.

Document identification

The name of this document (CTF2‑SPEC‑2.0) follows the specification of CTF2‑DOCID‑2.0.

1. Revision history

This document is the initial version of CTF2‑SPEC‑2.0.

The latest CTF 1 version is, as of 26 March 2024, CTF 1.8.3.

2. What’s CTF 2?

The Common Trace Format version 2 is a binary trace format designed to be very fast to write without compromising great flexibility.

The intention of CTF 2 is that applications written in any programming language, and running on any system (be it Linux or bare metal, for example), can generate traces natively.

A CTF 2 trace has all its data streams described by a metadata stream. Given the rich set of supported data field types, this makes it possible for a CTF 2 producer to append binary data structures as is to data streams without further data transformation. Indeed, the length, alignment, byte order, and bit order of fixed-length bit array fields are all configurable parameters within the metadata stream.

CTF 2 is transport-agnostic: this document doesn’t specify how to transport or store CTF 2 streams. Other documents can specify such conventions, and conform CTF 2 producers and consumers may or may not adhere to them.

CTF 2 is a major revision of CTF 1, bringing many improvements, such as:

Using JSON text sequences for the metadata stream.
Simplifying the metadata stream.

For example, field class aliases may only exist at the root level.
Adding new field classes and improving existing ones.
Supporting the UTF-16 and UTF-32 encodings of string fields.
Using roles instead of reserved structure member names to identify meaningful fields.
Adding the attribute and extension features to extend and customize the format.

and more, while remaining backward compatible at the data stream level.

3. Common definitions

Common CTF 2 definitions:

Byte

A group of eight bits operated on as a unit.

The bits are indexed such that, if the byte represents an 8-bit unsigned integer, bit 0 is the least significant and bit 7 is the most significant.

In the diagrams of this document, when a byte is represented as eight contiguous squares, the rightmost one is always bit 0:

Class

A set of values (instances) which share common properties.

For example, a fixed-length unsigned integer field class with an 8-bit length property is the set of the all the fixed-length unsigned integer fields from binary 00000000 to 11111111 (integers 0 to 255).

This specification often states that some class describes instances. For example, an event record class describes event records.

Consumer

A software or hardware system which consumes (reads) the streams of a trace.

A trace consumer is often a trace viewer or a trace analyzer.

Namespace

A string of which the purpose is to avoid naming conflicts.

This document doesn’t specify the format of a namespace. CTF 2 recommends to at least include a domain name owned by the organization defining the objects under a namespace as well as its registration year, for example example.com,1995.

The diamon.org,2012 namespace and its std alias are reserved for official DiaMon Workgroup CTF 2 documents.

Producer

A software or hardware system which produces (writes) the streams of a trace.

A trace producer is often a tracer.

Sequence

A set of related items which follow each other in a particular order.

Stream

A sequence of bytes.

4. Trace composition

A trace is:

One metadata stream.
One or more data streams.

As a reminder, this specification defines a stream as a sequence of bytes.

This document doesn’t specify how to transport or store CTF 2 streams. A producer could serialize all streams as a single file on the file system, or it could send the streams over the network using TCP, to name a few examples.

4.1. Metadata stream (overview)

A metadata stream describes trace data streams with JSON objects.

A metadata stream describes things such as:

The class of the data stream default clocks.
The names of event record classes.
The classes of event record fields.

Multiple traces MAY share the same metadata stream.

See Metadata stream for the full metadata stream specification.

4.2. Data stream

A data stream is a sequence of one or more data packets:

In the metadata stream, a data stream class describes data streams.

A packet MUST contain one or more bytes of data.

Although a packet MAY contain padding (garbage data) at the end itself, from the point of view of a data stream, there’s no padding between packets. In other words, the byte following the last byte of a packet is the first byte of the next packet.

A data stream MAY have, conceptually:

One default, monotonic clock: Described by a clock class in the metadata stream.

Packets and event records MAY contain snapshots, named timestamps, of the default clock of their data stream.
One counter of discarded event records: Indicates the number of event records which the producer needed to discard for different reasons.

For example, a tracer could discard an event record when it doesn’t fit some buffer and there’s no other available buffer.

A packet MAY contain a snapshot of this counter.

See Data stream decoding procedure to learn how to decode a CTF 2 data stream.

4.2.1. Packet

A packet is a segment of a data stream.

A packet contains a sequence of data fields or padding (garbage data). In the metadata stream, field classes describe data fields.

A packet P, contained in a data stream S, contains, in this order:

OPTIONAL: A header structure field, described at the trace class level in the metadata stream, which contains, in this order:
1. OPTIONAL: A fixed-length unsigned integer field which encodes the packet magic number value (0xc1fc1fc1, or 3,254,525,889).
2. In any order:
  - OPTIONAL: One or more static-length BLOB fields which encode the metadata stream UUID.
  - OPTIONAL: One or more unsigned integer fields which encode the current numeric ID of the class of S.
  - OPTIONAL: One or more unsigned integer fields which encode the current numeric ID of S.
OPTIONAL: A context structure field, described at the data stream class level in the metadata stream, which contains, in any order:
- OPTIONAL: One or more unsigned integer fields which encode the current total length of P, in bits (always a multiple of 8).
- OPTIONAL: One or more unsigned integer fields which encode the current content length of P, in bits.
- OPTIONAL: One or more unsigned integer fields which encode the current beginning timestamp or partial timestamp of P.
- OPTIONAL: One or more unsigned integer fields which encode the current end timestamp of P.
- OPTIONAL: One or more unsigned integer fields which encode the current snapshot of the discarded event record counter of S at the end of P.
- OPTIONAL: One or more unsigned integer fields which encode the current sequence number of P within S.
- OPTIONAL: User fields.
Zero or more event records.

A packet MUST contain one or more bytes of data.

A packet MAY contain padding (garbage data) after its last event record. The length of this padding is the difference between its total length and its content length (as found in its context structure field).

Packets are independent of each other: if one removes a packet from a data stream, a consumer can still decode the whole data stream. This is why:

Packets MAY contain snapshots of the discarded event record counter of their data stream.
Packets and event records MAY contain timestamps which are snapshots of the default clock of their data stream.

If the packet context fields of the packets of a data stream contain a packet sequence number field, a consumer can recognize missing packets.

See Packet decoding procedure to learn how to decode a CTF 2 packet.

4.2.2. Event record

An event record is the result of a producer writing a record with OPTIONAL user data when an event occurs during its execution.

A packet contains zero or more event records.

An event record class describes the specific parts of event records.

An event record E, contained in a data stream S, contains, in this order:

OPTIONAL: A header structure field, described at the data stream class level in the metadata stream, which contains, in any order:
- OPTIONAL: One or more unsigned integer fields which encode the numeric ID of the class of E which has the class of S as its parent.
- OPTIONAL: One or more unsigned integer fields which encode a timestamp or a partial timestamp.
OPTIONAL: A common context structure field, described at the data stream class level in the metadata stream, which contains user fields.
OPTIONAL: A specific context structure field, described at the event record class level in the metadata stream, which contains user fields.
OPTIONAL: A payload structure field, described at the event record class level in the metadata stream, which contains user fields.

An event record MUST contain one or more bits of data.

The default clock timestamp of an event record, that is, the value of the default clock of its data stream after its header field F, if F exists, is encoded/decoded MUST be greater than or equal to the default clock timestamp of the previous event record E, if E exists, within the same data stream.

See Event record decoding procedure to learn how to decode a CTF 2 event record.

5. Metadata stream

A metadata stream is a JSON text sequence, as specified by RFC 7464, of fragments:

In prose: any number of JSON texts, each encoded in UTF-8 [RFC3629], each preceded by one ASCII RS character, and each followed by a line feed (LF).

Since RS is an ASCII control character, it may only appear in JSON strings in escaped form (see [RFC7159]), and since RS may not appear in JSON texts in any other form, RS unambiguously delimits the start of any element in the sequence.

RS is sufficient to unambiguously delimit all top-level JSON value types other than numbers.

The Unicode codepoint of the ASCII record separator (RS) character is U+001E.

Together, the fragments of a metadata stream contain all the information about the data streams of one or more traces.

A fragment is a JSON object; its allowed properties depend on its type property.

Table 1. Common properties of a fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be one of: `"preamble"` F is a preamble fragment. `"field-class-alias"` F is a field class alias fragment. `"trace-class"` F is a trace class fragment. `"clock-class"` F is a clock class fragment. `"data-stream-class"` F is a data stream class fragment. `"event-record-class"` F is a event record class fragment.	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. For any fragment except a preamble fragment, any extension which exists under this property MUST also be declared in the preamble fragment of the same metadata stream.	No	`{}`

The metadata stream is a JSON text sequence of fragments instead of a single JSON object containing nested objects to enable real-time, or “live”, tracing: during tracing, a consumer can always decode event records having known event record classes while a producer can always add new event record classes to a data stream class by appending additional fragments to the metadata stream. Once a producer appends a fragment to a metadata stream, the fragment is considered “frozen”, in that it never needs to change.

A metadata stream:

MUST start with a preamble fragment.
MUST contain exactly one preamble fragment.
MAY contain one or more field class alias fragments.
MAY contain one trace class fragment.
MUST contain one or more data stream class fragments which, if any trace class fragment T exists, MUST follow T.
MAY contain one or more event record class fragments which, if their parent data stream class D exists, MUST follow D.

Example 1. Partial metadata stream.

In the sample below, the string <RS> represents a single “record separator (RS)” (U+001E) codepoint and the string [...] represents continuation.

<RS>{
  "type": "preamble",
  "version": 2
}
<RS>[...]

This section doesn’t specify how a metadata stream translates into data stream encoding and decoding rules; it only describes objects and their properties.

See Data stream decoding procedure to learn how to decode a data stream.

5.1. Extensions

A producer MAY add extensions to many metadata stream JSON objects.

The purpose of an extension is to add core features to CTF 2 or to modify existing core features, as specified by this document. In other words, an extension MAY alter the format itself.

This document doesn’t specify what an extension exactly is.

The preamble fragment of the metadata stream contains extension declarations:

Any extension in metadata stream objects MUST be declared, by namespace and name, in the preamble fragment.

Declaring an extension is said to enable it.
If a consumer doesn’t support any declared extension, it MUST NOT consume the data streams of the trace.

The consumer SHOULD report unsupported extensions as an error.

Extensions are a single JSON object, where each property is:

Name	A namespace
Value	A namespaced extensions object

A namespaced extensions object is a JSON object, where each property is:

Name	An extension name
Value	A JSON value

The metadata stream JSON objects which MAY contain extensions as their extensions property are:

Any fragment.

An extension in the preamble fragment also makes it declared/enabled.
Any field class.
A structure field member class.
A variant field class option.

Example 2. Three extensions under two namespaces.

{
  "my.tracer": {
    "piano": {
      "keys": 88,
      "temperament": "equal"
    },
    "ramen": 23
  },
  "abc/xyz": {
    "sax": {
      "variant": "alto"
    }
  }
}

5.2. Attributes

A producer MAY add custom attributes to many metadata stream JSON objects.

This document doesn’t specify what an attribute exactly is.

Unlike extensions, a consumer MUST NOT consider attributes to successfully decode data streams.

Attributes are a single JSON object, where each property is:

Name	A namespace
Value	A JSON value

The metadata stream JSON objects which MAY contain attributes as their attributes property are:

Any fragment.
Any field class.
A structure field member class.
A variant field class option.

Example 3. Attributes under two namespaces.

{
  "my.tracer": {
    "max-count": 45,
    "module": "sys"
  },
  "abc/xyz": true
}

5.3. Field classes

A field class describes fields, that is, sequences of bits as found in a data stream.

A field class contains all the properties a consumer needs to decode a given field.

A field is a field class instance.

This document specifies the following types of field classes:

Abstract field classes

One cannot use the following field classes directly: they are bases for other, concrete field classes:

Abstract integer field class
Abstract string field class
Abstract array field class
Abstract BLOB field class

Fixed/static-length scalar field classes

Fixed-length bit array field class
Fixed-length bit map field class
Fixed-length boolean field class
Fixed-length integer field class
Fixed-length floating point number field class
Static-length string field class
Static-length BLOB field class

Variable/dynamic-length scalar field classes

Variable-length integer field class
Null-terminated string field class
Dynamic-length string field class
Dynamic-length BLOB field class

Compound field classes

The following field classes MAY contain other field classes:

Structure field class
Static-length array field class
Dynamic-length array field class
Optional field class
Variant field class

The following diagram shows the relations between field classes; a field class box A containing another field class box B means B is an A:

A field class is a JSON object; its properties depend on its type property.

Table 2. Common properties of a field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be one of: `"fixed-length-bit-array"` F is a fixed-length bit array field class. `"fixed-length-bit-map"` F is a fixed-length bit map field class. `"fixed-length-boolean"` F is a fixed-length boolean field class. `"fixed-length-unsigned-integer"` `"fixed-length-signed-integer"` F is a fixed-length integer field class. `"fixed-length-floating-point-number"` F is a fixed-length floating point number field class. `"variable-length-unsigned-integer"` `"variable-length-signed-integer"` F is a variable-length integer field class. `"null-terminated-string"` F is a null-terminated string field class. `"static-length-string"` F is a static-length string field class. `"static-length-blob"` F is a static-length BLOB field class. `"dynamic-length-string"` F is a dynamic-length string field class. `"dynamic-length-blob"` F is a dynamic-length BLOB field class. `"structure"` F is a structure field class. `"static-length-array"` F is a static-length array field class. `"dynamic-length-array"` F is a dynamic-length array field class. `"optional"` F is a optional field class. `"variant"` F is a variant field class.	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

The following fragment properties MUST have a structure field class (or the name of a structure field class alias) as their value:

Trace class fragment

packet-header-field-class

Data stream class fragment

packet-context-field-class
event-record-header-field-class
event-record-common-context-field-class

Event record class fragment

specific-context-field-class
payload-field-class

5.3.1. Field location

A field location is a means for a consumer to locate a field which it needs to decode another, subsequent field.

A consumer needs to locate another field to decode instances of the following classes:

Dynamic-length array field class
Dynamic-length string field class
Dynamic-length BLOB field class: Needs a fixed-length unsigned integer or variable-length unsigned integer length field.
Optional field class: Needs a fixed-length boolean, fixed-length integer, or variable-length integer selector field.
Variant field class: Needs a fixed-length integer or variable-length integer selector field.

