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Introduction

EBML, short for Extensible Binary Meta Language, specifies a binary and octet (byte) aligned format inspired by the principle of XML (a framework for structuring data).

The goal of this document is to define a generic, binary, space-efficient format that can be used to define more complex formats (such as containers for multimedia content) using an EBML Schema. The definition of the EBML format recognizes the idea behind HTML and XML as a good one: separate structure and semantics allowing the same structural layer to be used with multiple, possibly widely differing semantic layers. Except for the EBML Header and a few global elements this specification does not define particular EBML format semantics; however this specification is intended to define how other EBML-based formats can be defined.

EBML uses a simple approach of building Elements upon three pieces of data (tag, length, and value) as this approach is well known, easy to parse, and allows selective data parsing. The EBML structure additionally allows for hierarchical arrangement to support complex structural formats in an efficient manner.

Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [@!RFC2119].

This document defines specific terms in order to define the format and application of EBML. Specific terms are defined below:

Child Element: A Child Element is a relative term to describe the EBML Elements immediately contained within a Master Element.

EBML: Extensible Binary Meta Language

Element Data: The value(s) of the EBML Element which is identified by its Element ID and Element Data Size. The form of the Element Data is defined by this document and the corresponding EBML Schema of the Element's EBML Document Type.

Element Data Size: An expression, encoded as a Variable Size Integer, of the length in octets of Element Data.

EBML Body: All data of an EBML Document following the EBML Header may be considered the EBML Body.

EBML Class: A representation of the octet length of an Element ID.

EBML Document: An EBML Document is a datastream comprised of only two components, an EBML Header and an EBML Body.

EBML Document Type: An EBML Document Type is a name provided by an EBML Schema for a particular implementation of EBML for a data format (examples: matroska and webm).

EBML Element: A foundation block of data that contains three parts: an Element ID, an Element Data Size, and Element Data.

EBML Header: The EBML Header is a declaration that provides processing instructions and identification of the EBML Body. The EBML Header may be considered as analogous to an XML Declaration [@!W3C.REC-xml-20081126] (see section 2.8 on Prolog and Document Type Declaration).

EBML Schema: A standardized definition for the structure of an EBML Document Type.

EBML Stream: An EBML Stream is a file that consists of one or more EBML Documents that are concatenated together.

Element ID: The Element ID is a binary value, encoded as a Variable Size Integer, used to uniquely identify a defined EBML Element within a specific EBML Schema.

Element Name: The official human-readable name of the EBML Element.

Element Path: The hierarchy of Parent Element where the EBML Element is expected to be found in the EBML Body.

Empty Element: An Empty Element is an EBML Element that has an Element Data Size with all VINT_DATA bits set to zero which indicates that the Element Data of the Element is zero octets in length.

Master Element: The Master Element contains zero, one, or many other EBML Elements.

Parent Element: A relative term to describe the Master Element which contains a specified element. For any specified EBML Element that is not at Root Level, the Parent Element refers to the Master Element in which that EBML Element is contained.

Root Element: A mandatory, non-repeating EBML Element which occurs at the top level of the path hierarchy within an EBML Body and contains all other EBML Elements of the EBML Body, excepting optional Void Elements.

Root Level: The starting level in the hierarchy of an EBML Document.

Top-Level Element: An EBML Element defined to only occur as a Child Element of the Root Element.

Unknown-Sized Element: An Element with an unknown Element Data Size.

Variable Size Integer: A compact variable-length binary value which defines its own length.

VINT: Also known as Variable Size Integer.

VINTMAX: The maximum possible value that can be stored as Element Data Size.

Security Considerations

EBML itself does not offer any kind of security and does not provide confidentiality. EBML does not provide any kind of authorization. EBML only offers marginally useful and effective data integrity options, such as CRC elements.

Even if the semantic layer offers any kind of encryption, EBML itself could leak information at both the semantic layer (as declared via the DocType element) and within the EBML structure (you can derive the presence of EBML elements even with an unknown semantic layer with a heuristic approach; not without errors, of course, but with a certain degree of confidence).

Attacks on an EBML Reader could include:

  • Invalid Element IDs that are longer than the limit stated in the EBMLMaxIDLength Element of the EBML Header.
  • Invalid Element IDs that are not encoded in the shortest-possible way.
  • Invalid Element IDs comprised of reserved values.
  • Invalid Element Data Size values that are longer than the limit stated in the EBMLMaxSizeLength Element of the EBML Header.
  • Invalid Element Data Size values (e.g. extending the length of the EBML Element beyond the scope of the Parent Element; possibly triggering access-out-of-bounds issues).
  • Very high lengths in order to force out-of-memory situations resulting in a denial of service, access-out-of-bounds issues etc.
  • Missing EBML Elements that are mandatory and have no declared default value.
  • Usage of 0x00 octets in EBML Elements with a string type.
  • Usage of invalid UTF-8 encoding in EBML Elements of UTF-8 type (e.g. in order to trigger access-out-of-bounds or buffer overflow issues).
  • Usage of invalid data in EBML Elements with a date type.

