Internet Engineering Task Force (IETF) S. Lhomme
Request for Comments: 8794
Category: Standards Track D. Rice
ISSN: 2070-1721
M. Bunkus
July 2020
Extensible Binary Meta Language
Abstract
This document defines the Extensible Binary Meta Language (EBML)
format as a binary container format designed for audio/video storage.
EBML is designed as a binary equivalent to XML and uses a storage-
efficient approach to build nested Elements with identifiers,
lengths, and values. Similar to how an XML Schema defines the
structure and semantics of an XML Document, this document defines how
EBML Schemas are created to convey the semantics of an EBML Document.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8794.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Notation and Conventions
3. Structure
4. Variable-Size Integer
4.1. VINT_WIDTH
4.2. VINT_MARKER
4.3. VINT_DATA
4.4. VINT Examples
5. Element ID
6. Element Data Size
6.1. Data Size Format
6.2. Unknown Data Size
6.3. Data Size Values
7. EBML Element Types
7.1. Signed Integer Element
7.2. Unsigned Integer Element
7.3. Float Element
7.4. String Element
7.5. UTF-8 Element
7.6. Date Element
7.7. Master Element
7.8. Binary Element
8. EBML Document
8.1. EBML Header
8.2. EBML Body
9. EBML Stream
10. EBML Versioning
10.1. EBML Header Version
10.2. EBML Document Version
11. Elements semantics
11.1. EBML Schema
11.1.1. EBML Schema Example
11.1.2. "<EBMLSchema>" Element
11.1.3. "<EBMLSchema>" Namespace
11.1.4. "<EBMLSchema>" Attributes
11.1.5. "<element>" Element
11.1.6. "<element>" Attributes
11.1.7. "<documentation>" Element
11.1.8. "<documentation>" Attributes
11.1.9. "<implementation_note>" Element
11.1.10. "<implementation_note>" Attributes
11.1.11. "<restriction>" Element
11.1.12. "<enum>" Element
11.1.13. "<enum>" Attributes
11.1.14. "<extension>" Element
11.1.15. "<extension>" Attributes
11.1.16. XML Schema for EBML Schema
11.1.17. Identically Recurring Elements
11.1.18. Textual expression of floats
11.1.19. Note on the use of default attributes to define
Mandatory EBML Elements
11.2. EBML Header Elements
11.2.1. EBML Element
11.2.2. EBMLVersion Element
11.2.3. EBMLReadVersion Element
11.2.4. EBMLMaxIDLength Element
11.2.5. EBMLMaxSizeLength Element
11.2.6. DocType Element
11.2.7. DocTypeVersion Element
11.2.8. DocTypeReadVersion Element
11.2.9. DocTypeExtension Element
11.2.10. DocTypeExtensionName Element
11.2.11. DocTypeExtensionVersion Element
11.3. Global Elements
11.3.1. CRC-32 Element
11.3.2. Void Element
12. Considerations for Reading EBML Data
13. Terminating Elements
14. Guidelines for Updating Elements
14.1. Reducing Element Data in Size
14.1.1. Adding a Void Element
14.1.2. Extending the Element Data Size
14.1.3. Terminating Element Data
14.2. Considerations when Updating Elements with Cyclic
Redundancy Check (CRC)
15. Backward and Forward Compatibility
15.1. Backward Compatibility
15.2. Forward Compatibility
16. Security Considerations
17. IANA Considerations
17.1. EBML Element IDs Registry
17.2. EBML DocTypes Registry
18. Normative References
19. Informative References
Authors' Addresses
1. Introduction
EBML, short for Extensible Binary Meta Language, specifies a binary
format aligned with octets (bytes) and 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
using an EBML Schema. EBML is used by the multimedia container,
Matroska [Matroska]. The applicability of EBML for other use cases
is beyond the scope of this document.
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, such as the audio/video
container formats Matroska and WebM [WebM].
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.
A typical EBML file has the following structure:
EBML Header (master)
+ DocType (string)
+ DocTypeVersion (unsigned integer)
EBML Body Root (master)
+ ElementA (utf-8)
+ Parent (master)
+ ElementB (integer)
+ Parent (master)
+ ElementB (integer)
2. Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document defines specific terms in order to define the format
and application of "EBML". Specific terms are defined below:
"EBML": Extensible Binary Meta Language
"EBML Document Type": A name provided by an "EBML Schema" to
designate a particular implementation of "EBML" for a data format
(e.g., Matroska and WebM).
"EBML Schema": A standardized definition for the structure of an
"EBML Document Type".
"EBML Document": A datastream comprised of only two components, an
"EBML Header" and an "EBML Body".
"EBML Reader": A data parser that interprets the semantics of an
"EBML Document" and creates a way for programs to use "EBML".
"EBML Stream": A file that consists of one or more "EBML Documents"
that are concatenated together.
"EBML Header": A declaration that provides processing instructions
and identification of the "EBML Body". The "EBML Header" is
analogous to an XML Declaration [XML] (see Section 2.8 on "Prolog
and Document Type Declaration").
"EBML Body": All data of an "EBML Document" following the "EBML
Header".
"Variable-Size Integer": A compact variable-length binary value that
defines its own length.
"VINT": Also known as "Variable-Size Integer".
"EBML Element": A foundation block of data that contains three
parts: an "Element ID", an "Element Data Size", and "Element
Data".
"Element ID": A binary value, encoded as a "Variable-Size Integer",
used to uniquely identify a defined "EBML Element" within a
specific "EBML Schema".
"Element Data Size": An expression, encoded as a "Variable-Size
Integer", of the length in octets of "Element Data".
"VINTMAX": The maximum possible value that can be stored as "Element
Data Size".
"Unknown-Sized Element": An "Element" with an unknown "Element Data
Size".
"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".
"Root Level": The starting level in the hierarchy of an "EBML
Document".
"Root Element": A mandatory, nonrepeating "EBML Element" that 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".