Let T be an anteriorly decoded field which a consumer needs to decode another field S.

A field location is a JSON object.

Table 3. Properties of a field location F.
Name	Type	Description	Required?	Default
`origin`	JSON string	Known origin of F, from where to start the field location procedure amongst: `"packet-header"` Header of the packet of S. `"packet-context"` Context of the packet of S. `"event-record-header"` Header of the event record of S. `"event-record-common-context"` Common context of the event record of S. `"event-record-specific-context"` Specific context of the event record of S. `"event-record-payload"` Payload of the event record of S. In other words, T MUST be in the same packet or event record as S.	No	Start the field location procedure from the structure field containing S.
`path`	JSON array, each element being one of: A JSON string `null`	Each element is one of, from the origin of F: A JSON string The name of a structure field member to follow to locate T. `null` The parent structure field of the current (in the field location procedure context) structure field. The value of this property MUST NOT be empty. The last element of this property MUST NOT be `null`.	Yes

Example 4. Field location with a known origin.

{
  "origin": "event-record-header",
  "path": ["id"]
}

Example 5. Field location without a known origin.

{
  "path": ["streak", "free"]
}

Example 6. Field location without a known origin, using a parent structure field.

{
  "path": [null, null, "mini", "parking", "stop"]
}

5.3.2. Integer range set

An integer range set is a JSON array of integer ranges.

An integer range set MUST contain one or more integer ranges.

An integer range is a JSON array of two elements:

The lower bound of the range (JSON integer, included).
The upper bound of the range (JSON integer, included).

An integer range represents all the integer values from the lower bound of the range to its upper bound.

The upper bound of an integer range MUST be greater than or equal to its lower bound.

If both the lower and upper bounds of an integer range are equal, then the integer range represents a single integer value.

Example 7. Integer ranges.

[3, 67]

[-45, 101]

Single integer value.

[42, 42]

Example 8. Integer range set containing three integer ranges.

[[3, 67], [-45, 1], [42, 42]]

5.3.3. Roles

Some fixed-length unsigned integer, variable-length unsigned integer, and static-length BLOB field class instances can have roles.

A role is specific semantics attached to the fields (instances) of a field class. For example, the packet-magic-number role of a fixed-length unsigned integer field class indicates that the value of its instances MUST be the packet magic number (0xc1fc1fc1).

Roles are a JSON array of role names (JSON strings).

See Trace class fragment and Data stream class fragment which indicate accepted roles within their root field classes.

5.3.4. Fixed-length bit array field class

A fixed-length bit array field class describes fixed-length bit array fields and is a base of a fixed-length bit map field class, a fixed-length boolean field class, a fixed-length integer field class, and a fixed-length floating point number field class.

A fixed-length bit array field is a simple array of contiguous bits, without any attached integer type semantics.

The length, or number of bits, of a fixed-length bit array field is a property (length) of its class.

Table 4. Common properties of a fixed-length bit array field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"fixed-length-bit-array"`.	Yes
`length`	JSON integer	Number of bits of an instance of F. The value of this property MUST be greater than zero.	Yes
`byte-order`	JSON string	Byte order of an instance of F. The value of this property MUST be one of: `"big-endian"` Big-endian. Within a data stream byte, the bits of an instance of F are decoded from the most significant to the least significant bits of the byte. `"little-endian"` Little-endian. Within a data stream byte, the bits of an instance of F are decoded from the least significant to the most significant bits of the byte.	Yes
`bit-order`	JSON string	Bit order of an instance of F. The value of this property MUST be one of: `"first-to-last"` The first bit to decode of an instance of F is the first element of the resulting array value. `"last-to-first"` The first bit to decode of an instance of F is the last element of the resulting array value.	No	`"first-to-last"` if the value of the `byte-order` property of F is `"little-endian"`, or `"last-to-first"` otherwise.
`alignment`	JSON integer	Alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two.	No	`1`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 9. Minimal fixed-length bit array field class.

{
  "type": "fixed-length-bit-array",
  "length": 16,
  "byte-order": "little-endian"
}

Example 10. Fixed-length bit array field class with instances aligned to 32 bits.

{
  "type": "fixed-length-bit-array",
  "length": 48,
  "byte-order": "big-endian",
  "alignment": 32
}

Example 11. Fixed-length bit array field class with a non-default bit order.

{
  "type": "fixed-length-bit-array",
  "length": 32,
  "byte-order": "little-endian",
  "bit-order": "last-to-first"
}

Example 12. Fixed-length bit array field class with attributes.

{
  "type": "fixed-length-bit-array",
  "length": 16,
  "byte-order": "little-endian",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.5. Fixed-length bit map field class

A fixed-length bit map field class is a fixed-length bit array field class which describes fixed-length bit map fields.

A fixed-length bit map field is a fixed-length bit array field with one or more associated bit names thanks to the flags property of its class.

Table 5. Properties of a fixed-length bit map field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"fixed-length-bit-map"`.	Yes
`length`	JSON integer	Number of bits of an instance of F. The value of this property MUST be greater than zero. Property inherited from the fixed-length bit array field class.	Yes
`byte-order`	JSON string	Byte order of an instance of F. The value of this property MUST be one of: `"big-endian"` Big-endian. Within a data stream byte, the bits of an instance of F are decoded from the most significant to the least significant bits of the byte. `"little-endian"` Little-endian. Within a data stream byte, the bits of an instance of F are decoded from the least significant to the most significant bits of the byte. Property inherited from the fixed-length bit array field class.	Yes
`bit-order`	JSON string	Bit order of an instance of F. The value of this property MUST be one of: `"first-to-last"` The first bit to decode of an instance of F is the first element of the resulting array value. `"last-to-first"` The first bit to decode of an instance of F is the last element of the resulting array value. Property inherited from the fixed-length bit array field class.	No	`"first-to-last"` if the value of the `byte-order` property of F is `"little-endian"`, or `"last-to-first"` otherwise.
`alignment`	JSON integer	Alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. Property inherited from the fixed-length bit array field class.	No	`1`
`flags`	Bit map field class flags	Flags of F. The value of this property MUST contain one or more properties.	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 13. Fixed-length bit map field class with a single flag.

{
  "type": "fixed-length-bit-map",
  "length": 16,
  "byte-order": "little-endian",
  "flags": {
    "FEATURE": [[3, 3]]
  }
}

Example 14. Fixed-length bit map field class with instances aligned to 32 bits and three flags.

{
  "type": "fixed-length-bit-map",
  "length": 48,
  "byte-order": "big-endian",
  "alignment": 32,
  "flags": {
    "shock": [[3, 3]]
    "cord": [[4, 4], [8, 10]]
    "learn": [[2, 7]]
  }
}

Example 15. Fixed-length bit map field class with attributes.

{
  "type": "fixed-length-bit-map",
  "length": 16,
  "byte-order": "little-endian",
  "flags": {
    "FEATURE": [[3, 3]]
  },
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.5.1. Bit map field class flags

Bit map field class flags map names to bit index range sets.

Bit map field class flags are a JSON object, where each property is:

Name

Flag name.

Value

Flag ranges of bit indexes (integer range set).

The indexes refer to the decoded bit array indexes.

The bit index ranges of two given flags MAY overlap.

Bit map field class flags MUST contain one or more properties.

Let F be some fixed-length bit map field class, K be the value of the flags property of F, and V be some decoded value of F, then if element (bit) I of V is true and I is an element of one of the ranges of a property E of K, then the name of E is said to be active for V.

Example 16. Bit map field class flags with four flags.

In this example, for the bit array value

[false, true, false, false, false, true, false, true]

the active flag names are Mercury, Earth, and Mars.

{
  "Mercury": [[7, 7]],
  "Venus": [[6, 6], [2, 3]],
  "Earth": [[5, 7]],
  "Mars": [[0, 1]]
}

5.3.6. Fixed-length boolean field class

A fixed-length boolean field class is a fixed-length bit array field class which describes fixed-length boolean fields.

A fixed-length boolean field is a fixed-length bit array field which has the following semantics:

If all the bits of the field are cleared (zero): The value of the fixed-length boolean field is false.
Otherwise: The value of the fixed-length boolean field is true.

Table 6. Properties of a fixed-length boolean field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"fixed-length-boolean"`.	Yes
`length`	JSON integer	Number of bits of an instance of F. The value of this property MUST be greater than zero. Property inherited from the fixed-length bit array field class.	Yes
`byte-order`	JSON string	Byte order of an instance of F. The value of this property MUST be one of: `"big-endian"` Big-endian. Within a data stream byte, the bits of an instance of F are decoded from the most significant to the least significant bits of the byte. `"little-endian"` Little-endian. Within a data stream byte, the bits of an instance of F are decoded from the least significant to the most significant bits of the byte. The value of this property has no effect on the decoded value of an instance of F. Property inherited from the fixed-length bit array field class.	Yes
`bit-order`	JSON string	Bit order of an instance of F. The value of this property MUST be one of: `"first-to-last"` The first bit to decode of an instance of F is the first element of the resulting array value. `"last-to-first"` The first bit to decode of an instance of F is the last element of the resulting array value. The value of this property has no effect on the decoded value of an instance of F. Property inherited from the fixed-length bit array field class.	No	`"first-to-last"` if the value of the `byte-order` property of F is `"little-endian"`, or `"last-to-first"` otherwise.
`alignment`	JSON integer	Alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. Property inherited from the fixed-length bit array field class.	No	`1`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 17. Minimal fixed-length boolean field class.

{
  "type": "fixed-length-boolean",
  "length": 16,
  "byte-order": "little-endian"
}

Example 18. Fixed-length boolean field class with instances aligned to 32 bits.

{
  "type": "fixed-length-boolean",
  "length": 48,
  "byte-order": "big-endian",
  "alignment": 32
}

Example 19. Fixed-length boolean field class with attributes.

{
  "type": "fixed-length-boolean",
  "length": 16,
  "byte-order": "little-endian",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.7. Abstract integer field class

An abstract integer field class is a base of a fixed-length integer field class and a variable-length integer field class.

This field class is abstract in that it only exists to show the relation between different integer field classes in this document: a packet cannot contain an abstract integer field.

Name Type Description Required? Default

preferred-display-base

JSON integer

Preferred base to display the value of an instance of F.

The value of this property MUST be one of:

`2`	Binary base.
`8`	Octal base.
`10`	Decimal base.
`16`	Hexadecimal base.

This property exists to remain backward compatible with CTF 1: it’s not strictly needed to decode an instance of F and has no semantic value.

10

mappings

Integer field class mappings

Mappings of F.

{}

5.3.7.1. Integer field class mappings

Integer field class mappings map names to integer range sets.

Integer field class mappings are a JSON object, where each property is:

Name	Mapping name.
Value	Mapped ranges of integers (integer range set).

The integer ranges of two given mappings MAY overlap.

Example 20. Integer field class mappings with three mappings.

In this example, the fortune and building mappings overlap with the values 4 and 5, and the building and journal mappings overlap with the value 80.

{
  "fortune": [[3, 67], [-45, 1], [84, 84]],
  "building": [[4, 5], [75, 82]],
  "journal": [[100, 2305], [80, 80]]
}

5.3.8. Fixed-length integer field class

A fixed-length integer field class is both an abstract integer field class and a fixed-length bit array field class which describes fixed-length integer fields.

A fixed-length integer field is a fixed-length bit array field which has integer semantics.

If the value of the type property of a fixed-length integer is "fixed-length-signed-integer", then its instances follow the two’s complement format.

Name Type Description Required? Default

type

JSON string

Type of F.

The value of this property MUST be one of:

"fixed-length-unsigned-integer": The instances of F are fixed-length unsigned integer fields.
"fixed-length-signed-integer": The instances of F are fixed-length signed integer fields.

Yes

length

JSON integer

Number of bits of an instance of F.

The value of this property MUST be greater than zero.

Property inherited from the fixed-length bit array field class.

Yes

byte-order

JSON string

Byte order of an instance of F.

The value of this property MUST be one of:

"big-endian": Big-endian.

Within a data stream byte, the bits of an instance of F are decoded from the most significant to the least significant bits of the byte.
"little-endian": Little-endian.

Within a data stream byte, the bits of an instance of F are decoded from the least significant to the most significant bits of the byte.

Property inherited from the fixed-length bit array field class.

Yes

bit-order

JSON string

Bit order of an instance of F.

The value of this property MUST be one of:

"first-to-last": The first bit to decode of an instance of F is the first element (least significant bit of the encoded integer value) of the resulting array value.
"last-to-first": The first bit to decode of an instance of F is the last element (most significant bit of the encoded integer value) of the resulting array value.

Property inherited from the fixed-length bit array field class.

"first-to-last" if the value of the byte-order property of F is "little-endian", or "last-to-first" otherwise.

alignment

JSON integer

Alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance.

The value of this property MUST be a positive power of two.

Property inherited from the fixed-length bit array field class.

1

preferred-display-base

JSON integer

Preferred base to display the value of an instance of F.

The value of this property MUST be one of:

`2`	Binary base.
`8`	Octal base.
`10`	Decimal base.
`16`	Hexadecimal base.

This property exists to remain backward compatible with CTF 1: it’s not strictly needed to decode an instance of F and has no semantic value.

Property inherited from the abstract integer field class.

10

mappings

Integer field class mappings

Mappings of F.

Property inherited from the abstract integer field class.

{}

roles

Roles

Roles of an instance of F.

See Trace class fragment and Data stream class fragment which indicate accepted roles within their root field classes.

This property MAY only exist when the type property of F is "fixed-length-unsigned-integer".

[]

attributes

Attributes

Attributes of F.

{}

extensions

Extensions

Extensions of F.

Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.

{}

Example 21. Minimal fixed-length unsigned integer field class.

{
  "type": "fixed-length-unsigned-integer",
  "length": 16,
  "byte-order": "little-endian"
}

Example 22. Fixed-length signed integer field class with instances aligned to 32 bits.

{
  "type": "fixed-length-signed-integer",
  "length": 48,
  "byte-order": "big-endian",
  "alignment": 32
}

Example 23. Fixed-length unsigned integer field class with instances to be preferably displayed with a hexadecimal base.

{
  "type": "fixed-length-unsigned-integer",
  "length": 48,
  "byte-order": "big-endian",
  "preferred-display-base": 16
}

Example 24. Fixed-length signed integer field class with two mappings.