Side channel attacks could exploit:

  • The semantic equivalence of the same string stored in a String Element or UTF-8 Element with and without zero-bit padding.
  • The semantic equivalence of VINT_DATA within Element Data Size with to different lengths due to left-padding zero bits.
  • Data contained within a Master Element which is not itself part of an EBML Element.
  • Extraneous copies of Identically Recurring Element.
  • Copies of Identically Recurring Element within a Parent Element that contain invalid CRC-32 Elements.
  • Use of Void Elements.

Structure

EBML uses a system of Elements to compose an EBML Document. EBML Elements incorporate three parts: an Element ID, an Element Data Size, and Element Data. The Element Data, which is described by the Element ID, includes either binary data, one or many other EBML Elements, or both.

Variable Size Integer

The Element ID and Element Data Size are both encoded as a Variable Size Integer, developed according to a UTF-8 like system. The Variable Size Integer is composed of a VINT_WIDTH, VINT_MARKER, and VINT_DATA, in that order. Variable Size Integers SHALL left-pad the VINT_DATA value with zero bits so that the whole Variable Size Integer is octet-aligned. Variable Size Integers SHALL be referred to as VINT for shorthand.

VINT_WIDTH

Each Variable Size Integer begins with a VINT_WIDTH which consists of zero or many zero-value bits. The count of consecutive zero-values of the VINT_WIDTH plus one equals the length in octets of the Variable Size Integer. For example, a Variable Size Integer that starts with a VINT_WIDTH which contains zero consecutive zero-value bits is one octet in length and a Variable Size Integer that starts with one consecutive zero-value bit is two octets in length. The VINT_WIDTH MUST only contain zero-value bits or be empty.

Within the EBML Header the VINT_WIDTH MUST NOT exceed three bits in length (meaning that the Variable Size Integer MUST NOT exceed four octets in length). Within the EBML Body, when VINTs are used to express an Element ID, the maximum length allowed for the VINT_WIDTH is one less than the value set in the EBMLMaxIDLength Element. Within the EBML Body, when VINTs are used to express an Element Data Size, the maximum length allowed for the VINT_WIDTH is one less than the value set in the EBMLMaxSizeLength Element.

VINT_MARKER

The VINT_MARKER serves as a separator between the VINT_WIDTH and VINT_DATA. Each Variable Size Integer MUST contain exactly one VINT_MARKER. The VINT_MARKER MUST be one bit in length and contain a bit with a value of one. The first bit with a value of one within the Variable Size Integer is the VINT_MARKER.

VINT_DATA

The VINT_DATA portion of the Variable Size Integer includes all data that follows (but not including) the VINT_MARKER until end of the Variable Size Integer whose length is derived from the VINT_WIDTH. The bits required for the VINT_WIDTH and the VINT_MARKER combined use one out of eight bits of the total length of the Variable Size Integer. Thus a Variable Size Integer of 1 octet length supplies 7 bits for VINT_DATA, a 2 octet length supplies 14 bits for VINT_DATA, and a 3 octet length supplies 21 bits for VINT_DATA. If the number of bits required for VINT_DATA are less than the bit size of VINT_DATA, then VINT_DATA SHOULD be zero-padded to the left to a size that fits. The VINT_DATA value MUST be expressed as a big-endian unsigned integer.

VINT Examples

This table shows examples of Variable Size Integers with lengths from 1 to 5 octets. The Size column refers to the size of the VINT_DATA in bits. The Representation column depicts a binary expression of Variable Size Integers where VINT_WIDTH is depicted by '0', the VINT_MARKER as '1', and the VINT_DATA as 'x'.

Octet Length Size Representation
1 2^7 1xxx xxxx
2 2^14 01xx xxxx xxxx xxxx
3 2^21 001x xxxx xxxx xxxx xxxx xxxx
4 2^28 0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx
5 2^35 0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx

Data encoded as a Variable Size Integer MAY be rendered at octet lengths larger than needed to store the data. In this table a binary value of 0b10 is shown encoded as different Variable Size Integers with lengths from one octet to four octet. All four encoded examples have identical semantic meaning though the VINT_WIDTH and the padding of the VINT_DATA vary.

Binary Value Octet Length As Represented in Variable Size Integer
10 1 1000 0010
10 2 0100 0000 0000 0010
10 3 0010 0000 0000 0000 0000 0010
10 4 0001 0000 0000 0000 0000 0000 0000 0010

Element ID

The Element ID MUST be encoded as a Variable Size Integer. By default, Element IDs are encoded in lengths from one octet to four octets, although Element IDs of greater lengths are used if the octet length of the longest Element ID of the EBML Document is declared in the EBMLMaxIDLength Element of the EBML Header (see the section on the EBMLMaxIDLength Element). The VINT_DATA component of the Element ID MUST NOT be set to either all zero values or all one values. The VINT_DATA component of the Element ID MUST be encoded at the shortest valid length. For example, an Element ID with binary encoding of 1011 1111 is valid, whereas an Element ID with binary encoding of 0100 0000 0011 1111 stores a semantically equal VINT_DATA but is invalid because a shorter VINT encoding is possible. Additionally, an Element ID with binary encoding of 1111 1111 is invalid since the VINT_DATA section is set to all one values, whereas an Element ID with binary encoding of 0100 0000 0111 1111 stores a semantically equal VINT_DATA and is the shortest VINT encoding is possible.