"Top-Level Element": An "EBML Element" defined to only occur as a
"Child Element" of the "Root Element".
"Master Element": The "Master Element" contains zero, one, or many
other "EBML Elements".
"Child Element": A "Child Element" is a relative term to describe
the "EBML Elements" immediately contained within a "Master
Element".
"Parent Element": A relative term to describe the "Master Element"
that 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 directly
contained.
"Descendant Element": A relative term to describe any "EBML
Elements" contained within a "Master Element", including any of
the "Child Elements" of its "Child Elements", and so on.
"Void Element": An "Element" used to overwrite data or reserve space
within a "Master Element" for later use.
"Element Name": The 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 "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.
3. 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 more other EBML
Elements, or both.
4. Variable-Size Integer
The Element ID and Element Data Size are both encoded as a Variable-
Size Integer. The Variable-Size Integer is composed of a VINT_WIDTH,
VINT_MARKER, and VINT_DATA, in that order. Variable-Size Integers
MUST left-pad the VINT_DATA value with zero bits so that the whole
Variable-Size Integer is octet aligned. The Variable-Size Integer
will be referred to as VINT for shorthand.
4.1. VINT_WIDTH
Each Variable-Size Integer starts with a VINT_WIDTH followed by a
VINT_MARKER. VINT_WIDTH is a sequence of zero or more bits of value
"0" and is terminated by the VINT_MARKER, which is a single bit of
value "1". The total length in bits of both VINT_WIDTH and
VINT_MARKER is the total length in octets in of the Variable-Size
Integer.
The single bit "1" starts a Variable-Size Integer with a length of
one octet. The sequence of bits "01" starts a Variable-Size Integer
with a length of two octets. "001" starts a Variable-Size Integer
with a length of three octets, and so on, with each additional "0"
bit adding one octet to the length of the Variable-Size Integer.
4.2. 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 is 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.
4.3. VINT_DATA
The VINT_DATA portion of the Variable-Size Integer includes all data
following (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 use one out
of every 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 is less than the bit size of VINT_DATA, then
VINT_DATA MUST be zero-padded to the left to a size that fits. The
VINT_DATA value MUST be expressed as a big-endian unsigned integer.
4.4. VINT Examples
Table 1 shows examples of Variable-Size Integers with lengths from 1
to 5 octets. The "Usable Bits" column refers to the number of bits
that can be used in the VINT_DATA. 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 | Usable Bits | Representation |
+==============+=============+===============================+
| 1 | 7 | 1xxx xxxx |
+--------------+-------------+-------------------------------+
| 2 | 14 | 01xx xxxx xxxx xxxx |
+--------------+-------------+-------------------------------+
| 3 | 21 | 001x xxxx xxxx xxxx xxxx xxxx |
+--------------+-------------+-------------------------------+
| 4 | 28 | 0001 xxxx xxxx xxxx xxxx xxxx |
| | | xxxx xxxx |
+--------------+-------------+-------------------------------+
| 5 | 35 | 0000 1xxx xxxx xxxx xxxx xxxx |
| | | xxxx xxxx xxxx xxxx |
+--------------+-------------+-------------------------------+
Table 1: VINT examples depicting usable bits
A Variable-Size Integer may be rendered at octet lengths larger than
needed to store the data in order to facilitate overwriting it at a
later date -- e.g., when its final size isn't known in advance. In
Table 2, an integer "2" (with a corresponding binary value of 0b10)
is shown encoded as different Variable-Size Integers with lengths
from one octet to four octets. All four encoded examples have
identical semantic meaning, though the VINT_WIDTH and the padding of
the VINT_DATA vary.
+=========+==============+=====================+====================+
| Integer | Octet | As Represented in | As Represented in |
| | Length | VINT (binary) | VINT (hexadecimal) |
+=========+==============+=====================+====================+
| 2 | 1 | 1000 0010 | 0x82 |
+---------+--------------+---------------------+--------------------+
| 2 | 2 | 0100 0000 0000 0010 | 0x4002 |
+---------+--------------+---------------------+--------------------+
| 2 | 3 | 0010 0000 0000 0000 | 0x200002 |
| | | 0000 0010 | |
+---------+--------------+---------------------+--------------------+
| 2 | 4 | 0001 0000 0000 0000 | 0x10000002 |
| | | 0000 0000 0000 0010 | |
+---------+--------------+---------------------+--------------------+
Table 2: VINT examples depicting the same integer value rendered
at different VINT lengths
5. Element ID
An Element ID is a Variable-Size Integer. By default, Element IDs
are from one octet to four octets in length, although Element IDs of
greater lengths MAY be used if the EBMLMaxIDLength Element of the
EBML Header is set to a value greater than four (see Section 11.2.4).
The bits of the VINT_DATA component of the Element ID MUST NOT be all
"0" values or all "1" 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-possible VINT encoding.
Table 3 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 |
+------------+-------------+----------------+--------------------+
Table 3: Examples of valid and invalid Element IDs
The range and count of possible Element IDs are determined by their
octet length. Examples of this are provided in Table 4.
+=========================+================+=================+
| Element ID Octet Length | Range of Valid | Number of Valid |
| | Element IDs | Element IDs |
+=========================+================+=================+
| 1 | 0x81 - 0xFE | 126 |
+-------------------------+----------------+-----------------+
| 2 | 0x407F - | 16,256 |
| | 0x7FFE | |
+-------------------------+----------------+-----------------+
| 3 | 0x203FFF - | 2,080,768 |
| | 0x3FFFFE | |
+-------------------------+----------------+-----------------+
| 4 | 0x101FFFFF - | 268,338,304 |
| | 0x1FFFFFFE | |
+-------------------------+----------------+-----------------+
Table 4: Examples of count and range for Element IDs at
various octet lengths
6. Element Data Size
6.1. Data Size Format
The Element Data Size expresses the length in octets of Element Data.