{
  "type": "fixed-length-signed-integer",
  "length": 32,
  "byte-order": "little-endian",
  "mappings": {
    "banana": [[-27399, -1882], [8, 199], [101, 101]],
    "orange": [[67, 67], [43, 1534]]
  }
}

Example 25. Fixed-length signed integer field class with attributes.

{
  "type": "fixed-length-signed-integer",
  "length": 16,
  "byte-order": "little-endian",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.9. Fixed-length floating point number field class

A fixed-length floating point number field class is a fixed-length bit array field class which describes fixed-length floating point number fields.

A fixed-length floating point number field is a fixed-length bit array which has floating point number semantics.

A fixed-length floating point number is encoded as per IEEE 754-2008 binary interchange format.

Table 9. Properties of a fixed-length floating point number field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"fixed-length-floating-point-number"`.	Yes
`length`	JSON integer	Number of bits of an instance of F. The value of this property MUST be one of: `16` The instances of F are binary16 floating point numbers, as per the IEEE 754-2008 binary interchange format. `32` The instances of F are binary32 floating point numbers. `64` The instances of F are binary64 floating point numbers. `128` The instances of F are binary128 floating point numbers. K, where K is greater than 128 and a multiple of 32 The instances of F are binaryK floating point numbers. Property inherited from the fixed-length bit array field class.	Yes
`byte-order`	JSON string	Byte order of an instance of F. The value of this property MUST be one of: `"big-endian"` Big-endian. Within a data stream byte, the bits of an instance of F are decoded from the most significant to the least significant bits of the byte. `"little-endian"` Little-endian. Within a data stream byte, the bits of an instance of F are decoded from the least significant to the most significant bits of the byte. Property inherited from the fixed-length bit array field class.	Yes
`bit-order`	JSON string	Bit order of an instance of F. The value of this property MUST be one of: `"first-to-last"` The first bit to decode of an instance of F is the first element (least significant bit of the mantissa) of the resulting array value. `"last-to-first"` The first bit to decode of an instance of F is the last element (sign bit) of the resulting array value. Property inherited from the fixed-length bit array field class.	No	`"first-to-last"` if the value of the `byte-order` property of F is `"little-endian"`, or `"last-to-first"` otherwise.
`alignment`	JSON integer	Alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. Property inherited from the fixed-length bit array field class.	No	`1`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 26. Minimal binary32 fixed-length floating point number field class.

{
  "type": "fixed-length-floating-point-number",
  "length": 32,
  "byte-order": "little-endian"
}

Example 27. binary64 fixed-length floating point number field class with instances aligned to 32 bits.

{
  "type": "fixed-length-floating-point-number",
  "length": 64,
  "byte-order": "big-endian",
  "alignment": 32
}

Example 28. binary192 fixed-length floating point number field class with attributes.

{
  "type": "fixed-length-floating-point-number",
  "length": 192,
  "byte-order": "little-endian",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.10. Variable-length integer field class

A variable-length integer field class is an abstract integer field class which describes variable-length integer fields.

A variable-length integer field is a sequence of bytes with a variable length which contains a multiple of 7 effective bits encoding an unsigned or signed integer value.

A variable-length integer field is encoded as per LEB128.

If the value of the type property of a variable-length integer field class is "variable-length-signed-integer", then its instances follow the two’s complement format.

Name Type Description Required? Default

type

JSON string

Type of F.

The value of this property MUST be one of:

"variable-length-unsigned-integer": The instances of F are variable-length unsigned integer fields.
"variable-length-signed-integer": The instances of F are variable-length signed integer fields.

Yes

preferred-display-base

JSON integer

Preferred base to display the value of an instance of F.

The value of this property MUST be one of:

`2`	Binary base.
`8`	Octal base.
`10`	Decimal base.
`16`	Hexadecimal base.

This property exists to remain backward compatible with CTF 1: it’s not strictly needed to decode an instance of F and has no semantic value.

Property inherited from the abstract integer field class.

10

mappings

Integer field class mappings

Mappings of F.

Property inherited from the abstract integer field class.

{}

roles

Roles

Roles of an instance of F.

See Trace class fragment and Data stream class fragment which indicate accepted roles within their root field classes.

This property MAY only exist when the type property of F is "variable-length-unsigned-integer".

[]

attributes

Attributes

Attributes of F.

{}

extensions

Extensions

Extensions of F.

Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.

{}

Example 29. Minimal variable-length unsigned integer field class.

{
  "type": "variable-length-unsigned-integer"
}

Example 30. Variable-length signed integer field class with instances to be preferably displayed with a hexadecimal base.

{
  "type": "variable-length-signed-integer",
  "preferred-display-base": 16
}

Example 31. Variable-length unsigned integer field class with three mappings.

{
  "type": "variable-length-unsigned-integer",
  "mappings": {
    "lime": [[3, 3]],
    "kiwi": [[8, 8]],
    "blueberry": [[11, 11]]
  }
}

Example 32. Variable-length unsigned integer field class with attributes.

{
  "type": "variable-length-unsigned-integer",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.11. Abstract string field class

An abstract string field class is a base of a null-terminated string field class, a static-length string field class, and a dynamic-length string field class.

This field class is abstract in that it only exists to show the relation between different string field classes in this document: a packet cannot contain an abstract string field.

Name Type Description Required? Default

encoding

JSON string

Character encoding of an instance of F.

The value of this property MUST be one of:

`utf-8`	UTF-8.
`utf-16be`	UTF-16BE (big endian).
`utf-16le`	UTF-16LE (little endian).
`utf-32be`	UTF-32BE (big endian).
`utf-32le`	UTF-32LE (little endian).

"utf-8"

5.3.12. Null-terminated string field class

A null-terminated string field class is an abstract string field class which describes null-terminated string fields.

A null-terminated string field is, in this order:

Zero or more contiguous encoded Unicode codepoints, following the encoding property of its class.
One “NULL” (U+0000) codepoint, following the encoding property of its class.

Name Type Description Required? Default

type

JSON string

Type of F.

The value of this property MUST be "null-terminated-string".

Yes

encoding

JSON string

Character encoding of an instance of F.

The value of this property MUST be one of:

`utf-8`	UTF-8.
`utf-16be`	UTF-16BE (big endian).
`utf-16le`	UTF-16LE (little endian).
`utf-32be`	UTF-32BE (big endian).
`utf-32le`	UTF-32LE (little endian).

Property inherited from the abstract string field class.

"utf-8"

attributes

Attributes

Attributes of F.

{}

extensions

Extensions

Extensions of F.

Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.

{}

Example 33. Minimal UTF-8 null-terminated string field class.

{
  "type": "null-terminated-string"
}

Example 34. UTF-16LE null-terminated string field class.

{
  "type": "null-terminated-string",
  "encoding": "utf-16le"
}

Example 35. Null-terminated string field class with attributes.

{
  "type": "null-terminated-string",
  "attributes": {
    "my.tracer": {
      "is-nice": true
    }
  }
}

5.3.13. Static-length string field class

A static-length string field class is an abstract string field class which describes static-length string fields.

A static-length string field is a sequence of zero or more contiguous encoded Unicode codepoints. All the encoded codepoints of a static-length string field before the first “NULL” (U+0000) codepoint, if any, form the resulting string value. The first U+0000 codepoint, if any, and all the following bytes are considered padding (garbage data).

The length, or number of bytes, of a static-length string field is a property (length) of its class.

Name Type Description Required? Default

type

JSON string

Type of F.

The value of this property MUST be "static-length-string".

Yes

length

JSON integer

Number of bytes contained in an instance of F.

In general, this doesn’t mean the number of encoded Unicode codepoints.

The value of this property MUST be greater than or equal to zero.

Yes

encoding

JSON string

Character encoding of an instance of F.

The value of this property MUST be one of:

`utf-8`	UTF-8.
`utf-16be`	UTF-16BE (big endian).
`utf-16le`	UTF-16LE (little endian).
`utf-32be`	UTF-32BE (big endian).
`utf-32le`	UTF-32LE (little endian).

Property inherited from the abstract string field class.

"utf-8"

attributes

Attributes

Attributes of F.

{}

extensions

Extensions

Extensions of F.

Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.

{}

Example 36. Empty UTF-8 static-length string field class.

{
  "type": "static-length-string",
  "length": 0
}

Example 37. UTF-8 static-length string field class with instances having 100 bytes.

{
  "type": "static-length-string",
  "length": 100
}

Example 38. UTF-32BE static-length string field class with instances having 64 bytes.

{
  "type": "static-length-string",
  "encoding": "utf32-be",
  "length": 100
}

Example 39. Static-length string field class with attributes.

{
  "type": "static-length-string",
  "length": 13,
  "attributes": {
    "my.tracer": null
  }
}

5.3.14. Dynamic-length string field class

A dynamic-length string field class is an abstract string field class which describes dynamic-length string fields.

A static-length string field is a sequence of zero or more contiguous encoded Unicode codepoints. All the encoded codepoints of a dynamic-length string field before the first “NULL” (U+0000) codepoint, if any, form the resulting string value. The first U+0000 codepoint, if any, and all the following bytes are considered padding (garbage data).

The length, or number of bytes, of a dynamic-length string field is the value of another, anterior (already encoded/decoded) length field. A consumer can locate this length field thanks to the length-field-location property of the dynamic-length string field class.

Name Type Description Required? Default

type

JSON string

Type of F.

The value of this property MUST be "dynamic-length-string".

Yes

length-field-location

Field location

Location of an anterior field of which the value is the number of bytes contained in an instance of F.

In general, this doesn’t mean the number of encoded Unicode codepoints.

Any length field MUST be an instance of one of:

Fixed-length unsigned integer field class
Variable-length unsigned integer field class

Yes

encoding

JSON string

Character encoding of an instance of F.

The value of this property MUST be one of:

`utf-8`	UTF-8.
`utf-16be`	UTF-16BE (big endian).
`utf-16le`	UTF-16LE (little endian).
`utf-32be`	UTF-32BE (big endian).
`utf-32le`	UTF-32LE (little endian).

Property inherited from the abstract string field class.

"utf-8"

attributes

Attributes

Attributes of F.

{}

extensions

Extensions

Extensions of F.

Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.

{}

Example 40. UTF-8 dynamic-length string field class.

{
  "type": "dynamic-length-string",
  "length-field-location": {
    "origin": "event-record-payload",
    "path": ["length"]
  }
}

Example 41. UTF-16BE dynamic-length string field class.

{
  "type": "dynamic-length-string",
  "length-field-location": {
    "path": [null, "attrs", "len"]
  },
  "encoding": "utf-16be"
}

Example 42. Dynamic-length string field class with attributes.

{
  "type": "dynamic-length-string",
  "length-field-location": {
    "path": ["name-length"]
  },
  "attributes": {
    "my.tracer": 177
  }
}

5.3.15. Abstract BLOB field class

An abstract BLOB field class is a base of a static-length BLOB field class and a dynamic-length BLOB field class.

This field class is abstract in that it only exists to show the relation between different BLOB field classes in this document: a packet cannot contain an abstract BLOB field.

Table 15. Common properties of a BLOB field class F.
Name	Type	Description	Required?	Default
`media-type`	JSON string	IANA media type of an instance of F.	No	`"application/octet-stream"`

5.3.16. Static-length BLOB field class

A static-length BLOB field class is an abstract BLOB field class which describes static-length BLOB fields.

A static-length BLOB field is a sequence of zero or more contiguous bytes with an associated IANA media type (given by the media-type property of its class).

The length, or number of bytes, of a static-length BLOB field is a property (length) of its class.

Table 16. Properties of a static-length BLOB field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"static-length-blob"`.	Yes
`length`	JSON integer	Number of bytes contained in an instance of F. The value of this property MUST be greater than or equal to zero.	Yes
`media-type`	JSON string	IANA media type of an instance of F. Property inherited from the abstract BLOB field class.	No	`"application/octet-stream"`
`roles`	Roles	Roles of an instance of F. See Trace class fragment and Data stream class fragment which indicate accepted roles within their root field classes.	No	`[]`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 43. Empty static-length BLOB field class with instances having a default IANA media type.

{
  "type": "static-length-blob",
  "length": 0
}

Example 44. Static-length TIFF BLOB field class with instances having 511,267 bytes.

{
  "type": "static-length-blob",
  "length": 511267,
  "media-type": "image/tif"
}

Example 45. Static-length CSV BLOB field class with attributes.

{
  "type": "static-length-blob",
  "length": 2400,
  "media-type": "text/csv",
  "attributes": {
    "my.tracer": {
      "csv-cols": 12
    }
  }
}

5.3.17. Dynamic-length BLOB field class

A dynamic-length BLOB field class is an abstract BLOB field class which describes dynamic-length BLOB fields.

A dynamic-length BLOB field is a sequence of zero or more contiguous bytes with an associated IANA media type.

The length, or number of bytes, of a dynamic-length BLOB field is the value of another, anterior (already encoded/decoded) length field. A consumer can locate this length field thanks to the length-field-location property of the dynamic-length BLOB field class.

Table 17. Properties of a dynamic-length BLOB field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"dynamic-length-blob"`.	Yes
`length-field-location`	Field location	Location of an anterior field of which the value is the number of bytes contained in an instance of F. Any length field MUST be an instance of one of: Fixed-length unsigned integer field class Variable-length unsigned integer field class	Yes
`media-type`	JSON string	IANA media type of an instance of F. Property inherited from the abstract BLOB field class.	No	`"application/octet-stream"`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 46. Dynamic-length BLOB field class with instances having a default IANA media type.

{
  "type": "dynamic-length-blob",
  "length-field-location": {
    "origin": "event-record-payload",
    "path": ["length"]
  }
}

Example 47. Dynamic-length JPEG BLOB field class with attributes.

{
  "type": "dynamic-length-blob",
  "length-field-location": {
    "path": [null, "length"]
  }
  "media-type": "image/jpeg",
  "attributes": {
    "my.tracer": {
      "quality": 85
    }
  }
}

5.3.18. Structure field class

A structure field class describes structure fields.

A structure field is a sequence of zero or more structure field members. A structure field member is a named field.

Table 18. Properties of a structure field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"structure"`.	Yes
`member-classes`	JSON array of structure field member classes	Classes of the members of an instance of F. The `name` property of each member class MUST be unique within the member class names of F.	No	`[]`
`minimum-alignment`	JSON integer	Minimum alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. The effective alignment of the first bit of an instance of F MAY be greater than the value of this property.	No	`1`
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 48. Empty structure field class: instances have no members.