The following table details these specific examples further:

VINT_WIDTH VINT_MARKER VINT_DATA Element ID Status
        | 1            |        0000000 | Invalid: VINT_DATA MUST NOT be set to all 0

0 | 1 | 00000000000000 | Invalid: VINT_DATA MUST NOT be set to all 0 | 1 | 0000001 | Valid 0 | 1 | 00000000000001 | Invalid: A shorter VINT_DATA encoding is available. | 1 | 0111111 | Valid 0 | 1 | 00000000111111 | Invalid: A shorter VINT_DATA encoding is available. | 1 | 1111111 | Invalid: VINT_DATA MUST NOT be set to all 1 0 | 1 | 00000001111111 | Valid

The octet length of an Element ID determines its EBML Class.

EBML Class Octet Length Number of Possible Element IDs
Class A 1 2^7 - 2 = 126
Class B 2 2^14 - 2^7 - 1 = 16,255
Class C 3 2^21 - 2^14 - 1 = 2,080,767
Class D 4 2^28 - 2^21 - 1 = 266,338,303

Element Data Size

The Element Data Size expresses the length in octets of Element Data. The Element Data Size itself MUST be encoded as a Variable Size Integer. By default, Element Data Sizes can be encoded in lengths from one octet to eight octets, although Element Data Sizes of greater lengths MAY be used if the octet length of the longest Element Data Size of the EBML Document is declared in the EBMLMaxSizeLength Element of the EBML Header (see the section on the EBMLMaxSizeLength Element). Unlike the VINT_DATA of the Element ID, the VINT_DATA component of the Element Data Size is not mandated to be encoded at the shortest valid length. For example, an Element Data Size with binary encoding of 1011 1111 or a binary encoding of 0100 0000 0011 1111 are both valid Element Data Sizes and both store a semantically equal value (both 0b00000000111111 and 0b0111111, the VINT_DATA sections of the examples, represent the integer 63).

Although an Element ID with all VINT_DATA bits set to zero is invalid, an Element Data Size with all VINT_DATA bits set to zero is allowed for EBML Element Types which do not mandate a non-zero length (see the section on EBML Element Types). An Element Data Size with all VINT_DATA bits set to zero indicates that the Element Data is zero octets in length. Such an EBML Element is referred to as an Empty Element. If an Empty Element has a default value declared then the EBML Reader MUST interpret the value of the Empty Element as the default value. If an Empty Element has no default value declared then the EBML Reader MUST interpret the value of the Empty Element as defined as part of the definition of the corresponding EBML Element Type associated with the Element ID.

An Element Data Size with all VINT_DATA bits set to one is reserved as an indicator that the size of the EBML Element is unknown. The only reserved value for the VINT_DATA of Element Data Size is all bits set to one. An EBML Element with an unknown Element Data Size is referred to as an Unknown-Sized Element. Only Master Elements SHALL be Unknown-Sized Elements. Master Elements MUST NOT use an unknown size unless the unknownsizeallowed attribute of their EBML Schema is set to true (see the section on the unknownsizeallowed attribute). The use of Unknown-Sized Elements allows for an EBML Element to be written and read before the size of the EBML Element is known. Unknown-Sized Element MUST NOT be used or defined unnecessarily; however if the Element Data Size is not known before the Element Data is written, such as in some cases of data streaming, then Unknown-Sized Elements MAY be used. The end of an Unknown-Sized Element is determined by the beginning of the next element, defined by this document or the corresponding EBML Schema, that is not a valid sub-element of the Unknown-Sized Element.

For Element Data Sizes encoded at octet lengths from one to eight, this table depicts the range of possible values that can be encoded as an Element Data Size. An Element Data Size with an octet length of 8 is able to express a size of 2^56-2 or 72,057,594,037,927,934 octets (or about 72 petabytes). The maximum possible value that can be stored as Element Data Size is referred to as VINTMAX.

Octet Length Possible Value Range
1 0 to 2^7-2
2 0 to 2^14-2
3 0 to 2^21-2
4 0 to 2^28-2
5 0 to 2^35-2
6 0 to 2^42-2
7 0 to 2^49-2
8 0 to 2^56-2

If the length of Element Data equals 2^(n*7)-1 then the octet length of the Element Data Size MUST be at least n+1. This rule prevents an Element Data Size from being expressed as a reserved value. For example, an EBML Element with an octet length of 127 MUST NOT be encoded in an Element Data Size encoding with a one octet length. The following table clarifies this rule by showing a valid and invalid expression of an Element Data Size with a VINT_DATA of 127 (which is equal to 2^(1*7)-1).

VINT_WIDTH VINT_MARKER VINT_DATA Element Data Size Status
        | 1            |        1111111 | Reserved (meaning Unknown)

0 | 1 | 00000001111111 | Valid (meaning 127 octets)

EBML Element Types

EBML Elements are defined by an EBML Schema which MUST declare one of the following EBML Element Types for each EBML Element. An EBML Element Type defines a concept of storing data within an EBML Element that describes such characteristics as length, endianness, and definition.

EBML Elements which are defined as a Signed Integer Element, Unsigned Integer Element, Float Element, or Date Element use big endian storage.