The Element Data Size itself is 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 Section 11.2.5). 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 that do not mandate a nonzero length
(see Section 7). 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 use the value of the Empty Element for the corresponding EBML
Element Type of the Element ID, 0 for numbers and an empty string for
strings.
6.2. Unknown Data Size
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 a Master Element is
allowed to be of unknown size, and it can only be so if the
"unknownsizeallowed" attribute of its EBML Schema is set to true (see
Section 11.1.6.10).
The use of Unknown-Sized Elements allows an EBML Element to be
written and read before the size of the EBML Element is known.
Unknown-Sized Elements MUST only be used if the Element Data Size is
not known before the Element Data is written, such as in some cases
of datastreaming. The end of an Unknown-Sized Element is determined
by whichever comes first:
* Any EBML Element that is a valid Parent Element of the Unknown-
Sized Element according to the EBML Schema, Global Elements
excluded.
* Any valid EBML Element according to the EBML Schema, Global
Elements excluded, that is not a Descendant Element of the
Unknown-Sized Element but shares a common direct parent, such as a
Top-Level Element.
* Any EBML Element that is a valid Root Element according to the
EBML Schema, Global Elements excluded.
* The end of the Parent Element with a known size has been reached.
* The end of the EBML Document, either when reaching the end of the
file or because a new EBML Header started.
Consider an Unknown-Sized Element whose EBML path is
"\root\level1\level2\elt". When reading a new Element ID, assuming
the EBML Path of that new Element is valid, here are some possible
and impossible ways that this new Element is ending "elt":
+==================================+============================+
| EBML Path of new element | Status |
+==================================+============================+
| \root\level1\level2 | Ends the Unknown-Sized |
| | Element, as it is a new |
| | Parent Element |
+----------------------------------+----------------------------+
| \root\level1 | Ends the Unknown-Sized |
| | Element, as it is a new |
| | Parent Element |
+----------------------------------+----------------------------+
| \root | Ends the Unknown-Sized |
| | Element, as it is a new |
| | Root Element |
+----------------------------------+----------------------------+
| \root2 | Ends the Unknown-Sized |
| | Element, as it is a new |
| | Root Element |
+----------------------------------+----------------------------+
| \root\level1\level2\other | Ends the Unknown-Sized |
| | Element, as they share the |
| | same parent |
+----------------------------------+----------------------------+
| \root\level1\level2\elt | Ends the Unknown-Sized |
| | Element, as they share the |
| | same parent |
+----------------------------------+----------------------------+
| \root\level1\level2\elt\inside | Doesn't end the Unknown- |
| | Sized Element; it's a |
| | child of "elt" |
+----------------------------------+----------------------------+
| \root\level1\level2\elt\<global> | Global Element is valid; |
| | it's a child of "elt" |
+----------------------------------+----------------------------+
| \root\level1\level2\<global> | Global Element cannot be |
| | interpreted with this |
| | path; while parsing "elt", |
| | a Global Element can only |
| | be a child of "elt" |
+----------------------------------+----------------------------+
Table 5: Examples of determining the end of an Unknown-Sized
Element
6.3. Data Size Values
For Element Data Sizes encoded at octet lengths from one to eight,
Table 6 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 |
+--------------+----------------------+
Table 6: Possible range of values
that can be stored in VINTs, by
octet length
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 the unknown-size value.
Table 7 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) and 16,383 (which is equal to 2^(2*7)-1).
+============+=============+=======================+==============+
| VINT_WIDTH | VINT_MARKER | VINT_DATA | Element Data |
| | | | Size Status |
+============+=============+=======================+==============+
| | 1 | 1111111 | Reserved |
| | | | (meaning |
| | | | Unknown) |
+------------+-------------+-----------------------+--------------+
| 0 | 1 | 00000001111111 | Valid |
| | | | (meaning 127 |
| | | | octets) |
+------------+-------------+-----------------------+--------------+
| 00 | 1 | 000000000000001111111 | Valid |
| | | | (meaning 127 |
| | | | octets) |
+------------+-------------+-----------------------+--------------+
| 0 | 1 | 11111111111111 | Reserved |
| | | | (meaning |
| | | | Unknown) |
+------------+-------------+-----------------------+--------------+
| 00 | 1 | 000000011111111111111 | Valid |
| | | | (16,383 |
| | | | octets) |
+------------+-------------+-----------------------+--------------+
Table 7: Demonstration of VINT_DATA reservation for VINTs of
unknown size
7. EBML Element Types
EBML Elements are defined by an EBML Schema (see Section 11.1), 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 that are defined as a Signed Integer Element, Unsigned
Integer Element, Float Element, or Date Element use big-endian
storage.
7.1. 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) that could be negative,
positive, or zero. Signed Integers are 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.
7.2. 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) that 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.
7.3. 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 in the 32-bit and
64-bit binary interchange format, as defined in [IEEE.754].
7.4. 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,
with an exception made for termination (see Section 13).
7.5. 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
[RFC3629], with an exception made for termination (see Section 13).
7.6. 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.
7.7. Master Element
A Master Element MUST declare a length in octets from zero to VINTMAX
or be of unknown length. See Section 6 for rules that apply to
elements of unknown length.
The Master Element contains zero or more other elements. EBML
Elements contained within a Master Element MUST have the
EBMLParentPath of their Element Path equal to the EBMLFullPath of the
Master Element Element Path (see Section 11.1.6.2). 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.
The EBML Schema identifies what Element IDs are valid within the
Master Elements 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.
7.8. 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.
8. EBML Document
An EBML Document is composed 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.
8.1. 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 MUST contain a single Master Element with an Element
Name of "EBML" and Element ID of "0x1A45DFA3" (see Section 11.2.1);
the Master Element may have any number of additional EBML Elements
within it. The EBML Header of an EBML Document that uses an
EBMLVersion of 1 MUST only contain EBML Elements that are defined as
part of this document.