{
  "type": "structure"
}

Example 49. Structure field class with three member classes.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "Villeray",
      "field-class": {
        "type": "null-terminated-string"
      }
    },
    {
      "name": "Berri",
      "field-class": {
        "type": "fixed-length-unsigned-integer",
        "length": 32,
        "byte-order": "little-endian",
        "preferred-display-base": 2
      },
      "attributes": {
        "my.tracer": {
          "is-mask": true
        }
      }
    },
    {
      "name": "Faillon",
      "field-class": {
        "type": "fixed-length-boolean",
        "length": 8,
        "byte-order": "little-endian"
      }
    }
  ]
}

Example 50. Structure field class with instances minimally aligned to 64 bits.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "St-Denis",
      "field-class": {
        "type": "null-terminated-string"
      }
    },
    {
      "name": "Lajeunesse",
      "field-class": {
        "type": "fixed-length-unsigned-integer",
        "length": 32,
        "byte-order": "big-endian",
        "alignment": 32
      }
    }
  ],
  "minimum-alignment": 64
}

Example 51. Structure field class with attributes.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "Henri-Julien",
      "field-class": {
        "type": "fixed-length-signed-integer",
        "length": 48,
        "byte-order": "little-endian"
      }
    },
    {
      "name": "Casgrain",
      "field-class": {
        "type": "static-length-string",
        "length": 32
      }
    }
  ],
  "attributes": {
    "my.tracer": {
      "version": 4
    }
  }
}

5.3.18.1. Structure field member class

A structure field member class describes structure field members.

A structure field member class is a JSON object.

Table 19. Properties of a structure field member class M.
Name	Type	Description	Required?	Default
`name`	JSON string	Name of M.	Yes
`field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the field class of M. JSON string The effective field class of the previously occurring field class alias named V is the field class of M.	Yes
`attributes`	Attributes	Attributes of M.	No	`{}`
`extensions`	Extensions	Extensions of M. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 52. Null-terminated string field class member class named cat.

{
  "name": "cat",
  "field-class": {
    "type": "null-terminated-string"
  }
}

Example 53. Variable-length signed integer field class member class named dog with attributes.

{
  "name": "dog",
  "field-class": {
    "type": "variable-length-signed-integer",
    "preferred-display-base": 8
  },
  "attributes": {
    "my.tracer": {
      "uuid": [
        243, 97, 0, 184, 236, 54, 72, 97,
        141, 107, 169, 214, 171, 137, 115, 201
      ],
      "is-pid": true
    }
  }
}

5.3.19. Abstract array field class

An abstract array field class is a base of a static-length array field class and a dynamic-length array field class.

This field class is abstract in that it only exists to show the relation between different array field classes in this document: a packet cannot contain an abstract array field.

Table 20. Common properties of an array field class F.
Name	Type	Description	Required?	Default
`element-field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the class of the element fields contained in an instance of F. JSON string The effective field class of the previously occurring field class alias named V is the class of the element fields contained in an instance of F.	Yes
`minimum-alignment`	JSON integer	Minimum alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. The effective alignment of the first bit of an instance of F MAY be greater than the value of this property.	No	`1`

5.3.20. Static-length array field class

A static-length array field class is an abstract array field class which describes static-length array fields.

A static-length array field is a sequence of zero or more element fields.

The length, or number of element fields, of a static-length array field is a property (length) of its class.

Table 21. Properties of a static-length array field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"static-length-array"`.	Yes
`element-field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the class of the element fields contained in an instance of F. JSON string The effective field class of the previously occurring field class alias named V is the class of the element fields contained in an instance of F. Property inherited from the abstract array field class.	Yes
`minimum-alignment`	JSON integer	Minimum alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. The effective alignment of the first bit of an instance of F MAY be greater than the value of this property. Property inherited from the abstract array field class.	No	`1`
`length`	JSON integer	Number of element fields contained in an instance of F. The value of this property MUST be greater than or equal to zero.	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 54. Empty static-length array field class.

{
  "type": "static-length-array",
  "element-field-class": {
    "type": "fixed-length-signed-integer",
    "length": 16,
    "byte-order": "little-endian",
    "alignment": 16
  },
  "length": 0
}

Example 55. Static-length array field class with instances having 100 null-terminated string fields.

{
  "type": "static-length-array",
  "element-field-class": {
    "type": "null-terminated-string"
  },
  "length": 100
}

Example 56. Static-length array field class with attributes.

{
  "type": "static-length-array",
  "element-field-class": {
    "type": "variable-length-unsigned-integer"
  },
  "length": 13,
  "attributes": {
    "my.tracer": true
  }
}

Example 57. Static-length array field class with instances minimally aligned to 32 bits.

With the following static-length array field class, a producer can write a single 32-bit-aligned, 32-bit little-endian integer value, and have consumers decode it as an array of 32 flags (booleans).

{
  "type": "static-length-array",
  "length": 32,
  "minimum-alignment": 32,
  "element-field-class": {
    "type": "fixed-length-boolean",
    "length": 1,
    "byte-order": "little-endian"
  }
}

5.3.21. Dynamic-length array field class

A dynamic-length array field class is an abstract array field class which describes dynamic-length array fields.

A dynamic-length array field is a sequence of zero or more element fields.

The length, or number of element fields, of a dynamic-length array field is the value of another, anterior (already encoded/decoded) length field. A consumer can locate this length field thanks to the length-field-location property of the dynamic-length array field class.

Table 22. Properties of a dynamic-length array field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"dynamic-length-array"`.	Yes
`element-field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the class of the element fields contained in an instance of F. JSON string The effective field class of the previously occurring field class alias named V is the class of the element fields contained in an instance of F. Property inherited from the abstract array field class.	Yes
`minimum-alignment`	JSON integer	Minimum alignment of the first bit of an instance of F relative to the beginning of the packet which contains this instance. The value of this property MUST be a positive power of two. The effective alignment of the first bit of an instance of F MAY be greater than the value of this property. Property inherited from the abstract array field class.	No	`1`
`length-field-location`	Field location	Location of an anterior field of which the value is the number of element fields contained in an instance of F. Any length field MUST be an instance of one of: Fixed-length unsigned integer field class Variable-length unsigned integer field class	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 58. Dynamic-length array field class.

{
  "type": "dynamic-length-array",
  "element-field-class": {
    "type": "fixed-length-unsigned-integer",
    "length": 32,
    "byte-order": "big-endian",
    "alignment": 16
  },
  "length-field-location": {
    "origin": "event-record-payload",
    "path": ["length"]
  }
}

Example 59. Dynamic-length array field class with attributes.

{
  "type": "dynamic-length-array",
  "element-field-class": {
    "type": "variable-length-unsigned-integer"
  },
  "length-field-location": {
    "path": ["from-user", "common-length"]
  },
  "attributes": {
    "my.tracer": 177
  }
}

5.3.22. Optional field class

An optional field class describes optional fields.

An optional field is, depending on the value of another, anterior (already encoded/decoded) selector field, one of:

An instance of a given field class (field-class property of the optional field class).

In this case, the optional field is said to be enabled.
A zero-bit field (no field).

In this case, the optional field is said to be disabled.

A consumer can locate the selector field thanks to the selector-field-location property of the optional field class.

Table 23. Properties of an optional field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"optional"`.	Yes
`field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the class of an instance of F when it’s enabled. JSON string The effective field class of the previously occurring field class alias named V is the class of an instance of F when it’s enabled.	Yes
`selector-field-location`	Field location	Location of an anterior field of which the value indicates whether or not an instance of F is enabled. A selector field S MUST be an instance of one of: Fixed-length boolean field class An instance of F is enabled when S is true. Fixed-length integer field class Variable-length integer field class An instance of F is enabled when the value of S is an element of any of the integer ranges of the `selector-field-ranges` property of F. For a given instance of F, the `type` property of the classes of all the possible selector fields MUST be one of: `"fixed-length-boolean"` Any of: `"fixed-length-unsigned-integer"` `"variable-length-unsigned-integer"` Any of: `"fixed-length-signed-integer"` `"variable-length-signed-integer"`	Yes
`selector-field-ranges`	Integer range set	Ranges of integers which the value of a selector field MUST be an element of to enable an instance of F.	Yes, if the selector field is an instance of a fixed-length integer field class or a variable-length integer field class.	None if the selector field is an instance of a fixed-length boolean field class.
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 60. Optional static-length array field class with a boolean selector field class.

{
  "type": "optional",
  "selector-field-location": {
    "origin": "event-record-payload",
    "path": ["has-ip"]
  },
  "field-class": {
    "type": "static-length-array",
    "element-field-class": {
      "type": "fixed-length-unsigned-integer",
      "length": 8,
      "byte-order": "little-endian",
      "alignment": 8
    },
    "length": 16
  }
}

Example 61. Optional null-terminated string with a fixed-length signed integer selector field class.

{
  "type": "optional",
  "selector-field-location": {
    "origin": "event-record-payload",
    "path": ["has-ip"]
  },
  "selector-field-ranges": [[-12, -12], [-5, 0], [15, 35]],
  "field-class": {
    "type": "null-terminated-string"
  }
}

5.3.23. Variant field class

A variant field class describes variant fields.

A variant field is, depending on the value of another, anterior (already encoded/decoded) selector field, the instance of a specific, effective field class amongst one or more variant field class options.

A consumer can locate the selector field thanks to the selector-field-location property of the variant field class.

Table 24. Properties of a variant field class F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"variant"`.	Yes
`options`	JSON array of variant field class options	Options containing the possible effective classes of an instance of F. This array MUST contain one or more elements. The `name` property of each option, if it’s set, MUST be unique within the option names of F. The integer ranges (`selector-field-ranges` property) of two given options MUST NOT intersect.	Yes
`selector-field-location`	Field location	Location of an anterior field of which the value indicates which option of F contains the effective class of an instance of F. For a given instance of F, the `type` property of the classes of all the possible selector fields MUST be one of: Any of: `"fixed-length-unsigned-integer"` `"variable-length-unsigned-integer"` Any of: `"fixed-length-signed-integer"` `"variable-length-signed-integer"`	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 62. Variant field class with two options.

{
  "type": "variant",
  "selector-field-location": {
    "origin": "event-record-payload",
    "path": ["sel"]
  },
  "options": [
    {
      "selector-field-ranges": [[5, 5]],
      "field-class": {
        "type": "null-terminated-string"
      }
    },
    {
      "selector-field-ranges": [[8, 8]],
      "field-class": {
        "type": "fixed-length-signed-integer",
        "length": 16,
        "byte-order": "little-endian",
        "preferred-display-base": 8
      }
    }
  ]
}

Example 63. Variant field class within an optional field class which share the same selector field location.

This example shows that an optional field class and a contained variant field class MAY share the same selector field location.

In this example, depending on the value of the selector field:

0	The optional field is not enabled.
1	The optional field is enabled and is a variant field. The variant field is an instance of a null-terminated string field class (effective class).
2	The optional field is enabled and is a variant field. The variant field is an instance of a variable-length signed integer field class (effective class).

{
  "type": "optional",
  "selector-field-location": {
    "origin": "event-record-payload",
    "path": ["sel"]
  },
  "selector-field-ranges": [[1, 255]],
  "field-class": {
    "type": "variant",
    "selector-field-location": {
      "origin": "event-record-payload",
      "path": ["sel"]
    },
    "options": [
      {
        "selector-field-ranges": [[1, 1]],
        "field-class": {
          "type": "null-terminated-string"
        }
      },
      {
        "selector-field-ranges": [[2, 2]],
        "field-class": {
          "type": "variable-length-signed-integer",
          "preferred-display-base": 16
        }
      }
    ]
  }
}

Example 64. Variant field class with attributes.

{
  "type": "variant",
  "selector-field-location": {
    "origin": "event-record-specific-context",
    "path": ["sel"]
  },
  "options": [
    {
      "selector-field-ranges": [[5, 5], [10, 10], [15, 15]],
      "field-class": {
        "type": "static-length-string",
        "length": 20
      }
    },
    {
      "selector-field-ranges": [[0, 4], [6, 9], [11, 14], [16, 127]],
      "field-class": {
        "type": "fixed-length-floating-point-number",
        "length": 32,
        "byte-order": "big-endian"
      }
    }
  ],
  "attributes": {
    "my.tracer": {
      "owner": "Jimmy",
      "id": 199990
    }
  }
}

5.3.23.1. Variant field class option

A variant field class option contains a possible effective class of a variant field.

A variant field class option O also contains the ranges of integer values (selector-field-ranges property) of which the value of a selector field MUST be an element of for the effective class of a variant field to be the field class of O.

A variant field class option is a JSON object.

Table 25. Properties of a variant field class option O contained in a variant field class F.
Name	Type	Description	Required?	Default
`field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the field class of O. JSON string The effective field class of the previously occurring field class alias named V is the field class of O.	Yes
`selector-field-ranges`	Integer range set	Ranges of integers which the value of a selector field MUST be an element of for the effective class of an instance of F to be the field class (`field-class` property) of O.	Yes
`name`	JSON string	Name of O. This property exists to remain backward compatible with CTF 1: it’s not strictly needed to decode an instance of F and has no semantic value.	No	O is unnamed
`attributes`	Attributes	Attributes of O.	No	`{}`
`extensions`	Extensions	Extensions of O. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Example 65. Unnamed null-terminated string field class option.

{
  "field-class": {
    "type": "null-terminated-string"
  },
  "selector-field-ranges": [[3, 9]]
}

Example 66. Variable-length signed integer field class option named juice with attributes.

{
  "name": "juice",
  "field-class": {
    "type": "variable-length-signed-integer",
    "preferred-display-base": 16
  },
  "selector-field-ranges": [[-4, 4], [9, 9], [100, 200]],
  "attributes": {
    "my.tracer": {
      "uuid": [
        243, 97, 0, 184, 236, 54, 72, 97,
        141, 107, 169, 214, 171, 137, 115, 201
      ],
      "is-did": true
    }
  }
}

5.4. Preamble fragment

A preamble fragment indicates:

The CTF 2 major version (2).

CTF 2 doesn’t have a minor version: a producer can use attributes and extensions to add features to, or change features of, the format which this document specifies.
OPTIONAL: The UUID of the whole metadata stream.