Signed Integer Element

A Signed Integer Element MUST declare a length from zero to eight octets. If the EBML Element is not defined to have a default value, then a Signed Integer Element with a zero-octet length represents an integer value of zero.

A Signed Integer Element stores an integer (meaning that it can be written without a fractional component) which could be negative, positive, or zero. Signed Integers MUST be stored with two's complement notation with the leftmost bit being the sign bit. Because EBML limits Signed Integers to 8 octets in length a Signed Integer Element stores a number from −9,223,372,036,854,775,808 to +9,223,372,036,854,775,807.

Unsigned Integer Element

An Unsigned Integer Element MUST declare a length from zero to eight octets. If the EBML Element is not defined to have a default value, then an Unsigned Integer Element with a zero-octet length represents an integer value of zero.

An Unsigned Integer Element stores an integer (meaning that it can be written without a fractional component) which could be positive or zero. Because EBML limits Unsigned Integers to 8 octets in length an Unsigned Integer Element stores a number from 0 to 18,446,744,073,709,551,615.

Float Element

A Float Element MUST declare a length of either zero octets (0 bit), four octets (32 bit) or eight octets (64 bit). If the EBML Element is not defined to have a default value, then a Float Element with a zero-octet length represents a numerical value of zero.

A Float Element stores a floating-point number as defined in [@!IEEE.754.1985].

String Element

A String Element MUST declare a length in octets from zero to VINTMAX. If the EBML Element is not defined to have a default value, then a String Element with a zero-octet length represents an empty string.

A String Element MUST either be empty (zero-length) or contain printable ASCII characters [@!RFC0020] in the range of 0x20 to 0x7E. Octets with all bits set to zero MAY follow the string value when needed, such as reducing the length of a stored string while maintaining the same Element Data Size. A string with one or more octets with all bits set to zero and a string without one or more octets with all bits set to zero are semantically equal.

UTF-8 Element

A UTF-8 Element MUST declare a length in octets from zero to VINTMAX. If the EBML Element is not defined to have a default value, then a UTF-8 Element with a zero-octet length represents an empty string.

A UTF-8 Element contains only a valid Unicode string as defined in [@!RFC2279]. Octets with all bits set to zero MAY follow the string value when needed, such as reducing the length of a stored UTF-8 data while maintaining the same Element Data Size. A UTF-8 value with one or more octets with all bits set to zero and a UTF-8 value without one or more octets with all bits set to zero are semantically equal.

Date Element

A Date Element MUST declare a length of either zero octets or eight octets. If the EBML Element is not defined to have a default value, then a Date Element with a zero-octet length represents a timestamp of 2001-01-01T00:00:00.000000000 UTC [@!RFC3339].

The Date Element stores an integer in the same format as the Signed Integer Element that expresses a point in time referenced in nanoseconds from the precise beginning of the third millennium of the Gregorian Calendar in Coordinated Universal Time (also known as 2001-01-01T00:00:00.000000000 UTC). This provides a possible expression of time from 1708-09-11T00:12:44.854775808 UTC to 2293-04-11T11:47:16.854775807 UTC.

Master Element

A Master Element MUST declare a length in octets from zero to VINTMAX. The Master Element MAY also use an unknown length. See the section on Element Data Size for rules that apply to elements of unknown length.

The Master Element contains zero, one, or many other elements. EBML Elements contained within a Master Element MUST have the EBMLParentPath of their Element Path equals to the EBMLReferencePath of the Master Element Element Path (see the section on the EBML Path). Element Data stored within Master Elements SHOULD only consist of EBML Elements and SHOULD NOT contain any data that is not part of an EBML Element. When EBML is used in transmission or streaming, data that is not part of an EBML Element is permitted to be present within a Master Element if unknownsizeallowed is enabled within the definition for that Master Element. In this case, the EBML Reader should skip data until a valid Element ID of the same EBMLParentPath or the next upper level Element Path of the Master Element is found. What Element IDs are considered valid within a Master Element is identified by the EBML Schema for that version of the EBML Document Type. Any data contained within a Master Element that is not part of a Child Element MUST be ignored.

Binary Element

A Binary Element MUST declare a length in octets from zero to VINTMAX.

The contents of a Binary Element should not be interpreted by the EBML Reader.

EBML Document

An EBML Document is comprised of only two components, an EBML Header and an EBML Body. An EBML Document MUST start with an EBML Header that declares significant characteristics of the entire EBML Body. An EBML Document consists of EBML Elements and MUST NOT contain any data that is not part of an EBML Element.

EBML Header

The EBML Header is a declaration that provides processing instructions and identification of the EBML Body. The EBML Header of an EBML Document is analogous to the XML Declaration of an XML Document.

The EBML Header documents the EBML Schema (also known as the EBML DocType) that is used to semantically interpret the structure and meaning of the EBML Document. Additionally the EBML Header documents the versions of both EBML and the EBML Schema that were used to write the EBML Document and the versions required to read the EBML Document.

The EBML Header consists of a single Master Element with an Element Name of EBML and Element ID of 0x1A45DFA3 (see the definition of the EBML Element). The EBML Header MUST only contain EBML Elements that are defined as part of this document.