Elements within an EBML Header can be at most 4 octets long, except
for the EBML Element with Element Name "EBML" and Element ID
"0x1A45DFA3" (see Section 11.2.1); this Element can be up to 8 octets
long.
8.2. 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 reached at whichever comes
first: the beginning of a new EBML Header at the Root Level or the
end of the file. This document defines precisely which EBML Elements
are to be used within the EBML Header but does not name or define
which EBML Elements are to be used within the EBML Body. The
definition of which EBML Elements are to be used within the EBML Body
is defined by an EBML Schema.
Within the EBML Body, the maximum octet length allowed for any
Element ID is set by the EBMLMaxIDLength Element of the EBML Header,
and the maximum octet length allowed for any Element Data Size is set
by the EBMLMaxSizeLength Element of the EBML Header.
9. EBML Stream
An EBML Stream is a file that consists of one or more EBML Documents
that are concatenated together. An occurrence of an EBML Header at
the Root Level marks the beginning of an EBML Document.
10. EBML Versioning
An EBML Document handles 2 different versions: the version of the
EBML Header and the version of the EBML Body. Both versions are
meant to be backward compatible.
10.1. EBML Header Version
The version of the EBML Header is found in EBMLVersion. An EBML
parser can read an EBML Header if it can read either the EBMLVersion
version or a version equal or higher than the one found in
EBMLReadVersion.
10.2. EBML Document Version
The version of the EBML Body is found in EBMLDocTypeVersion. A
parser for the particular DocType format can read the EBML Document
if it can read either the EBMLDocTypeVersion version of that format
or a version equal or higher than the one found in
EBMLDocTypeReadVersion.
11. Elements semantics
11.1. EBML Schema
An EBML Schema is a well-formed XML Document [XML] 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 is analogous to the relationship of an XML Schema
[XML-SCHEMA] to an XML Document [XML]. An EBML Schema MUST be
clearly associated with one or more EBML Document Types. An EBML
Document Type is identified by a string stored within the EBML Header
in the DocType Element -- for example, Matroska or WebM (see
Section 11.2.6). The DocType value for an EBML Document Type MUST be
unique, persistent, and described in the IANA registry (see
Section 17.2).
An EBML Schema MUST declare exactly one EBML Element at Root Level
(referred to as the Root Element) that occurs exactly once within an
EBML Document. The Void Element MAY also occur at Root Level but is
not a Root Element (see Section 11.3.2).
The EBML Schema MUST document all Elements of the EBML Body. The
EBML Schema does not document Global Elements that are defined by
this document (namely, the Void Element and the CRC-32 Element).
The EBML Schema MUST NOT use the Element ID "0x1A45DFA3", which is
reserved for the EBML Header for the purpose of resynchronization.
An EBML Schema MAY constrain the use of EBML Header Elements (see
Section 11.2) by adding or constraining that Element's "range"
attribute. For example, an EBML Schema MAY constrain the
EBMLMaxSizeLength to a maximum value of "8" or MAY constrain the
EBMLVersion to only support a value of "1". If an EBML Schema adopts
the EBML Header Element as is, then it is not required to document
that Element within the EBML Schema. If an EBML Schema constrains
the range of an EBML Header Element, then that Element MUST be
documented within an "<element>" node of the EBML Schema. This
document provides an example of an EBML Schema; see Section 11.1.1.
11.1.1. EBML Schema Example
<?xml version="1.0" encoding="utf-8"?>
<EBMLSchema xmlns="urn:ietf:rfc:8794"
docType="files-in-ebml-demo" version="1">
<!-- constraints to the range of two EBML Header Elements -->
<element name="EBMLReadVersion" path="\EBML\EBMLReadVersion"
id="0x42F7" minOccurs="1" maxOccurs="1" range="1" default="1"
type="uinteger"/>
<element name="EBMLMaxSizeLength"
path="\EBML\EBMLMaxSizeLength" id="0x42F3" minOccurs="1"
maxOccurs="1" range="8" default="8" type="uinteger"/>
<!-- Root Element-->
<element name="Files" path="\Files" id="0x1946696C"
type="master">
<documentation lang="en"
purpose="definition">Container of data and
attributes representing one or many files.</documentation>
</element>
<element name="File" path="\Files\File" id="0x6146"
type="master" minOccurs="1">
<documentation lang="en" purpose="definition">
An attached file.
</documentation>
</element>
<element name="FileName" path="\Files\File\FileName"
id="0x614E" type="utf-8"
minOccurs="1">
<documentation lang="en" purpose="definition">
Filename of the attached file.
</documentation>
</element>
<element name="MimeType" path="\Files\File\MimeType"
id="0x464D" type="string"
minOccurs="1">
<documentation lang="en" purpose="definition">
MIME type of the file.
</documentation>
</element>
<element name="ModificationTimestamp"
path="\Files\File\ModificationTimestamp" id="0x4654"
type="date" minOccurs="1">
<documentation lang="en" purpose="definition">
Modification timestamp of the file.
</documentation>
</element>
<element name="Data" path="\Files\File\Data" id="0x4664"
type="binary" minOccurs="1">
<documentation lang="en" purpose="definition">
The data of the file.
</documentation>
</element>
</EBMLSchema>
11.1.2. "<EBMLSchema>" Element
Within an EBML Schema, the XPath [XPath] of the "<EBMLSchema>"
element is "/EBMLSchema".
When used as an XML Document, the EBML Schema MUST use "<EBMLSchema>"
as the top-level element. The "<EBMLSchema>" element can contain
"<element>" subelements.
11.1.3. "<EBMLSchema>" Namespace
The namespace URI for elements of the EBML Schema is a URN as defined
by [RFC8141] that uses the namespace identifier 'ietf' defined by
[RFC2648] and extended by [RFC3688]. This URN is
"urn:ietf:rfc:8794".