The purpose of such a UUID is to associate data stream packets to a specific metadata stream through the metadata-stream-uuid role of a packet header static-length BLOB field.
OPTIONAL: Extension declarations.

An extension declaration is an initial extension of which the purpose is to declare that it’s enabled within the metadata stream.

Because an extension MAY alter the CTF 2 format itself, and because a preamble fragment is always the first metadata stream fragment, those extension declarations make it possible for a consumer to gracefully decline the data streams of the trace if it doesn’t support any declared extension.

The first fragment of a metadata stream MUST be a preamble fragment.

Table 26. Properties of a preamble fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"preamble"`.	Yes
`version`	JSON integer	CTF 2 major version. The value of this property MUST be `2`.	Yes
`uuid`	JSON array of 16 JSON integers	UUID of the whole metadata stream. The purpose of this UUID is to associate data stream packets to a specific metadata stream through the `metadata-stream-uuid` role of a packet header static-length BLOB field. The 16 JSON integers of this JSON array are the numeric values of the 16 bytes of the UUID.	No	The metadata stream has no UUID
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extension declarations of F. The name of each property is a namespace and its value is a namespaced extensions object. Within a namespaced extensions object, an extension named N is declared when it exists as a property named N, whatever the value of the property.	No	`{}`

Example 67. Minimal preamble fragment.

{
  "type": "preamble",
  "version": 2
}

Example 68. Preamble fragment with extension declarations.

The following preamble fragment declares the piano and ramen extensions under the my.tracer namespace.

{
  "type": "preamble",
  "version": 2,
  "extensions": {
    "my.tracer": {
      "piano": {
        "keys": 88,
        "temperament": "equal"
      },
      "ramen": null
    }
  }
}

Example 69. Preamble fragment with a metadata stream UUID.

{
  "type": "preamble",
  "version": 2,
  "uuid": [
    229, 62, 10, 184, 80, 161, 79, 10,
    183, 16, 181, 240, 187, 169, 196, 172
  ]
}

5.5. Field class alias fragment

A field class alias assigns a name to a field class.

The following JSON object properties MAY refer to the effective field class of a previously occurring field class alias using its name (JSON string):

Field class alias fragment

field-class

Trace class fragment

packet-header-field-class

Data stream class fragment

packet-context-field-class
event-record-header-field-class
event-record-common-context-field-class

Event record class fragment

specific-context-field-class
payload-field-class

Structure field member class

field-class

Abstract array field class

element-field-class

Optional field class

field-class

Variant field class option

field-class

Table 27. Properties of a field class alias fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"field-class-alias"`.	Yes
`name`	JSON string	Name of F.	Yes
`field-class`	Field class or JSON string	Depending on the type of a value V of this property: Field class V is the effective field class of F. JSON string The effective field class of the previously occurring field class alias named V is the effective field class of F.	Yes
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Within a metadata stream, two given field class alias fragments MUST NOT share the same name property value.

Example 70. Field class alias fragment assigning the name u8 to an 8-bit fixed-length unsigned integer field class.

{
  "type": "field-class-alias",
  "name": "u8",
  "field-class": {
    "type": "fixed-length-unsigned-integer",
    "length": 8,
    "byte-order": "little-endian"
  }
}

Example 71. Field class alias fragment assigning the name uint8 to an aliased field class named u8.

{
  "type": "field-class-alias",
  "name": "uint8",
  "field-class": "u8"
}

5.6. Trace class fragment

A trace class describes traces.

Within a metadata stream, if a trace class fragment T exists, then T MUST occur before any data stream class fragment.

Table 28. Properties of a trace class fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"trace-class"`.	Yes
`namespace`	JSON string	Namespace of an instance of F.	No	An instance of F has no namespace
`name`	JSON string	Name of an instance of F.	No	An instance of F has no name
`uid`	JSON string	Unique ID (any string) of an instance of F within some scope identified by the `namespace` (if any) and `name` (if any) properties.	No	An instance of F has no unique ID
`environment`	JSON object, where each property is: Name Environment entry name. Value Environment entry value (JSON integer or JSON string).	Environment of an instance of F. This property exists to remain backward compatible with CTF 1: this document doesn’t specify what an environment entry exactly is.	No	`{}`
`packet-header-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of all the packet header fields of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of all the packet header fields of an instance of F. Any field class contained in the packet header field class MUST satisfy at least one of: Have at least one valid role. Be a structure field class. Be an optional field class. Be a variant field class. Be the class of a field which is the selector field of an optional or variant field (anywhere within the same packet).	No	F has no packet header field class
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

5.6.1. Roles

If the packet-header-field-class property of a trace class fragment exists, then the field classes of its member classes MAY have the following roles:

Name Description Field class (F) constraints Other constraints

data-stream-class-id

Current data stream class ID.

The purpose of a data stream class ID field is to set the current ID of the class of the data stream of the current packet.

Fixed-length unsigned integer field class or variable-length unsigned integer field class.

data-stream-id

Current data stream ID.

The purpose of a data stream ID field is to set the current ID of the data stream of the current packet.

Combined with the ID of its class, such a field makes it possible to uniquely identify a data stream within a trace.

Fixed-length unsigned integer field class or variable-length unsigned integer field class.

metadata-stream-uuid

Metadata stream UUID.

Static-length BLOB field class with the following property value:

length

16

The uuid property of the preamble fragment MUST exist.

The 16 bytes of an instance of F MUST be equal to the 16 JSON integers of the uuid property of the preamble fragment.

packet-magic-number

Packet magic number.

The purpose of a packet magic number field is to confirm the beginning of a CTF 2 packet.

Fixed-length unsigned integer field class with the following property value:

length

32

An instance of F MUST be the first member of the packet header structure field.

The value of an instance of F value MUST be 0xc1fc1fc1 (3,254,525,889).

Example 72. Metadata stream fragments.

Preamble fragment.

{
  "type": "preamble",
  "version": 2,
  "uuid": [
    30, 201, 100, 148, 228, 2, 69, 70,
    147, 219, 233, 34, 43, 238, 108, 199
  ]
}

Trace class fragment.

{
  "type": "trace-class",
  "packet-header-field-class": {
    "type": "structure",
    "member-classes": [
      {
        "name": "the magic!",
        "field-class": {
          "type": "fixed-length-unsigned-integer",
          "length": 32,
          "byte-order": "little-endian",
          "preferred-display-base": 16,
          "roles": ["packet-magic-number"]
        }
      },
      {
        "name": "the UUID",
        "field-class": {
          "type": "static-length-blob",
          "length": 16,
          "roles": ["metadata-stream-uuid"]
        }
      },
      {
        "name": "my data stream class ID",
        "field-class": {
          "type": "fixed-length-unsigned-integer",
          "length": 8,
          "byte-order": "little-endian",
          "roles": ["data-stream-class-id"]
        }
      },
      {
        "name": "my data stream ID",
        "field-class": {
          "type": "variable-length-unsigned-integer",
          "roles": ["data-stream-id"]
        }
      }
    ]
  }
}

5.7. Clock class fragment

A clock class describes clocks.

A data stream MAY have a default clock.

Within a metadata stream, a clock class fragment MUST occur before any data stream class fragment which refers to it by internal ID with its default-clock-class-id property.

Table 30. Properties of a clock class fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"clock-class"`.	Yes
`id`	JSON string	Internal ID of F. The sole purpose of this property is to indicate the default clock class of a data stream class through its `default-clock-class-id` property.	Yes
`namespace`	JSON string	Namespace of an instance of F.	No	An instance of F has no namespace
`name`	JSON string	Name of an instance of F.	No	An instance of F has no name
`uid`	JSON string	Unique ID (any string) of an instance of F within some scope identified by the `namespace` (if any) and `name` (if any) properties.	No	An instance of F has no unique ID
`frequency`	JSON integer	Frequency of an instance of F (Hz). The value of this property MUST be greater than zero.	Yes
`origin`	JSON string or clock origin	Origin of an instance of F. The origin of a clock is what makes it possible to establish a correlation with other clocks having different identities, possibly amongst different traces, while also considering the `precision` and `accuracy` properties. The value of this property must be one of: `"unix-epoch"` The origin of an instance of F is the Unix epoch. A clock having the Unix epoch as its origin has a correlation with other clocks sharing this origin. A clock origin object CO The origin of an instance of F is described by CO. A clock having an origin described by CO has a correlation with other clocks having an origin described by a clock origin which is identical to CO.	No	An instance of F has no known origin: it has no correlation with any other clock having a different identity
`offset-from-origin`	Clock offset	Offset of an instance of F relative to its origin (see the `origin` property). Let: H be the value of the `frequency` property of F. O be the value of this property. S be the value of the `seconds` property of O. C be the value of the `cycles` property of O. Then the effective offset of an instance of F from its origin, in clock cycles, is S × H + C.	No	`{}`
`precision`	JSON integer	Precision of an instance of F (clock cycles). This property describes the random errors of an instance of F. The value of this property MUST be greater than or equal to zero.	No	The precision of an instance of F is unknown
`accuracy`	JSON integer	Accuracy of an instance of F (clock cycles). This property describes the systematic errors of an instance of F. The value of this property MUST be greater than or equal to zero.	No	The accuracy of an instance of F is unknown
`description`	JSON string	Textual description of F. This property exists to remain backward compatible with CTF 1: it’s not strictly needed to decode data streams and has no semantic value.	No	F has no textual description
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Two clock classes which satisfy all the following conditions are said to have the same identity:

Both share the same namespace property value, or both don’t have any namespace property.
Both have a name property and share the same value.
Both have a uid property and share the same value.

Two clock classes having the same identity, within the same metadata stream or in two different metadata streams, MUST also share:

The same frequency property value.
The same precision property value, if any.
The same accuracy property value, if any.
The same origin property value, if any.

The precision and accuracy properties of a clock class respectively describe the random and systematic errors of its instances. Moreover:

For a clock class CC, let:
- The value of the precision property of CC be P.
- The value of the accuracy property of CC be A
- V be the value of an instance of CC.
Then the range of possible true values of the instance is [V − P − A, V + P + A].
Let two clock classes CCA and CCB have the same identity: when computing value differences between instances of CCA and CCB, the value of their precision property is relevant, but the value of their accuracy property isn’t.
Let two clock classes CCA and CCB not have the same identity, but share the same origin property value: when computing value differences between instances of CCA and CCB, both the values of their precision and accuracy properties are relevant.

Within a metadata stream, two given clock class fragments MUST NOT share the same id property value.

Example 73. Minimal clock class fragment with 1-GHz instances having no known origin.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 1000000000
}

Example 74. Minimal clock class fragment with instances having the Unix epoch as their origin.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 1000000000,
  "origin": "unix-epoch"
}

Example 75. Clock class fragment with a specific identity.

{
  "type": "clock-class",
  "id": "my clock class",
  "namespace": "some-tracer",
  "name": "hot",
  "uid": "wheels.28",
  "frequency": 80000000
}

Example 76. Clock class fragment with instances having a custom origin.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 1000000000,
  "origin": {
    "name": "my origin",
    "uid": "60:57:18:a3:42:29"
  }
}

Example 77. Clock class fragment with instances having a specific offset from origin.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 1000000000,
  "offset-from-origin": {
    "seconds": 1605112699,
    "cycles": 2878388
  }
}

Example 78. Clock class fragment with instances having a known precision of 100 cycles.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 8000000,
  "precision": 100
}

Example 79. Clock class fragment with instances having a known accuracy of 1500 cycles.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 16000000,
  "accuracy": 1500
}

Example 80. Clock class fragment with attributes.

{
  "type": "clock-class",
  "id": "my clock class",
  "frequency": 32000000,
  "attributes": {
    "my.tracer": {
      "sys-name": "SOC23",
      "bus": {
        "name": "LMB5",
        "index": 5
      },
      "propagation-delay-ps": 177
    }
  }
}

5.7.1. Clock origin

A clock origin specifies the origin of the instances of a clock class.

A clock origin is a JSON object.

Table 31. Properties of a clock origin CO.
Name	Type	Description	Required?	Default
`namespace`	JSON string	Namespace of CO.	No	CO has no namespace
`name`	JSON string	Name of CO.	Yes
`uid`	JSON string	Unique ID (any string) of CO within some scope identified by the `namespace` (if any) and `name` properties.	Yes

Considering two clock class fragments CCA and CCB, which don’t need to be part of the same metadata stream, both having an origin property set to some clock origin object: an instance of CCA has a correlation with an instance of CCB if and only if the clock origin objects of CCA and CCB are identical.

Example 81. Clock origin without a namespace.

{
  "name": "my-origin",
  "uid": "42"
}

Example 82. Clock origin with a namespace.

{
  "namespace": "some-tracer",
  "name": "the-origin",
  "uid": "7e41f662-27b6-45d5-8d9b-0504daa7b04a"
}

5.7.2. Clock offset

A clock offset contains the offset of the instances of a clock class relative to their origin (see the origin property).

A clock offset is a JSON object.

Table 32. Properties of a clock offset contained in a clock class fragment F.
Name	Type	Description	Required?	Default
`seconds`	JSON integer	Offset, in seconds, of an instance of F relative to its origin.	No	`0`
`cycles`	JSON integer	Offset, in cycles, of an instance of F relative to its origin. The value of this property MUST be greater than or equal to zero. The value of this property MUST be less than the value of the `frequency` property of F.	No	`0`

Example 83. Minimal clock offset.

{}

Example 84. Clock offset with seconds and cycles.

{
  "seconds": 1605112699,
  "cycles": 2878388
}

Example 85. Clock offset with seconds only.

{
  "seconds": 1605111293
}

Example 86. Negative clock offset.

This example shows that a clock offset MAY be negative, that is, before the origin of the clock.

{
  "seconds": -18003,
  "cycles": 11928547
}

5.8. Data stream class fragment

A data stream class describes data streams.

Within a metadata stream, a data stream class fragment F MUST occur before any event record class fragment of which F is the parent.