All EBML Elements within the EBML Header MUST NOT use any Element ID with a length greater than 4 octets. All EBML Elements within the EBML Header MUST NOT use any Element Data Size with a length greater than 4 octets.

EBML Body

All data of an EBML Document following the EBML Header is the EBML Body. The end of the EBML Body, as well as the end of the EBML Document that contains the EBML Body, is considered as whichever comes first: the beginning of a new EBML Header at the Root Level or the end of the file. The EBML Body MUST consist only of EBML Elements and MUST NOT contain any data that is not part of an EBML Element. This document defines precisely what EBML Elements are to be used within the EBML Header, but does not name or define what EBML Elements are to be used within the EBML Body. The definition of what EBML Elements are to be used within the EBML Body is defined by an EBML Schema.

EBML Stream

An EBML Stream is a file that consists of one or many EBML Documents that are concatenated together. An occurrence of a EBML Header at the Root Level marks the beginning of an EBML Document.

Elements semantic

EBML Schema

An EBML Schema is an XML Document that defines the properties, arrangement, and usage of EBML Elements that compose a specific EBML Document Type. The relationship of an EBML Schema to an EBML Document may be considered analogous to the relationship of an XML Schema [@?W3C.REC-xmlschema-0-20010502] to an XML Document [@!W3C.REC-xml-20081126]. An EBML Schema MUST be clearly associated with one or many EBML Document Types. An EBML Schema must be expressed as well-formed XML. An EBML Document Type is identified by a string stored within the EBML Header in the DocType Element; for example matroska or webm (see the definition of the DocType Element). The DocType value for an EBML Document Type SHOULD be unique and persistent.

An EBML Schema MUST declare exactly one EBML Element at Root Level (referred to as the Root Element) that MUST occur exactly once within an EBML Document. The Void Element MAY also occur at Root Level but is not considered to be Root Elements (see the definition of the Void Element).

The EBML Schema does not itself document the EBML Header, but documents all data of the EBML Document that follows the EBML Header. The EBML Header itself is documented by this specification in the EBML Header Elements (see EBML Header Elements). The EBML Schema also does not document Global Elements that are defined by this document (namely the Void Element and the CRC-32 Element).

Element

As an XML Document, the EBML Schema MUST use <EBMLSchema> as the top level element. The <EBMLSchema> element MAY contain <element> sub-elements.

Attributes

Within an EBML Schema the <EBMLSchema> element uses the following attributes:

docType

The docType lists the official name of the EBML Document Type that is defined by the EBML Schema; for example, <EBMLSchema docType="matroska">.

The docType attribute is REQUIRED within the <EBMLSchema> Element.

version

The version lists an incremental non-negative integer that specifies the version of the docType documented by the EBML Schema. Unlike XML Schemas, an EBML Schema documents all versions of a docType's definition rather than using separate EBML Schemas for each version of a docType. EBML Elements may be introduced and deprecated by using the minver and maxver attributes of <element>.

The version attribute is REQUIRED within the <EBMLSchema> Element.

Element

Each <element> defines one EBML Element through the use of several attributes that are defined in EBML Schema Element Attributes. EBML Schemas MAY contain additional attributes to extend the semantics but MUST NOT conflict with the definitions of the <element> attributes defined within this document.

The <element> nodes contain a description of the meaning and use of the EBML Element stored within one or many <documentation> sub-elements. All <element> nodes MUST be sub-elements of the <EBMLSchema>.

Attributes

Within an EBML Schema the <element> uses the following attributes to define an EBML Element:

name

The name provides the official human-readable name of the EBML Element. The value of the name MUST be in the form of characters "A" to "Z", "a" to "z", "0" to "9", "-" and ".".

The name attribute is REQUIRED.

path

The path defines the allowed storage locations of the EBML Element within an EBML Document. This path MUST be defined with the full hierarchy of EBML Elements separated with a /. The top EBML Element in the path hierarchy being the first in the value. The syntax of the path attribute is defined using this Augmented Backus-Naur Form (ABNF) [@!RFC5234] with the case sensitive update [@!RFC7405] notation:

The path attribute is REQUIRED.

EBMLFullPath            = EBMLElementOccurrence "(" EBMLReferencePath ")"
EBMLReferencePath       = [EBMLParentPath] EBMLElementPath
EBMLParentPath          = EBMLFixedParent EBMLLastParent
EBMLFixedParent         = *(EBMLPathAtom)
EBMLElementPath         = EBMLPathAtom / EBMLPathAtomRecursive
EBMLPathAtom            = PathDelimiter EBMLAtomName
EBMLPathAtomRecursive   = "(1*(" EBMLPathAtom "))"
EBMLLastParent          = EBMLPathAtom / EBMLVariableParent
EBMLVariableParent      = "(" VariableParentOccurrence "\)"
EBMLAtomName            = 1*(EBMLNameChar)
EBMLNameChar            = ALPHA / DIGIT / "-" / "."
PathDelimiter           = "\"
EBMLElementOccurrence    = [EBMLMinOccurrence] "*" [EBMLMaxOccurrence]
EBMLMinOccurrence        = 1*DIGIT
EBMLMaxOccurrence        = 1*DIGIT
VariableParentOccurrence = [PathMinOccurrence] "*" [PathMaxOccurrence]
PathMinOccurrence        = 1*DIGIT
PathMaxOccurrence        = 1*DIGIT

The "*", "(" and ")" symbols MUST be interpreted as they are defined in the ABNF.