11.1.4. "<EBMLSchema>" Attributes
Within an EBML Schema, the "<EBMLSchema>" element uses the following
attributes to define an EBML Element:
11.1.4.1. docType
Within an EBML Schema, the XPath of the "@docType" attribute is
"/EBMLSchema/@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.
11.1.4.2. version
Within an EBML Schema, the XPath of the "@version" attribute is
"/EBMLSchema/@version".
The version lists a nonnegative 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.
11.1.4.3. ebml
Within an EBML Schema, the XPath of the "@ebml" attribute is
"/EBMLSchema/@ebml".
The "ebml" attribute is a positive integer that specifies the version
of the EBML Header (see Section 11.2.2) used by the EBML Schema. If
the attribute is omitted, the EBML Header version is 1.
11.1.5. "<element>" Element
Within an EBML Schema, the XPath of the "<element>" element is
"/EBMLSchema/element".
Each "<element>" defines one EBML Element through the use of several
attributes that are defined in Section 11.1.6. 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 more "<documentation>"
subelements, followed by optional "<implementation_note>"
subelements, followed by zero or one "<restriction>" subelement,
followed by optional "<extension>" subelements. All "<element>"
nodes MUST be subelements of the "<EBMLSchema>".
11.1.6. "<element>" Attributes
Within an EBML Schema, the "<element>" uses the following attributes
to define an EBML Element:
11.1.6.1. name
Within an EBML Schema, the XPath of the "@name" attribute is
"/EBMLSchema/element/@name".
The name provides the 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 first character of the name
MUST be in the form of an "A" to "Z", "a" to "z", or "0" to "9"
character.
The "name" attribute is REQUIRED.
11.1.6.2. path
Within an EBML Schema, the XPath of the "@path" attribute is
"/EBMLSchema/element/@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 is 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 = EBMLParentPath EBMLElement
EBMLParentPath = PathDelimiter [EBMLParents]
EBMLParents = 0*IntermediatePathAtom EBMLLastParent
IntermediatePathAtom = EBMLPathAtom / GlobalPlaceholder
EBMLLastParent = EBMLPathAtom / GlobalPlaceholder
EBMLPathAtom = [IsRecursive] EBMLAtomName PathDelimiter
EBMLElement = [IsRecursive] EBMLAtomName
PathDelimiter = "\"
IsRecursive = "+"
EBMLAtomName = ALPHA / DIGIT 0*EBMLNameChar
EBMLNameChar = ALPHA / DIGIT / "-" / "."
GlobalPlaceholder = "(" GlobalParentOccurrence "\)"
GlobalParentOccurrence = [PathMinOccurrence] "-" [PathMaxOccurrence]
PathMinOccurrence = 1*DIGIT ; no upper limit
PathMaxOccurrence = 1*DIGIT ; no upper limit
The "*", "(", and ")" symbols are interpreted as defined in
[RFC5234].
The EBMLAtomName of the EBMLElement part MUST be equal to the "@name"
attribute of the EBML Schema. If the EBMLElement part contains an
IsRecursive part, the EBML Element can occur within itself
recursively (see Section 11.1.6.11).
The starting PathDelimiter of EBMLParentPath corresponds to the root
of the EBML Document.
The "@path" value MUST be unique within the EBML Schema. The "@id"
value corresponding to this "@path" MUST NOT be defined for use
within another EBML Element with the same EBMLParentPath as this
"@path".
A path with a GlobalPlaceholder as the EBMLLastParent defines a
Global Element; see Section 11.3. If the element has no
EBMLLastParent part, or the EBMLLastParent part is not a
GlobalPlaceholder, then the Element is not a Global Element.
The GlobalParentOccurrence part is interpreted as the number of valid
EBMLPathAtom parts that can replace the GlobalPlaceholder in the
path. PathMinOccurrence represents the minimum number of
EBMLPathAtoms required to replace the GlobalPlaceholder.
PathMaxOccurrence represents the maximum number of EBMLPathAtoms
possible to replace the GlobalPlaceholder.
If PathMinOccurrence is not present, then that GlobalParentOccurrence
has a PathMinOccurrence value of 0. If PathMaxOccurrence is not
present, then there is no upper bound for the permitted number of
EBMLPathAtoms possible to replace the GlobalPlaceholder.
PathMaxOccurrence MUST NOT have the value 0, as it would mean no
EBMLPathAtom can replace the GlobalPlaceholder, and the EBMLFullPath
would be the same without that GlobalPlaceholder part.
PathMaxOccurrence MUST be bigger than, or equal to,
PathMinOccurrence.
For example, in "\a\(0-1\)global", the Element path "\a\x\global"
corresponds to an EBMLPathAtom occurrence of 1. The Element
"\a\x\y\global" corresponds to an EBMLPathAtom occurrence of 2, etc.
In those cases, "\a\x" or "\a\x\y" MUST be valid paths to be able to
contain the element "global".
Consider another EBML Path, "\a\(1-\)global". There has to be at
least one EBMLPathAtom between the "\a\" part and "global". So the
"global" EBML Element cannot be found inside the "\a" EBML Element,
as it means the resulting path "\a\global" has no EBMLPathAtom
between the "\a\" and "global". However, the "global" EBML Element
can be found inside the "\a\b" EBML Element, because the resulting
path, "\a\b\global", has one EBMLPathAtom between the "\a\" and
"global". Alternatively, it can be found inside the "\a\b\c" EBML
Element (two EBMLPathAtom), or inside the "\a\b\c\d" EBML Element
(three EBMLPathAtom), etc.
Consider another EBML Path, "\a\(0-1\)global". There has to be at
most one EBMLPathAtom between the "\a\" part and "global". So the
"global" EBML Element can be found inside either the "\a" EBML
Element (0 EBMLPathAtom replacing GlobalPlaceholder) or the "\a\b"
EBML Element (one replacement EBMLPathAtom). But it cannot be found
inside the "\a\b\c" EBML Element, because the resulting path,
"\a\b\c\global", has two EBMLPathAtom between "\a\" and "global".