Table 33. Properties of a data stream class fragment F.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"data-stream-class"`.	Yes
`id`	JSON integer	Numeric ID of F. The value of this property MUST be greater than or equal to zero.	No	`0`
`namespace`	JSON string	Namespace of an instance of F.	No	An instance of F has no namespace
`name`	JSON string	Name of an instance of F.	No	An instance of F has no name
`uid`	JSON string	Unique ID (any string) of an instance of F within some scope identified by the `namespace` (if any) and `name` (if any) properties.	No	An instance of F has no unique ID
`default-clock-class-id`	JSON string	Internal ID of the class of the default clock of an instance of F. Within the metadata stream containing F, the clock class fragment which has the value of this property as its `id` property MUST occur before F.	No	An instance of F has no default clock
`packet-context-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of all the packet context fields of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of all the packet context fields of an instance of F.	No	F has no packet context field class
`event-record-header-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of all the event record header fields of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of all the event record header fields of an instance of F. Any field class contained in the event record header field class MUST satisfy at least one of: Have at least one valid role. Be a structure field class. Be an optional field class. Be a variant field class. Be the class of a field which is the selector field of an optional or variant field (anywhere within the same event record).	No	F has no event record header field class
`event-record-common-context-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of all the event record common context fields of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of all the event record common context fields of an instance of F.	No	F has no event record common context field class
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Within a metadata stream, two given data stream class fragments MUST NOT share the same id property value.

5.8.1. Roles

If the packet-context-field-class property of a data stream class fragment exists, then the field classes of its member classes MAY have the following roles:

Table 34. Roles of field classes of the member classes of a packet context field class.
Name	Description	Field class (F) constraints	Other constraints
`default-clock-timestamp`	Current timestamp of the default clock of the data stream when the packet begins.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.	The data stream class has a `default-clock-class-id` property.
`discarded-event-record-counter-snapshot`	Current snapshot of the discarded event record counter of the data stream when the packet ends.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.
`packet-content-length`	Current content length (bits) of the packet.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.
`packet-end-default-clock-timestamp`	Current timestamp of the default clock of the data stream when the packet ends.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.	The data stream class has a `default-clock-class-id` property.
`packet-sequence-number`	Current sequence number of the packet within its data stream.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.
`packet-total-length`	Current total length (bits) of the packet.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.

If the event-record-header-field-class property of a data stream class fragment exists, then the field classes of its member classes MAY have the following roles:

Table 35. Roles of field classes of the member classes of an event record header field class.
Name	Description	Field class (F) constraints	Other constraints
`default-clock-timestamp`	Current timestamp of the default clock of the data stream when the event record occurs.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.	The data stream class has a `default-clock-class-id` property.
`event-record-class-id`	Current event record class ID. The purpose of a field having this role is to set the current ID of the class of the event record within its parent data stream class.	Fixed-length unsigned integer field class or variable-length unsigned integer field class.

5.9. Event record class fragment

An event record class describes event records.

The data stream class fragment of which the value of the id property matches the value of the data-stream-class-id property of an event record class fragment F is considered the parent of F.

Table 36. Properties of an event record class fragment F having the data stream class P as its parent.
Name	Type	Description	Required?	Default
`type`	JSON string	Type of F. The value of this property MUST be `"event-record-class"`.	Yes
`id`	JSON integer	Numeric ID of F within P. The value of this property MUST be greater than or equal to zero.	No	`0`
`data-stream-class-id`	JSON integer	Numeric ID of P. The value of this property MUST be greater than or equal to zero. Within the metadata stream, P MUST occur before F.	No	`0`
`namespace`	JSON string	Namespace of an instance of F.	No	An instance of F has no namespace
`name`	JSON string	Name of an instance of F.	No	An instance of F has no name
`uid`	JSON string	Unique ID (any string) of an instance of F within some scope identified by the `namespace` (if any) and `name` (if any) properties.	No	An instance of F has no unique ID
`specific-context-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of the specific context field of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of the specific context field of an instance of F.	No	F has no event record specific context field class
`payload-field-class`	Structure field class or JSON string	Depending on the type of a value V of this property: Structure field class V is the class of the payload field of an instance of F. JSON string The effective field class of the previously occurring structure field class alias named V is the class of the payload field of an instance of F.	No	F has no event record payload field class
`attributes`	Attributes	Attributes of F.	No	`{}`
`extensions`	Extensions	Extensions of F. Any extension which exists under this property MUST also be declared in the preamble fragment of the metadata stream.	No	`{}`

Within a metadata stream, two given event record class fragments MUST NOT share both the same id and data-stream-class-id property values.

6. Data stream decoding procedure

This section shows how to, procedurally, decode a CTF 2 data stream.

Decoding a data stream is the responsibility of a consumer.

This document does not detail the method for encoding a data stream since encoding offers significantly more flexibility than decoding. However, one can infer the encoding process based on the outlined decoding procedure.

To decode a data stream S:

While there’s remaining data in S:
- Decode one packet.

6.1. Packet decoding procedure

A consumer needs to keep a packet decoding state while decoding a packet. A packet decoding state comprises the following variables:

Table 37. Variables needed to decode a packet P within a data stream S.
Name	Type	Description	Initial value
X	Unsigned integer	Current decoding offset/position (bits) from the beginning of P.	0
DEF_CLK_VAL	Unsigned integer	If the class of S has a `default-clock-class-id` property: current value (clock cycles) of the default clock of S.	0
DISC_ER_SNAP	OPTIONAL unsigned integer	Current snapshot of the discarded event record counter of S at the end of P.	None
DSC	OPTIONAL data stream class	Current class of S.	None
DSC_ID	Unsigned integer	Current ID of the class of S.	0
DS_ID	OPTIONAL unsigned integer	Current ID of S.	None
LAST_BYTE_ORDER	OPTIONAL string	Byte order of the last decoded fixed-length bit array field.	None
PKT_CONTENT_LEN	Unsigned integer	Current content length (bits) of P.	∞
PKT_END_DEF_CLK_VAL	OPTIONAL unsigned integer	If the class of S has a `default-clock-class-id` property: current value (clock cycles) of the default clock of S at the end of P.	None
PKT_SEQ_NUM	OPTIONAL unsigned integer	Current sequence number of P.	None
PKT_TOTAL_LEN	Unsigned integer	Current total length (bits) of P.	∞

To decode a packet P within a data stream S:

If the packet-header-field-class property of the trace class fragment of the metadata stream exists, then decode the header field of P using the value of this property.

During the packet header field decoding procedure, after having decoded a field F as V, with F having the class C with a roles property:
- If C has the role data-stream-class-id, then set DSC_ID to V.
- If C has the role data-stream-id, then set DS_ID to V.
- If C has the role packet-magic-number, then validate that V is 0xc1fc1fc1 (3,254,525,889).
  
  A consumer SHOULD report an invalid packet magic number as an error.
- If C has the role metadata-stream-uuid, then validate that V matches the uuid property of the preamble fragment of the metadata stream of the trace of S.
  
  A consumer SHOULD report a metadata stream UUID mismatch as an error.
After having decoded the whole packet header field, if DS_ID is set, then it’s the ID of S within its class. In other words, two data streams MAY have the same ID if they’re instances of different data stream classes.
Set DSC to the data stream class having DSC_ID as the value of its id property.

If no data stream class has the ID DSC_ID, then report an error and abort the data stream decoding process.
If the packet-context-field-class property of DSC exists, then decode the context field of P using the value of this property.

During the packet context field decoding procedure, after having decoded a field F as V, with F having the class C with a roles property:
- If C has the role default-clock-timestamp, then update DEF_CLK_VAL from F.
- If C has the role discarded-event-record-counter-snapshot, then set DISC_ER_SNAP to V.
- If C has the role packet-content-length, then set PKT_CONTENT_LEN to V.
- If C has the role packet-end-default-clock-timestamp, then set PKT_END_DEF_CLK_VAL to V.
- If C has the role packet-sequence-number, then set PKT_SEQ_NUM to V.
- If C has the role packet-total-length, then set PKT_TOTAL_LEN to V.
After having decoded the whole packet context field:
- If set, DEF_CLK_VAL is the value of the default clock of S at the beginning of P.
- If set, PKT_END_DEF_CLK_VAL is the value of the default clock of S at the end of P.
- If both DEF_CLK_VAL and PKT_END_DEF_CLK_VAL are set, and if DEF_CLK_VAL > PKT_END_DEF_CLK_VAL (packet beginning timestamp is greater than packet end timestamp), then a consumer SHOULD report an error.
- If PKT_TOTAL_LEN is ∞ and PKT_CONTENT_LEN is not ∞, then set PKT_TOTAL_LEN to PKT_CONTENT_LEN.
- If PKT_CONTENT_LEN is ∞ and PKT_TOTAL_LEN is not ∞, then set PKT_CONTENT_LEN to PKT_TOTAL_LEN.
- If both PKT_TOTAL_LEN and PKT_CONTENT_LEN arenot ∞, and if PKT_CONTENT_LEN > PKT_TOTAL_LEN, then report an error and abort the data stream decoding process.
- If set, DISC_ER_SNAP is a snapshot of the discarded event record counter of S at the end of P.
- If set, PKT_SEQ_NUM is the sequence number of P.
While X < PKT_CONTENT_LEN and there’s remaining data in S:
- Decode an event record.
If PKT_TOTAL_LEN and PKT_CONTENT_LEN both are not ∞, then set X to PKT_TOTAL_LEN, effectively skipping end-of-packet padding.

6.2. Event record decoding procedure

A consumer needs to keep an event record decoding state while decoding an event record. An event record decoding state comprises the following variables:

Table 38. Variables needed to decode an event record E within a data stream S.
Name	Type	Description	Initial value
ERC_ID	Unsigned integer	Current ID of the class of E of which the parent is the class of S.	0
ERC	OPTIONAL event record class	Current class of E.	None

To decode an event record E within a data stream S:

If the event-record-header-field-class property of DSC exists, then decode the header field of E using the value of this property.

During the event record header field decoding procedure, after having decoded a field F as V, with F having the class C with a roles property:
- If C has the role event-record-class-id, then set ERC_ID to V.
- If C has the role default-clock-timestamp, then update DEF_CLK_VAL from F.
After having decoded the whole event record header field, DEF_CLK_VAL is the value of the default clock of S when E occurs.
Set ERC to the event record class having:
- DSC_ID as the value of its data-stream-class-id property.
- ERC_ID as the value of its id property.
If no event record class has the ID ERC_ID within a data stream class having the ID DSC_ID, then report an error and abort the data stream decoding process.
If the event-record-common-context-field-class property of DSC exists, then decode the common context field of E using the value of this property.
If the specific-context-field-class property of ERC exists, then decode the specific context field of E using the value of this property.
If the payload-field-class property of ERC exists, then decode the payload field of E using the value of this property.

6.3. Clock value update procedure

To update DEF_CLK_VAL from an unsigned integer field F having the unsigned integer value V and the class C:

Let L be an unsigned integer initialized to, depending on the type property of C:

"fixed-length-unsigned-integer"

The value of the length property of C.

"variable-length-unsigned-integer"

S ×7, where S is the number of bytes which F occupies with the data stream.
Let MASK be an unsigned integer initialized to 2^L − 1.
Let H be an unsigned integer initialized to DEF_CLK_VAL & ~MASK, where “&” is the bitwise AND operator and “~” is the bitwise NOT operator.
Let CUR be an unsigned integer initialized to DEF_CLK_VAL & MASK, where “&” is the bitwise AND operator.
Set DEF_CLK_VAL to:

If V ≥ CUR

H + V

Else

H + MASK + 1 + V

6.4. Field decoding procedure

The class of a field contains what’s needed to decode it as a value.

In a data stream, a field is a specific sequence of bits, whereas a value is the meaningful interpretation of these bits with associated semantics.

The types of values are:

Value type

Possible values

Nil

None.

Boolean

True or false.

Unsigned/signed integer

Integral quantity.

Real

Continuous quantity.

String

Sequence of Unicode codepoints.

Array

Sequence of values.

Structure

Sequence of named values (members).

To decode an instance of a field class F, depending on the value of its type property:

Value of the type property of F Decoding procedure of F

"fixed-length-bit-array"

Decode a fixed-length bit array field.

"fixed-length-bit-map"

Decode a fixed-length bit map field.

"fixed-length-boolean"

Decode a fixed-length boolean field.

"fixed-length-unsigned-integer"

Decode a fixed-length unsigned integer field.

"fixed-length-signed-integer"

Decode a fixed-length signed integer field.

"fixed-length-floating-point-number"

Decode a fixed-length floating point number field.

"variable-length-unsigned-integer"

Decode a variable-length unsigned integer field.

"variable-length-signed-integer"

Decode a variable-length signed integer.

"null-terminated-string"

Decode a null-terminated string field.

"static-length-string"

Decode a static-length string field.

"static-length-blob"

Decode a static-length BLOB field.

"dynamic-length-string"

Decode a dynamic-length string field.

"dynamic-length-blob"

Decode a dynamic-length BLOB field.

"structure"

Decode a structure field.

"static-length-array"

Decode a static-length array field.

"dynamic-length-array"

Decode a dynamic-length array field.

"optional"

Decode an optional field.

"variant"

Decode a variant field.

6.4.1. Alignment procedure

The decoding procedure of many fields require X to have a specific alignment.

The alignment requirement of an instance of a field class F is, depending on the value of its type property:

type property of F Alignment requirement of an instance of F

"fixed-length-bit-array"
"fixed-length-bit-map"
"fixed-length-boolean"
"fixed-length-unsigned-integer"
"fixed-length-signed-integer"
"fixed-length-floating-point-number"

The value of the alignment property of F.

"variable-length-unsigned-integer"
"variable-length-signed-integer"
"null-terminated-string"
"static-length-string"
"static-length-blob"
"dynamic-length-string"
"dynamic-length-blob"

"structure"

The maximum value of:

The value of the minimum-alignment property of F.
The alignment requirements of the instances of the field-class property of each member class of the member-classes property of F.

"static-length-array"
"dynamic-length-array"

The maximum value of:

The value of the minimum-alignment property of F.
The alignment requirement of an instance of the element-field-class property of F.

"optional"
"variant"

To align X to some alignment requirement A (bits):

Set X to ((X + A − 1) & −A), where “&” is the bitwise AND operator.
If X ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.

6.4.2. Field location procedure

To locate a previously decoded length or selector field using a field location FL while decoding another dependent field F:

Let V be:

If the origin property of FL exists

Depending on the value of the origin property of FL:

"packet-header"

The header structure of P (current packet).

"packet-context"

The context structure of P.

"event-record-header"

The header structure of E (current event record).

"event-record-common-context"

The common context structure of E.

"event-record-specific-context"

The specific context structure of E.

"event-record-payload"

The payload structure of E.

If the consumer cannot set V because there’s no such structure or because it’s not already decoded nor currently being decoded, then report an error and abort the data stream decoding process.