The EBMLPathAtom part of the EBMLElementPath MUST be equal to the name attribute of the EBML Schema.

The starting PathDelimiter of the path corresponds to the root of the EBML Document.

The EBMLElementOccurrence part is interpreted as an ABNF Variable Repetition. The repetition amounts correspond to how many times the EBML Element can be found in its parent Parent Element.

The EBMLMinOccurrence represents the minimum number of occurrences of this EBML Element within its Parent Element. Each instance of the Parent Element MUST contain at least this many instances of this EBML Element. If the EBML Element has an empty EBMLParentPath then EBMLMinOccurrence refers to constraints on the occurrence of the EBML Element within the EBML Document. If EBMLMinOccurrence is not present then that EBML Element is considered to have a EBMLMinOccurrence value of 0. The semantic meaning of EBMLMinOccurrence within an EBML Schema is considered analogous to the meaning of minOccurs within an XML Schema. EBML Elements with EBMLMinOccurrence set to "1" that also have a default value (see default) declared are not REQUIRED to be stored but are REQUIRED to be interpreted, see Note on the Use of default attributes to define Mandatory EBML Elements. An EBML Element defined with a EBMLMinOccurrence value greater than zero is called a Mandatory EBML Element.

The EBMLMaxOccurrence represents the maximum number of occurrences of this EBML Element within its Parent Element. Each instance of the Parent Element MUST contain at most this many instances of this EBML Element. If the EBML Element has an empty EBMLParentPath then EBMLMaxOccurrence refers to constraints on the occurrence of the EBML Element within the EBML Document. If EBMLMaxOccurrence is not present then that EBML Element is considered to have no maximum occurrence. The semantic meaning of EBMLMaxOccurrence within an EBML Schema path is considered analogous to the meaning of maxOccurs within an XML Schema.

The VariableParentOccurrence part is interpreted as an ABNF Variable Repetition. The repetition amounts correspond to the amount of unspecified Parent Element levels there can be between the EBMLFixedParent and the actual EBMLElementPath.

If the path contains a EBMLPathAtomRecursive part, the EBML Element can occur within itself recursively (see the recursive attribute).

id

The Element ID encoded as a Variable Size Integer expressed in hexadecimal notation prefixed by a 0x that is read and stored in big-endian order. To reduce the risk of false positives while parsing EBML Streams, the Element IDs of the Root Element and Top-Level Elements SHOULD be at least 4 octets in length. Element IDs defined for use at Root Level or directly under the Root Level MAY use shorter octet lengths to facilitate padding and optimize edits to EBML Documents; for instance, the Void Element uses an Element ID with a one octet length to allow its usage in more writing and editing scenarios.

The id attribute is REQUIRED.

minOccurs

An integer expressing the minimum number of occurrences of this EBML Element within its Parent Element. The minOccurs value MUST be equal to the EBMLMinOccurrence value of the path.

The minOccurs attribute is OPTIONAL. If the minOccurs attribute is not present then that EBML Element is considered to have a minOccurs value of 0.

maxOccurs

An integer expressing the maximum number of occurrences of this EBML Element within its Parent Element. The maxOccurs value MUST be equal to the EBMLMaxOccurrence value of the path.

The maxOccurs attribute is OPTIONAL. If the maxOccurs attribute is not present then that EBML Element is considered to have no maximum occurrence, similar to unbounded in the XML world.

range

A numerical range for EBML Elements which are of numerical types (Unsigned Integer, Signed Integer, Float, and Date). If specified the value of the EBML Element MUST be within the defined range. See section of Expressions of range for rules applied to expression of range values.

The range attribute is OPTIONAL. If the range attribute is not present then any value legal for the type attribute is valid.

size

A value to express the valid length of the Element Data as written measured in octets. The size provides a constraint in addition to the Length value of the definition of the corresponding EBML Element Type. This size MUST be expressed as either a non-negative integer or a range (see expression of range) that consists of only non-negative integers and valid operators.

The size attribute is OPTIONAL. If the size attribute is not present for that EBML Element then that EBML Element is only limited in size by the definition of the associated EBML Element Type.

default

If an Element is mandatory (has a EBMLMinOccurrence value greater than zero) but not written within its Parent Element or stored as an Empty Element, then the EBML Reader of the EBML Document MUST semantically interpret the EBML Element as present with this specified default value for the EBML Element. EBML Elements that are Master Elements MUST NOT declare a default value. EBML Elements with a minOccurs value greater than 1 MUST NOT declare a default value.

The default attribute is OPTIONAL.

type

The type MUST be set to one of the following values: 'integer' (signed integer), 'uinteger' (unsigned integer), 'float', 'string', 'date', 'utf-8', 'master', or 'binary'. The content of each type is defined within section on EBML Element Types.

The type attribute is REQUIRED.

unknownsizeallowed

A boolean to express if an EBML Element MAY be used as an Unknown-Sized Element (having all VINT_DATA bits of Element Data Size set to 1). EBML Elements that are not Master Elements MUST NOT set unknownsizeallowed to true. An EBML Element that is defined with an unknownsizeallowed attribute set to 1 MUST also have the unknownsizeallowed attribute of its Parent Element set to 1.