11.1.6.3. id
Within an EBML Schema, the XPath of the "@id" attribute is
"/EBMLSchema/element/@id".
The Element ID is encoded as a Variable-Size Integer. It is read and
stored in big-endian order. In the EBML Schema, it is expressed in
hexadecimal notation prefixed by a 0x. 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 length of one octet to allow its usage in more
writing and editing scenarios.
The Element ID of any Element found within an EBML Document MUST only
match a single "@path" value of its corresponding EBML Schema, but a
separate instance of that Element ID value defined by the EBML Schema
MAY occur within a different "@path". If more than one Element is
defined to use the same "@id" value, then the "@path" values of those
Elements MUST NOT share the same EBMLParentPath. Elements MUST NOT
be defined to use the same "@id" value if one of their common Parent
Elements could be an Unknown-Sized Element.
The "id" attribute is REQUIRED.
11.1.6.4. minOccurs
Within an EBML Schema, the XPath of the "@minOccurs" attribute is
"/EBMLSchema/element/@minOccurs".
"minOccurs" is a nonnegative integer expressing the minimum permitted
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 "minOccurs" refers to constraints on the
occurrence of the EBML Element within the EBML Document. EBML
Elements with "minOccurs" set to "1" that also have a default value
(see Section 11.1.6.8) declared are not REQUIRED to be stored but are
REQUIRED to be interpreted; see Section 11.1.19.
An EBML Element defined with a "minOccurs" value greater than zero is
called a Mandatory EBML Element.
The "minOccurs" attribute is OPTIONAL. If the "minOccurs" attribute
is not present, then that EBML Element has a "minOccurs" value of 0.
The semantic meaning of "minOccurs" within an EBML Schema is
analogous to the meaning of "minOccurs" within an XML Schema.
11.1.6.5. maxOccurs
Within an EBML Schema, the XPath of the "@maxOccurs" attribute is
"/EBMLSchema/element/@maxOccurs".
"maxOccurs" is a nonnegative integer expressing the maximum permitted
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, including the unwritten mandatory
element with a default value; see Section 11.1.19. If the EBML
Element has an empty EBMLParentPath, then "maxOccurs" refers to
constraints on the occurrence of the EBML Element within the EBML
Document.
The "maxOccurs" attribute is OPTIONAL. If the "maxOccurs" attribute
is not present, then there is no upper bound for the permitted number
of occurrences of this EBML Element within its Parent Element or
within the EBML Document, depending on whether or not the
EBMLParentPath of the EBML Element is empty.
The semantic meaning of "maxOccurs" within an EBML Schema is
analogous to the meaning of "maxOccurs" within an XML Schema; when it
is not present, it's similar to xml:maxOccurs="unbounded" in an XML
Schema.
11.1.6.6. range
Within an EBML Schema, the XPath of the "@range" attribute is
"/EBMLSchema/element/@range".
A numerical range for EBML Elements that 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 11.1.6.6.1 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.
11.1.6.6.1. Expression of range
The "range" attribute MUST only be used with EBML Elements that are
either signed integer, unsigned integer, float, or date. The
expression defines the upper, lower, exact, or excluded value of the
EBML Element and optionally an upper boundary value combined with a
lower boundary. The range expression may contain whitespace (using
the ASCII 0x20 character) for readability, but whitespace within a
range expression MUST NOT convey meaning.
To set a fixed value for the range, the value is used as the
attribute value. For example, "1234" means the EBML element always
has the value 1234. The value can be prefixed with "not" to indicate
that the fixed value MUST NOT be used for that Element. For example,
"not 1234" means the Element can use all values of its type except
1234.
The ">" sign is used for an exclusive lower boundary, and the ">="
sign is used for an inclusive lower boundary. For example, ">3"
means the Element value MUST be greater than 3, and ">=0x1p+0" means
the Element value MUST be greater than or equal to the floating value
1.0; see Section 11.1.18.
The "<" sign is used for an exclusive upper boundary, and the "<="
sign is used for an inclusive upper boundary. For example, "<-2"
means the Element value MUST be less than -2, and "<=10" means the
Element value MUST be less than or equal to 10.
The lower and upper bounds can be combined into an expression to form
a closed boundary. The lower boundary comes first, followed by the
upper boundary, separated by a comma. For example, ">3,<= 20" means
the Element value MUST be greater than 3 and less than or equal to
20.
A special form of lower and upper bounds using the "-" separator is
possible, meaning the Element value MUST be greater than, or equal
to, the first value and MUST be less than or equal to the second
value. For example, "1-10" is equivalent to ">=1,<=10". If the
upper boundary is negative, the "range" attribute MUST only use the
latter form.
11.1.6.7. length
Within an EBML Schema, the XPath of the "@length" attribute is
"/EBMLSchema/element/@length".
The "length" attribute is a value to express the valid length of the
Element Data as written, measured in octets. The length provides a
constraint in addition to the Length value of the definition of the
corresponding EBML Element Type. This length MUST be expressed as
either a nonnegative integer or a range (see Section 11.1.6.6.1) that
consists of only nonnegative integers and valid operators.
The "length" attribute is OPTIONAL. If the "length" attribute is not
present for that EBML Element, then that EBML Element is only limited
in length by the definition of the associated EBML Element Type.
11.1.6.8. default
Within an EBML Schema, the XPath of the "@default" attribute is
"/EBMLSchema/element/@default".
If an Element is mandatory (has a "minOccurs" 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. An unwritten mandatory Element with a
declared default value is semantically equivalent to that Element if
written with the default value stored as the Element Data. 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.
11.1.6.9. type
Within an EBML Schema, the XPath of the "@type" attribute is
"/EBMLSchema/element/@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 in Section 7.
The "type" attribute is REQUIRED.
11.1.6.10. unknownsizeallowed
Within an EBML Schema, the XPath of the "@unknownsizeallowed"
attribute is "/EBMLSchema/element/@unknownsizeallowed".