Otherwise

The most nested structure (immediate parent) containing the eventual value of F.
For each element FLE of the path property of FL:
1. Let V be, depending on FLE:
  
  A JSON string
  
  The value of the member named FLE within V.
  
  If no member is named FLE within V, then report an error and abort the data stream decoding process.
  
  If the member named FLE within V isn’t already decoded nor currently being decoded, then report an error and abort the data stream decoding process.
  
  null
  
  The most nested structure (immediate parent) containing V.
  
  If no structure contains V, then report an error and abort the data stream decoding process.
2. Depending on the type of V:
  Boolean
  
  Signed integer
  
  Unsigned integer
  
  If FLE isn’t the last element of FL, then report an error and abort the data stream decoding process.
  
  Structure
  
  Continue.
  
  Array
  
  While V is an array:
  
  If V isn’t currently being decoded, then report an error and abort the data stream decoding process.
  
  Set V to the element of V currently being decoded.
  
  Other
  
  Report an error and abort the data stream decoding process.

V is the located field.

Example 87. Dynamic-length array field and its length field in the same root field.

Assume the following JSON object is an event record payload structure field class.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "corn", (3)
      "field-class": {
        "type": "fixed-length-unsigned-integer",
        "length": 32,
        "byte-order": "little-endian"
      }
    },
    {
      "name": "inside",
      "field-class": {
        "type": "fixed-length-unsigned-integer",
        "length": 16,
        "byte-order": "little-endian"
      }
    },
    {
      "name": "carbon",
      "field-class": {
        "type": "dynamic-length-array", (1)
        "length-field-location": { (2)
          "origin": "event-record-payload",
          "path": ["corn"]
        },
        "element-field-class": {
          "type": "null-terminated-string"
        }
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.
3	Length member class.

Example 88. Dynamic-length array field and its length field in the same root field, within the same array field element.

Assume the following JSON object is an event record payload structure field class.

Both the dynamic-length array field and its length field exist within the same element of the static-length array field named nature.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "norm",
      "field-class": {
        "type": "null-terminated-string"
      }
    },
    {
      "name": "nature",
      "field-class": {
        "type": "static-length-array",
        "length": 43,
        "element-field-class": {
          "type": "structure",
          "member-classes": [
            {
              "name": "laser", (3)
              "field-class": {
                "type": "variable-length-unsigned-integer"
              }
            },
            {
              "name": "joystick",
              "field-class": {
                "type": "dynamic-length-array", (1)
                "length-field-location": { (2)
                  "path": ["laser"]
                },
                "element-field-class": {
                  "type": "null-terminated-string"
                }
              }
            }
          ]
        }
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.
3	Length member class.

Example 89. Dynamic-length array and its length field in the same root field, within the same variant field.

Assume the following JSON object is an event record payload structure field class.

Both the dynamic-length array field and its length field exist within the same option of the variant field named clinic.

Moreover, the selector field of the clinic variant field is the lawyer field.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "lawyer", (5)
      "field-class": {
        "type": "fixed-length-signed-integer",
        "length": 16,
        "byte-order": "little-endian"
      }
    },
    {
      "name": "clinic",
      "field-class": {
        "type": "variant",
        "selector-field-location": { (4)
          "origin": "event-record-payload",
          "path": ["lawyer"]
        },
        "options": [
          {
            "selector-field-ranges": [[0, 0]],
            "field-class": {
              "type": "null-terminated-string"
            }
          },
          {
            "selector-field-ranges": [[1, 4]],
            "field-class": {
              "type": "structure",
              "member-classes": [
                {
                  "name": "lemon", (3)
                  "field-class": {
                    "type": "fixed-length-unsigned-integer",
                    "length": 8,
                    "byte-order": "big-endian"
                  }
                },
                {
                  "name": "joystick",
                  "field-class": {
                    "type": "dynamic-length-array", (1)
                    "length-field-location": { (2)
                      "origin": "event-record-payload",
                      "path": ["clinic", "lemon"]
                    },
                    "element-field-class": {
                      "type": "null-terminated-string"
                    }
                  }
                }
              ]
            }
          },
          {
            "selector-field-ranges": [[5, 5], [7, 7]],
            "field-class": {
              "type": "fixed-length-boolean",
              "length": 8,
              "byte-order": "little-endian"
            }
          }
        ]
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.
3	Length member class.
4	Selector field location of the variant field class.
5	Selector member class.

Example 90. Dynamic-length array and its length field in the same root field; length field is a variant field.

Assume the following JSON object is an event record payload structure field class.

The length field of the dynamic-length array field is a variant field: it can be an 8-bit, a 16-bit, or a 32-bit fixed-length integer field, depending on the selection of the variant field.

Moreover, the selector field of the variant field is located in another root field (event record specific context).

{
  "type": "structure",
  "member-classes": [
    {
      "name": "glass", (3)
      "field-class": {
        "type": "variant",
        "selector-field-location": {
          "origin": "event-record-specific-context",
          "path": ["sel"]
        },
        "options": [
          {
            "selector-field-ranges": [[0, 0]],
            "field-class": {
              "type": "fixed-length-unsigned-integer", (4)
              "length": 8,
              "byte-order": "little-endian"
            }
          },
          {
            "selector-field-ranges": [[1, 1]],
            "field-class": {
              "type": "fixed-length-unsigned-integer", (4)
              "length": 16,
              "byte-order": "little-endian"
            }
          },
          {
            "selector-field-ranges": [[2, 2]],
            "field-class": {
              "type": "fixed-length-unsigned-integer", (4)
              "length": 32,
              "byte-order": "little-endian"
            }
          }
        ]
      }
    },
    {
      "name": "margin",
      "field-class": {
        "type": "dynamic-length-array", (1)
        "length-field-location": { (2)
          "origin": "event-record-payload",
          "path": ["glass"]
        },
        "element-field-class": {
          "type": "null-terminated-string"
        }
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.
3	Length member class.
4	Possible length field class.

Example 91. Dynamic-length array and its length field in the same root field; structure field containing length field is a variant field.

Assume the following JSON object is an event record payload structure field class.

The length field of the dynamic-length array field is within a structure field which is a variant field.

Moreover:

The selector field of the variant field is located in another root field (event record common context).
The field class of the third option of the glass variant field class contains a dynamic-length BLOB field class (lock member); the length field of its instance is the previous member (eagle) within the same structure field.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "glass",
      "field-class": {
        "type": "variant",
        "selector-field-location": {
          "origin": "event-record-common-context",
          "path": ["sel"]
        },
        "options": [
          {
            "selector-field-ranges": [[0, 0]],
            "field-class": {
              "type": "structure",
              "member-classes": [
                {
                  "name": "eagle",
                  "field-class": {
                    "type": "fixed-length-unsigned-integer", (3)
                    "length": 16,
                    "byte-order": "little-endian"
                  }
                },
                {
                  "name": "road",
                  "field-class": {
                    "type": "null-terminated-string"
                  }
                }
              ]
            }
          },
          {
            "selector-field-ranges": [[32, 172]],
            "field-class": {
              "type": "structure",
              "member-classes": [
                {
                  "name": "nuance",
                  "field-class": {
                    "type": "null-terminated-string"
                  }
                },
                {
                  "name": "eagle",
                  "field-class": {
                    "type": "fixed-length-unsigned-integer", (3)
                    "length": 24,
                    "byte-order": "big-endian"
                  }
                }
              ]
            }
          },
          {
            "selector-field-ranges": [[5, 5]],
            "field-class": {
              "type": "structure",
              "member-classes": [
                {
                  "name": "eagle", (5)
                  "field-class": {
                    "type": "variable-length-unsigned-integer" (3)
                  }
                },
                {
                  "name": "lock",
                  "field-class": {
                    "type": "dynamic-length-blob",
                    "length-field-location": { (4)
                      "origin": "event-record-payload",
                      "path": ["glass", "eagle"]
                    }
                  }
                }
              ]
            }
          }
        ]
      }
    },
    {
      "name": "margin",
      "field-class": {
        "type": "dynamic-length-array", (1)
        "length-field-location": { (2)
          "origin": "event-record-payload",
          "path": ["glass", "eagle"]
        },
        "element-field-class": {
          "type": "null-terminated-string"
        }
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.
3	Possible length field class.
4	Length field location of the dynamic-length BLOB field class.
5	Length field class for the dynamic-length BLOB field class.

Note that both the dynamic-length array and dynamic-length BLOB field classes have the same length field location.

Example 92. Dynamic-length array and its length field in another root field.

Assume the following JSON objects are the event record specific context and payload structure field classes of the same event record class.

The length field of the dynamic-length array field of the event record payload is within the event record specific context.

Event record specific context field class.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "cook",
      "field-class": {
        "type": "fixed-length-floating-point-number",
        "length": 64,
        "byte-order": "little-endian"
      }
    },
    {
      "name": "vegetable", (1)
      "field-class": {
        "type": "variable-length-unsigned-integer"
      }
    }
  ]
}

1	Length member class.

Event record payload field class.

{
  "type": "structure",
  "member-classes": [
    {
      "name": "avenue",
      "field-class": {
        "type": "dynamic-length-array", (1)
        "length-field-location": { (2)
          "origin": "event-record-specific-context",
          "path": ["vegetable"]
        },
        "element-field-class": {
          "type": "null-terminated-string"
        }
      }
    },
    {
      "name": "railroad",
      "field-class": {
        "type": "null-terminated-string"
      }
    }
  ]
}

1	Dynamic-length array field class.
2	Length field location of the dynamic-length array field class.

6.4.3. Fixed-length bit array field decoding procedure

For this whole section about decoding an instance of a fixed-length bit array field class F, let BYTE_ORDER be the value of the byte-order property of F.

To read a single data stream bit from an instance of F:

Let:
- BYTE_I be X / 8 (integral division; remainder discarded).
- BIT_I be:
  
  BYTE_ORDER is "big-endian"
  7 − (X mod 8)
  
  BYTE_ORDER is "little-endian"
  X mod 8
Depending on the value of the bit at the index BIT_I within the byte at the index BYTE_I from the beginning of P (the current packet being decoded):

0
The bit value is false.

1
The bit value is true.

To decode an instance of F:

Let:
1. L be the value of the length property of F.
2. BIT_ORDER be the value of the bit-order property of F.
3. V be an array of booleans of length L.
4. I be an unsigned integer initialized to 0.
Align X according to F.
If X + L ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
If X mod 8 ≠ 0 and LAST_BYTE_ORDER ≠ BYTE_ORDER, then report an error and abort the data stream decoding process.
While I < L:
1. Let DI be an unsigned integer initialized to:
  
  BIT_ORDER is "first-to-last"
  I
  
  BIT_ORDER is "last-to-first"
  L − I − 1
2. Set the element at the index DI of V to the current single bit value.
3. Set I to I + 1.
4. Add one to X.
Set LAST_BYTE_ORDER to BYTE_ORDER.

V is the decoded value.

To add to the decoding procedure above, note that:

The “reading direction” within a data stream byte depends on BYTE_ORDER.
The “filling direction” of V depends on BIT_ORDER.

Assuming X = 0, the following diagrams show which bit is selected by X within a 16-bit fixed-length bit array field depending on BYTE_ORDER and BIT_ORDER:

"big-endian" and "last-to-first"
"big-endian" and "first-to-last"
"little-endian" and "first-to-last"
"little-endian" and "last-to-first"

Example 93. Contiguous fixed-length bit array fields: big-endian versus little-endian.

This example shows the binary layout of contiguous big-endian and little-endian fixed-length bit array fields.

Assume that X = 16. All the fixed-length bit array fields of this example have an implicit one-bit alignment requirement.

Considering the following member classes of some structure field class:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 9,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 14,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "red",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "big-endian"
    }
  }
]

All the fixed-length bit array classes above have an implicit last-to-first bit order.

The binary layout is as such (X indicated in bytes):

Considering the following member classes of some structure field class, the only difference with the previous sample being the values of the byte-order property (and the implicit first-to-last bit order):

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "little-endian"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 9,
      "byte-order": "little-endian"
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 14,
      "byte-order": "little-endian"
    }
  },
  {
    "name": "red",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "little-endian"
    }
  }
]

The binary layout is as such (X indicated in bytes):

Example 94. Contiguous fixed-length bit array fields: first-to-last versus last-to-first bit order.

This example shows the binary layout of contiguous big-endian fixed-length bit array fields using the last-to-first (default) and first-to-last bit order.

Assume that X = 16. All the fixed-length bit array fields of this example have an implicit one-bit alignment requirement.

Considering the following member classes of some structure field class:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 9,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 14,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "red",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "big-endian"
    }
  }
]

All the fixed-length bit array classes above have an implicit last-to-first bit order.

The binary layout is as such (X indicated in bytes):

Considering the following member classes of some structure field class, the only difference with the previous sample being the explicit value of the bit-order property:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 9,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 14,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  },
  {
    "name": "red",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  }
]

The binary layout is as such (X indicated in bytes):

Example 95. Padding between fixed-length bit array fields.

This example shows how the alignment requirement of a fixed-length bit array field can translate into padding bits to skip during the decoding process.

Assume that X = 32.

Considering the following member classes of some structure field class:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 5,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "big-endian",
      "alignment": 8
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "big-endian",
      "bit-order": "first-to-last",
      "alignment": 4
    }
  }
]

The binary layout is as such (X indicated in bytes):

Considering the following member classes of some structure field class, the only difference with the previous sample being the values of the byte-order and bit-order properties:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 5,
      "byte-order": "little-endian",
      "bit-order": "last-to-first"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "little-endian",
      "alignment": 8
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 4,
      "byte-order": "little-endian",
      "bit-order": "last-to-first",
      "alignment": 4
    }
  }
]

The binary layout is as such (X indicated in bytes):

Example 96. Contiguous fixed-length bit array fields with different byte orders.

Step 3 of the decoding procedure above requires that a consumer stops the data stream decoding process if the byte order between two contiguous fixed-length bit array fields changes when X isn’t a multiple of 8.

In other words, a given data stream byte MUST NOT contain bits of two fixed-length bit array fields having different byte orders.

This example shows how contiguous fixed-length bit array fields may have different byte orders with correct alignment.

Assume that X = 16.