The unknownsizeallowed attribute is OPTIONAL. If the unknownsizeallowed attribute is not used then that EBML Element is not allowed to use an unknown Element Data Size.

recursive

A boolean to express if an EBML Element MAY be stored recursively. In this case the EBML Element MAY be stored within another EBML Element that has the same Element ID. Which itself can be stored in an EBML Element that has the same Element ID, and so on. EBML Elements that are not Master Elements MUST NOT set recursive to true.

If the path contains a EBMLPathAtomRecursive part then the recursive value MUST be true and false otherwise.

The recursive attribute is OPTIONAL. If the recursive attribute is not present then the EBML Element MUST NOT be used recursively.

minver

The minver (minimum version) attribute stores a non-negative integer that represents the first version of the docType to support the EBML Element.

The minver attribute is OPTIONAL. If the minver attribute is not present then the EBML Element has a minimum version of "1".

maxver

The maxver (maximum version) attribute stores a non-negative integer that represents the last or most recent version of the docType to support the element. maxver MUST be greater than or equal to minver.

The maxver attribute is OPTIONAL. If the maxver attribute is not present then the EBML Element has a maximum version equal to the value stored in the version attribute of .

Element

The <documentation> element provides additional information about the EBML Element.

Attributes

lang

A lang attribute which is set to the [@!RFC5646] value of the language of the element's documentation.

The lang attribute is OPTIONAL.

type

A type attribute distinguishes the meaning of the documentation. Values for the <documentation> sub-element's type attribute MUST include one of the following: definition, rationale, usage notes, and references.

The type attribute is OPTIONAL.

XML Schema for EBML Schema

<{{EBMLSchema.xsd}}

EBML Schema Example

<{{ebml_schema_example.xml}}

Identically Recurring Elements

An Identically Recurring Element is an EBML Element that MAY occur within its Parent Element more than once but that each recurrence within that Parent Element MUST be identical both in storage and semantics. Identically Recurring Elements are permitted to be stored multiple times within the same Parent Element in order to increase data resilience and optimize the use of EBML in transmission. For instance a pertinent Top-Level Element could be periodically resent within a data stream so that an EBML Reader which starts reading the stream from the middle could better interpret the contents. Identically Recurring Elements SHOULD include a CRC-32 Element as a Child Element; this is especially recommended when EBML is used for long-term storage or transmission. If a Parent Element contains more than one copy of an Identically Recurring Element which includes a CRC-32 Element as a Child Element then the first instance of the Identically Recurring Element with a valid CRC-32 value should be used for interpretation. If a Parent Element contains more than one copy of an Identically Recurring Element which does not contain a CRC-32 Element or if CRC-32 Elements are present but none are valid then the first instance of the Identically Recurring Element should be used for interpretation.

Expression of range

The range attribute MUST only be used with EBML Elements that are either signed integer, unsigned integer, float, or date. The range expression may contain whitespace for readability but whitespace within a range expression MUST NOT convey meaning. The expression of the range MUST adhere to one of the following forms:

  • x-y where x and y are integers or floats and y MUST be greater than x, meaning that the value MUST be greater than or equal to x and less than or equal to y. x MUST be less than y.
  • >x where x is an integer or float, meaning that the value MUST be greater than x.
  • >=x where x is an integer or float, meaning that the value MUST be greater than or equal to x.
  • <x where x is an integer or float, meaning that the value MUST be less than x.
  • <=x where x is an integer or float, meaning that the value MUST be less than or equal to x.
  • x where x is an integer or float, meaning that the value MUST be equal x.

The range may use the prefix not to indicate that the expressed range is negated. Please also see textual expression of floats.

Textual expression of Floats

When a float value is represented textually in an EBML Schema, such as within a default or range value, the float values MUST be expressed as Hexadecimal Floating-Point Constants as defined in the C11 standard [@!ISO.9899.2011] (see section 6.4.4.2 on Floating Constants). The following table provides examples of expressions of float ranges.

as decimal as Hexadecimal Floating-Point Constants
0.0-1.0 0x0p+1-0x1p+0
1.0-256.0 0x1p+0-0x1p+8
0.857421875 0x1.b7p-1
-1.0--0.857421875 -0x1p+0--0x1.b7p-1

Within an expression of a float range, as in an integer range, the - (hyphen) character is the separator between the minimal and maximum value permitted by the range. Hexadecimal Floating-Point Constants also use a - (hyphen) when indicating a negative binary power. Within a float range, when a - (hyphen) is immediately preceded by a letter p, then the - (hyphen) is a part of the Hexadecimal Floating-Point Constant which notes negative binary power. Within a float range, when a - (hyphen) is not immediately preceded by a letter p, then the - (hyphen) represents the separator between the minimal and maximum value permitted by the range.

Note on the Use of default attributes to define Mandatory EBML Elements

If a Mandatory EBML Element has a default value declared by an EBML Schema and the value of the EBML Element is equal to the declared default value then that EBML Element is not required to be present within the EBML Document if its Parent Element is present. In this case, the default value of the Mandatory EBML Element MUST be interpreted by the EBML Reader although the EBML Element is not present within its Parent Element.