This attribute is a boolean to express whether an EBML Element is
permitted to be 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.
An EBML Element with the "unknownsizeallowed" attribute set to 1 MUST
NOT have its "recursive" attribute 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.
11.1.6.11. recursive
Within an EBML Schema, the XPath of the "@recursive" attribute is
"/EBMLSchema/element/@recursive".
This attribute is a boolean to express whether an EBML Element is
permitted to be stored recursively. If it is allowed, 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 EBMLElement part of the "@path" contains an IsRecursive part,
then the "recursive" value MUST be true; otherwise, it MUST be false.
An EBML Element with the "recursive" attribute set to 1 MUST NOT have
its "unknownsizeallowed" attribute set to 1.
The "recursive" attribute is OPTIONAL. If the "recursive" attribute
is not present, then the EBML Element MUST NOT be used recursively.
11.1.6.12. recurring
Within an EBML Schema, the XPath of the "@recurring" attribute is
"/EBMLSchema/element/@recurring".
This attribute is a boolean to express whether or not an EBML Element
is defined as an Identically Recurring Element; see Section 11.1.17.
The "recurring" attribute is OPTIONAL. If the "recurring" attribute
is not present, then the EBML Element is not an Identically Recurring
Element.
11.1.6.13. minver
Within an EBML Schema, the XPath of the "@minver" attribute is
"/EBMLSchema/element/@minver".
The "minver" (minimum version) attribute stores a nonnegative 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".
11.1.6.14. maxver
Within an EBML Schema, the XPath of the "@maxver" attribute is
"/EBMLSchema/element/@maxver".
The "maxver" (maximum version) attribute stores a nonnegative 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 "<EBMLSchema>".
11.1.7. "<documentation>" Element
Within an EBML Schema, the XPaths of the "<documentation>" elements
are "/EBMLSchema/element/documentation" and
"/EBMLSchema/element/restriction/enum/documentation".
The "<documentation>" element provides additional information about
EBML Elements or enumeration values. Within the "<documentation>"
element, the following XHTML [XHTML] elements MAY be used: "<a>",
"<br>", and "<strong>".
11.1.8. "<documentation>" Attributes
11.1.8.1. lang
Within an EBML Schema, the XPath of the "@lang" attribute is
"/EBMLSchema/element/documentation/@lang".
The "lang" attribute is set to the value from [RFC5646] of the
language of the element's documentation.
The "lang" attribute is OPTIONAL.
11.1.8.2. purpose
Within an EBML Schema, the XPath of the "@purpose" attribute is
"/EBMLSchema/element/documentation/@purpose".
A "purpose" attribute distinguishes the meaning of the documentation.
Values for the "<documentation>" subelement's "purpose" attribute
MUST include one of the values listed in Table 8.
+============+=================================================+
| value of | definition |
| "purpose" | |
| attribute | |
+============+=================================================+
| definition | A "definition" is recommended for every defined |
| | EBML Element. This documentation explains the |
| | semantic meaning of the EBML Element. |
+------------+-------------------------------------------------+
| rationale | An explanation about the reason or catalyst for |
| | the definition of the Element. |
+------------+-------------------------------------------------+
| usage | Recommended practices or guidelines for both |
| notes | reading, writing, or interpreting the Element. |
+------------+-------------------------------------------------+
| references | Informational references to support the |
| | contextualization and understanding of the |
| | value of the Element. |
+------------+-------------------------------------------------+
Table 8: Definitions of the permitted values for the
"purpose" attribute of the documentation Element
The "purpose" attribute is REQUIRED.
11.1.9. "<implementation_note>" Element
Within an EBML Schema, the XPath of the "<implementation_note>"
element is "/EBMLSchema/element/implementation_note".
In some cases within an EBML Document Type, the attributes of the
"<element>" element are not sufficient to clearly communicate how the
defined EBML Element is intended to be implemented. For instance,
one EBML Element might only be mandatory if another EBML Element is
present. As another example, the default value of an EBML Element
might be derived from a related Element's content. In these cases
where the Element's definition is conditional or advanced
implementation notes are needed, one or many "<implementation_note>"
elements can be used to store that information. The
"<implementation_note>" refers to a specific attribute of the parent
"<element>" as expressed by the "note_attribute" attribute (see
Section 11.1.10.1).
11.1.10. "<implementation_note>" Attributes
11.1.10.1. note_attribute
Within an EBML Schema, the XPath of the "@note_attribute" attribute
is "/EBMLSchema/element/implementation_note/@note_attribute".
The "note_attribute" attribute references which of the attributes of
the "<element>" the "<implementation_note>" relates to. The
"note_attribute" attribute MUST be set to one of the following values
(corresponding to that attribute of the parent "<element>"):
"minOccurs", "maxOccurs", "range", "length", "default", "minver", or
"maxver". The "<implementation_note>" SHALL supersede the parent
"<element>"'s attribute that is named in the "note_attribute"
attribute. An "<element>" SHALL NOT have more than one
"<implementation_note>" of the same "note_attribute".
The "note_attribute" attribute is REQUIRED.
11.1.10.2. "<implementation_note>" Example
The following fragment of an EBML Schema demonstrates how an
"<implementation_note>" is used. In this case, an EBML Schema
documents a list of items that are described with an optional cost.
The Currency Element uses an "<implementation_note>" to say that the
Currency Element is REQUIRED if the Cost Element is set, otherwise
not.
<element name="Items" path="\Items" id="0x4025" type="master"
minOccurs="1" maxOccurs="1">
<documentation lang="en" purpose="definition">
A set of items.
</documentation>
</element>
<element name="Item" path="\Items\Item" id="0x4026"
type="master">
<documentation lang="en" purpose="definition">
An item.
</documentation>
</element>
<element name="Cost" path="\Items\Item\Cost" id="0x4024"
type="float" maxOccurs="1">
<documentation lang="en" purpose="definition">
The cost of the item, if any.