Considering the following member classes of some structure field class:

[
  {
    "name": "green",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 3,
      "byte-order": "big-endian",
      "bit-order": "first-to-last"
    }
  },
  {
    "name": "blue",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 5,
      "byte-order": "big-endian"
    }
  },
  {
    "name": "yellow",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 5,
      "byte-order": "little-endian",
      "bit-order": "last-to-first"
    }
  },
  {
    "name": "orange",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 8,
      "byte-order": "little-endian"
    }
  },
  {
    "name": "red",
    "field-class": {
      "type": "fixed-length-bit-array",
      "length": 6,
      "byte-order": "big-endian",
      "bit-order": "first-to-last",
      "alignment": 8
    }
  }
]

The binary layout is as such:

6.4.4. Fixed-length bit map field decoding procedure

To decode an instance of a fixed-length bit map field class:

Follow Fixed-length bit array field decoding procedure.

V is the decoded value.

Example 97. Fixed-length bit map field decoding.

Consider the following fixed-length bit map field class F:

{
  "type": "fixed-length-bit-map",
  "length": 16,
  "byte-order": "big-endian",
  "flags": {
    "RED": [
      [0, 2],
      [4, 4],
      [10, 10]
    ],
    "ORANGE": [[6, 6]],
    "GREEN": [
      [7, 7],
      [9, 9],
      [11, 11]
    ],
    "YELLOW": [
      [9, 9],
      [13, 14]
    ],
    "BLUE": [[15, 15]]
  }
]

Assume that X = 16 to decode the following instance of F:

Because the implicit value of the bit-order property of F is "last-to-first", then the value of element 3 of the resulting bit array is false while the value of element 4 is true.

The resulting bit array with the associated flags is:

The active flags are RED, GREEN, and YELLOW because at least one of their bit is true.

6.4.5. Fixed-length boolean field decoding procedure

To decode an instance of a fixed-length boolean field class:

Let VB be a boolean.
Follow Fixed-length bit array field decoding procedure.
If all the elements of V are false, then set VB to false.

Else, set VB to true.

VB is the decoded boolean value.

6.4.6. Fixed-length unsigned integer field decoding procedure

To decode an instance of a fixed-length unsigned integer field class:

Let VI be an unsigned integer.
Follow Fixed-length bit array field decoding procedure.
Set VI as the unsigned integer interpretation of V, where the first element of V is the least significant bit.

VI is the decoded unsigned integer value.

6.4.7. Fixed-length signed integer field decoding procedure

To decode an instance of a fixed-length signed integer field class:

Let VI be a signed integer.
Follow Fixed-length bit array field decoding procedure.
Set VI as the signed integer interpretation, following the two’s complement format, of V, where the first element of V is the least significant bit.

VI is the decoded signed integer value.

6.4.8. Fixed-length floating point number field decoding procedure

To decode an instance of a fixed-length floating point number field class:

Let VR be a real value.
Follow Fixed-length bit array field decoding procedure.
Set VR to the real number interpretation, following the IEEE 754-2008 binary interchange format, of V, where the first element of V is the least significant bit (least significant bit of the mantissa) and the last element of V is the sign bit.

VR is the decoded real value.

6.4.9. Variable-length unsigned integer field decoding procedure

To decode an instance of a variable-length unsigned integer field class F:

Let:
- B be a byte.
- A be an empty array of bytes.
- VI be an unsigned integer.
Align X according to F.
If X + 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Set B to the byte of P at the offset X.
Add 8 to X.
While bit 7 of B is set:
1. Append B to A.
2. If X + 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
3. Set B to the byte of P at the offset X.
4. Add 8 to X.
Decode A as VI following the unsigned LEB128 (ULEB128) format.

VI is the decoded unsigned integer value.

Example 98. 3-byte variable-length unsigned integer field decoding.

Consider the following variable-length unsigned integer field class F:

{
  "type": "variable-length-unsigned-integer"
}

The following diagram shows the three bytes of an instance of F and the resulting bits.

Note that the data bits of the last byte (in red) become the first elements of the resulting bits because LEB128 is a little-endian encoding.

The decoded unsigned integer value is 1,876,916.

6.4.10. Variable-length signed integer field decoding procedure

To decode an instance of a variable-length signed integer field class F:

Let:
- B be a byte.
- A be an empty array of bytes.
- VI be an unsigned integer.
Align X according to F.
If X + 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Set B to the byte of P at the offset X.
Add 8 to X.
While bit 7 of B is set:
1. Append B to A.
2. If X + 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
3. Set B to the byte of P at the offset X.
4. Add 8 to X.
Decode A as VI following the signed LEB128 format.

VI is the decoded signed integer value.

Example 99. 3-byte variable-length signed integer field decoding.

Consider the following variable-length signed integer field class F:

{
  "type": "variable-length-signed-integer"
}

The following diagram shows the three bytes of an instance of F and the resulting bits.

Note that the data bits of the last byte (in red) become the first elements of the resulting bits because LEB128 is a little-endian encoding.

The decoded signed integer value is −220,236.

6.4.11. Null-terminated string field decoding procedure

To decode an instance of a null-terminated string field class F:

Let:
1. UL be an unsigned integer initialized to, depending on the value of the encoding property of F:
  
  utf-8
  1
  
  utf-16be
  utf-16le
  2
  
  utf-32be
  utf-32le
  4
2. B be an array of UL byte(s).
3. A be an empty sequence of bytes.
4. V be a string.
Align X according to F.
If X + UL × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Set B to the next UL byte(s) of data from P at the offset X.
Add UL × 8 to X.
While at least one bit within the byte(s) of B is set:
1. Append the byte(s) of B to A.
2. If X + UL × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
3. Set B to the next UL byte(s) of data from P at the offset X.
4. Add UL × 8 to X.
Decode A as V following, depending on the value of the encoding property of F:

utf-8
UTF-8.

utf-16be
UTF-16BE (big endian).

utf-16le
UTF-16LE (little endian).

utf-32be
UTF-32BE (big endian).

utf-32le
UTF-32LE (little endian).

V is the decoded string value.

Example 100. 22-byte UTF-8 null-terminated string field decoding.

Consider the following null-terminated string field class F:

{
  "type": "null-terminated-string"
}

Instances of F have an implicit UTF-8 encoding.

The following diagram shows packet bytes including a 22-byte instance of F (in blue) and its resulting string value.

The offset of the null-terminated string field, from the beginning of the packet, is 0xfc23 bytes, which means X = 516,376.

The field contains 22 UTF-8 bytes, including the trailing “NULL” (U+0000) codepoint.

The resulting string value contains 19 Unicode codepoints.

After the field is decoded, X = 516,552.

Example 101. 20-byte UTF-16LE null-terminated string field decoding.

Consider the following null-terminated string field class F:

{
  "type": "null-terminated-string",
  "encoding": "utf-16le"
}

Instances of F have an UTF-16LE encoding.

The following diagram shows packet bytes including a 20-byte instance of F (in blue) and its resulting string value.

The offset of the null-terminated string field, from the beginning of the packet, is 0xfc23 bytes, which means X = 516,376.

The field contains 20 UTF-16LE bytes, including the trailing “NULL” (U+0000) codepoint.

The resulting string value contains nine Unicode codepoints.

After the field is decoded, X = 516,536.

6.4.12. Static-length string field decoding procedure

To decode an instance of a static-length string field class F:

Let:
1. UL be a unsigned integer initialized to, depending on the value of the encoding property of F:
  
  utf-8
  1
  
  utf-16be
  utf-16le
  2
  
  utf-32be
  utf-32le
  4
2. L be the length property of F.
3. B be an array of UL byte(s).
4. A be an empty sequence of bytes.
5. V be a string.
Align X according to F.
If X + L × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Let XI be X.
While X < XI + L × 8:
1. If X + UL × 8 > XI + L × 8, then report an error and abort the data stream decoding process.
2. Set B to the next UL byte(s) of data from P at the offset X.
3. Add UL × 8 to X.
4. If at least one bit of B is set, then
  
  Append the byte(s) of B to A.
  
  Else
  
  Set X to XI + L × 8.
Decode A as V following, depending on the value of the encoding property of F:

utf-8
UTF-8.

utf-16be
UTF-16BE (big endian).

utf-16le
UTF-16LE (little endian).

utf-32be
UTF-32BE (big endian).

utf-32le
UTF-32LE (little endian).

V is the decoded string value.

Example 102. 18-byte UTF-8 static-length string field decoding.

Consider the following static-length string field class F:

{
  "type": "static-length-string",
  "length": 18
}

The following diagram shows packet bytes including a 18-byte instance of F (in green) and its resulting string value.

The offset of the static-length string field, from the beginning of the packet, is 0x8c46 bytes, which means X = 287,280.

The field contains 14 UTF-8 bytes, a null terminating byte, and three garbage data bytes to ignore.

The resulting string value contains seven Unicode codepoints.

After the field is decoded, X = 287,424.

6.4.13. Static-length BLOB field decoding procedure

To decode an instance of a static-length BLOB field class F:

Let:
1. L be the length property of F.
2. V be an array of bytes of length L.
Align X according to F.
If X + L × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Read L bytes of data from P at the offset X as V.
Add L × 8 to X.

V is the decoded BLOB value.

6.4.14. Dynamic-length string field decoding procedure

To decode an instance of a dynamic-length string field class F:

Let:
1. UL be an unsigned integer initialized to, depending on the value of the encoding property of F:
  
  utf-8
  1
  
  utf-16be
  utf-16le
  2
  
  utf-32be
  utf-32le
  4
2. L be the value of the previously decoded unsigned integer field of which the value of the length-field-location property of F indicates the location.
3. B be an array of UL byte(s).
4. A be an empty sequence of bytes.
5. V be a string.
Align X according to F.
If X + L × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Let XI be X.
While X < XI + L × 8:
1. If X + UL × 8 > XI + L × 8, then report an error and abort the data stream decoding process.
2. Set B to the next UL byte(s) of data from P at the offset X.
3. Add UL × 8 to X.
4. If at least one bit of B is set, then
  
  Append the byte(s) of B to A.
  
  Else
  
  Set X to XI + L × 8.
Decode A as V following, depending on the value of the encoding property of F:

utf-8
UTF-8.

utf-16be
UTF-16BE (big endian).

utf-16le
UTF-16LE (little endian).

utf-32be
UTF-32BE (big endian).

utf-32le
UTF-32LE (little endian).

V is the decoded string value.

6.4.15. Dynamic-length BLOB field decoding procedure

To decode an instance of a dynamic-length BLOB field class F:

Let:
1. L be the value of the previously decoded unsigned integer field of which the value of the length-field-location property of F indicates the location.
2. V be an array of bytes of length L.
Align X according to F.
If X + L × 8 ≥ PKT_CONTENT_LEN, then report an error and abort the data stream decoding process.
Read L bytes of data from P at the offset X as V.
Add L × 8 to X.

V is the decoded BLOB value.

6.4.16. Structure field decoding procedure

To decode an instance of a structure field class F:

Let:
- M be the member-classes property of F.
- V be an empty structure.
Align X according to F.
For each member class MC of M:
1. Let:
  - MF be the field-class property of MC.
  - MN be the name property of MC.
2. Decode one instance of MF, appending the resulting value as a member of V named MN.

V is the decoded value.

6.4.17. Static-length array field decoding procedure

To decode an instance of a static-length array field class F:

Let:
1. L be the length property of F.
2. EF be the element-field-class property of F.
3. I be an unsigned integer initialized to 0.
4. V be an array of values of length L.
Align X according to F.
While I < L:
1. Decode one instance of EF as element I of V.
2. Add one to I.

V is the decoded value.

6.4.18. Dynamic-length array field decoding procedure

To decode an instance of a dynamic-length array field class F:

Let:
1. L be the value of the previously decoded unsigned integer field of which the value of the length-field-location property of F indicates the location.
2. EF be the element-field-class property of F.
3. I be an unsigned integer initialized to 0.
4. V be an array of values of length L
Align X according to F.
While I < L:
1. Decode one instance of EF as element I of V.
2. Add one to I.

V is the decoded value.

Example 103. 5-element dynamic-length array field decoding.

Consider the following event record payload structure field class F:

{
  "type": "structure",
  "member-classes": [
    {
      "name": "len",
      "field-class": {
        "type": "fixed-length-unsigned-integer",
        "length": 16,
        "byte-order": "big-endian"
      }
    },
    {
      "name": "id",
      "field-class": {
        "type": "null-terminated-string"
      }
    },
    {
      "name": "vals",
      "field-class": {
        "type":
        "type": "dynamic-length-array",
        "length-field-location": {
          "origin": "event-record-payload",
          "path": ["len"]
        },
        "element-field-class": {
          "type": "fixed-length-unsigned-integer",
          "length": 32,
          "byte-order": "little-endian",
          "alignment": 32
        }
      }
    }
  ]
}

The following diagram shows packet bytes including an instance of F (starting in green and ending in blue) and the resulting signed integer values of its vals member.

The offset of the len fixed-length unsigned integer field, from the beginning of the packet, is 0x42c1 bytes, which means X = 136,712.

The offset of the vals dynamic-length array field, from the beginning of the packet, is 0x42c8 bytes, which means X = 136,768.

The unsigned integer value of the len field is 5, which means the vals field contains five fixed-length signed integer fields.

After the structure field is decoded, X = 136,928.

6.4.19. Optional field decoding procedure

To decode an instance of an optional field class F:

Let:
- SEL be the value of the previously decoded boolean or integer field of which the value of the selector-field-location property of F indicates the location.
- OF be the field-class property of F.
- V be a nil value.
If the class of SEL is a fixed-length boolean field class and SEL is true, then:
- Decode one instance of OF as V.
Else, if SEL is an element of any integer range of the selector-field-ranges property of F, then:
- Decode one instance of OF as V.

V is the decoded value.

6.4.20. Variant field decoding procedure

To decode an instance of an variant field class F:

Let:
1. SEL be the value of the previously decoded integer field of which the value of the selector-field-location property of F indicates the location.
2. OPTS be the options property of F.
3. OF be the field-class property of the variant field class option OPT of OPTS of which SEL is an element of any integer range of the selector-field-ranges property of OPT.
4. V be a value.
Decode one instance of OF as V.

V is the decoded value.

BYTE_ORDER is `"big-endian"`	7 − (X mod 8)
BYTE_ORDER is `"little-endian"`	X mod 8

BIT_ORDER is `"first-to-last"`	I
BIT_ORDER is `"last-to-first"`	L − I − 1