If a Mandatory EBML Element has no default value declared by an EBML Schema and its Parent Element is present then the EBML Element MUST be present as well. If a Mandatory EBML Element has a default value declared by an EBML Schema and its Parent Element is present and the value of the EBML Element is NOT equal to the declared default value then the EBML Element MUST be present.

This table clarifies if a Mandatory EBML Element MUST be written, according to if the default value is declared, if the value of the EBML Element is equal to the declared default value, and if the Parent Element is used.

Is the default value declared? Is the value equal to default? Is the Parent Element present? Then is storing the EBML Element REQUIRED?
Yes Yes Yes No
Yes Yes No No
Yes No Yes Yes
Yes No No No
No n/a Yes Yes
No n/a No No

EBML Header Elements

This document contains definitions of all EBML Elements of the EBML Header.

EBML Element

name: EBML

path: 1*1(\EBML)

id: 0x1A45DFA3

minOccurs: 1

maxOccurs: 1

type: Master Element

description: Set the EBML characteristics of the data to follow. Each EBML Document has to start with this.

EBMLVersion Element

name: EBMLVersion

path: 1*1(\EBML\EBMLVersion)

id 0x4286

minOccurs: 1

maxOccurs: 1

range: not 0

default: 1

type: Unsigned Integer

description: The version of EBML specifications used to create the EBML Document. The version of EBML defined in this document is 1, so EBMLVersion SHOULD be 1.

EBMLReadVersion Element

name: EBMLReadVersion

path: 1*1(\EBML\EBMLReadVersion)

id: 0x42F7

minOccurs: 1

maxOccurs: 1

range: 1

default: 1

type: Unsigned Integer

description: The minimum EBML version an EBML Reader has to support to read this EBML Document. The EBMLReadVersion Element MUST be less than or equal to EBMLVersion.

EBMLMaxIDLength Element

name: EBMLMaxIDLength

path: 1*1(\EBML\EBMLMaxIDLength)

id 0x42F2

minOccurs: 1

maxOccurs: 1

range: >=4

default: 4

type: Unsigned Integer

description: The EBMLMaxIDLength Element stores the maximum length in octets of the Element IDs to be found within the EBML Body. An EBMLMaxIDLength Element value of four is RECOMMENDED, though larger values are allowed.

EBMLMaxSizeLength Element

name: EBMLMaxSizeLength

path: 1*1(\EBML\EBMLMaxSizeLength)

id 0x42F3

minOccurs: 1

maxOccurs: 1

range: not 0

default: 8

type: Unsigned Integer

description: The EBMLMaxSizeLength Element stores the maximum length in octets of the expression of all Element Data Sizes to be found within the EBML Body. To be clear the EBMLMaxSizeLength Element documents the maximum 'length' of all Element Data Size expressions within the EBML Body and not the maximum 'value' of all Element Data Size expressions within the EBML Body. EBML Elements that have an Element Data Size expression which is larger in octets than what is expressed by EBMLMaxSizeLength ELEMENT SHALL be considered invalid.

DocType Element

name: DocType

path: 1*1(\EBML\DocType)

id 0x4282

minOccurs: 1

maxOccurs: 1

size: >0

type: String

description: A string that describes and identifies the content of the EBML Body that follows this EBML Header.

DocTypeVersion Element

name: DocTypeVersion

path: 1*1(\EBML\DocTypeVersion)

id 0x4287

minOccurs: 1

maxOccurs: 1

default: 1

type: Unsigned Integer

description: The version of DocType interpreter used to create the EBML Document.

DocTypeReadVersion Element

name: DocTypeReadVersion

path: 1*1(\EBML\DocTypeReadVersion)

id 0x4285

minOccurs: 1

maxOccurs: 1

default: 1

type: Unsigned Integer

description: The minimum DocType version an EBML Reader has to support to read this EBML Document. The value of the DocTypeReadVersion Element MUST be less than or equal to the value of the DocTypeVersion Element.

Global elements (used everywhere in the format)

name: CRC-32

path: *1((1*\)\CRC-32)

id: 0xBF

minOccurs: 0

maxOccurs: 1

size: 4

type: Binary

description: The CRC-32 Element contains a 32-bit Cyclic Redundancy Check value of all the Element Data of the Parent Element as stored except for the CRC-32 Element itself. When the CRC-32 Element is present, the CRC-32 Element MUST be the first ordered EBML Element within its Parent Element for easier reading. All Top-Level Elements of an EBML Document that are Master Elements SHOULD include a CRC-32 Element as a Child Element. The CRC in use is the IEEE-CRC-32 algorithm as used in the [@!ISO.3309.1979] standard and in section 8.1.1.6.2 of [@!ITU.V42.1994], with initial value of 0xFFFFFFFF. The CRC value MUST be computed on a little endian bitstream and MUST use little endian storage.

Void Element

name: Void

path: *((*\)\Void)

id: 0xEC

minOccurs: 0

type: Binary

description: Used to void damaged data, to avoid unexpected behaviors when using damaged data. The content is discarded. Also used to reserve space in a sub-element for later use.

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