</documentation>
</element>
<element name="Currency" path="\Items\Item\Currency" id="0x403F"
type="string" maxOccurs="1">
<documentation lang="en" purpose="definition">
The currency of the item's cost.
</documentation>
<implementation_note note_attribute="minOccurs">
Currency MUST be set (minOccurs=1) if the associated Item stores
a Cost, else Currency MAY be unset (minOccurs=0).
</implementation_note>
</element>
11.1.11. "<restriction>" Element
Within an EBML Schema, the XPath of the "<restriction>" element is
"/EBMLSchema/element/restriction".
The "<restriction>" element provides information about restrictions
to the allowable values for the EBML Element, which are listed in
"<enum>" elements.
11.1.12. "<enum>" Element
Within an EBML Schema, the XPath of the "<enum>" element is
"/EBMLSchema/element/restriction/enum".
The "<enum>" element stores a list of values allowed for storage in
the EBML Element. The values MUST match the type of the EBML Element
(for example, "<enum value="Yes">" cannot be a valid value for an
EBML Element that is defined as an unsigned integer). An "<enum>"
element MAY also store "<documentation>" elements to further describe
the "<enum>".
11.1.13. "<enum>" Attributes
11.1.13.1. label
Within an EBML Schema, the XPath of the "@label" attribute is
"/EBMLSchema/element/restriction/enum/@label".
The label provides a concise expression for human consumption that
describes what the value of "<enum>" represents.
The "label" attribute is OPTIONAL.
11.1.13.2. value
Within an EBML Schema, the XPath of the "@value" attribute is
"/EBMLSchema/element/restriction/enum/@value".
The value represents data that MAY be stored within the EBML Element.
The "value" attribute is REQUIRED.
11.1.14. "<extension>" Element
Within an EBML Schema, the XPath of the "<extension>" element is
"/EBMLSchema/element/extension".
The "<extension>" element provides an unconstrained element to
contain information about the associated EBML "<element>", which is
undefined by this document but MAY be defined by the associated EBML
Document Type. The "<extension>" element MUST contain a "type"
attribute and also MAY contain any other attribute or subelement as
long as the EBML Schema remains as a well-formed XML Document. All
"<extension>" elements MUST be subelements of the "<element>".
11.1.15. "<extension>" Attributes
11.1.15.1. type
Within an EBML Schema, the XPath of the "@type" attribute is
"/EBMLSchema/element/extension/@type".
The "type" attribute should reference a name or identifier of the
project or authority associated with the contents of the
"<extension>" element.
The "type" attribute is REQUIRED.
11.1.16. XML Schema for EBML Schema
The following provides an XML Schema [XML-SCHEMA] for facilitating
verification of an EBML Schema described in Section 11.1.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="urn:ietf:rfc:8794"
targetNamespace="urn:ietf:rfc:8794"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xhtml="http://www.w3.org/1999/xhtml"
elementFormDefault="qualified" version="01">
<!-- for HTML in comments -->
<xs:import namespace="http://www.w3.org/1999/xhtml"
schemaLocation="http://www.w3.org/MarkUp/SCHEMA/xhtml11.xsd"/>
<xs:element name="EBMLSchema" type="EBMLSchemaType"/>
<xs:complexType name="EBMLSchemaType">
<xs:sequence>
<xs:element name="element" type="elementType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="docType" use="required"/>
<xs:attribute name="version" use="required" type="xs:integer"/>
<xs:attribute name="ebml" type="xs:positiveInteger"
default="1"/>
</xs:complexType>
<xs:complexType name="elementType">
<xs:sequence>
<xs:element name="documentation" type="documentationType"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="implementation_note" type="noteType"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="restriction" type="restrictionType"
minOccurs="0" maxOccurs="1"/>
<xs:element name="extension" type="extensionType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="name" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:pattern value="[0-9A-Za-z.-]([0-9A-Za-z.-])*"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="path" use="required">
<!-- <xs:simpleType>
<xs:restriction base="xs:integer">
<xs:pattern value="[0-9]*\*[0-9]*()"/>
</xs:restriction>
</xs:simpleType> -->
</xs:attribute>
<xs:attribute name="id" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:pattern value="0x([0-9A-F][0-9A-F])+"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="minOccurs" default="0">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="maxOccurs" default="1">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="range"/>
<xs:attribute name="length"/>
<xs:attribute name="default"/>
<xs:attribute name="type" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="integer"/>
<xs:enumeration value="uinteger"/>
<xs:enumeration value="float"/>
<xs:enumeration value="string"/>
<xs:enumeration value="date"/>
<xs:enumeration value="utf-8"/>
<xs:enumeration value="master"/>
<xs:enumeration value="binary"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="unknownsizeallowed" type="xs:boolean"
default="false"/>
<xs:attribute name="recursive" type="xs:boolean"
default="false"/>
<xs:attribute name="recurring" type="xs:boolean"
default="false"/>
<xs:attribute name="minver" default="1">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="maxver">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="restrictionType">
<xs:sequence>
<xs:element name="enum" type="enumType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="extensionType">
<xs:sequence>
<xs:any processContents="skip"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="type" use="required"/>
<xs:anyAttribute processContents="skip"/>
</xs:complexType>
<xs:complexType name="enumType">
<xs:sequence>
<xs:element name="documentation" type="documentationType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="label"/>
<xs:attribute name="value" use="required"/>
</xs:complexType>
<xs:complexType name="documentationType" mixed="true">
<xs:sequence>
<xs:element name="a" type="xhtml:xhtml.a.type"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="br" type="xhtml:xhtml.br.type"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="strong" type="xhtml:xhtml.strong.type"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="lang"/>
<xs:attribute name="purpose" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="definition"/>
<xs:enumeration value="rationale"/>
<xs:enumeration value="references"/>
<xs:enumeration value="usage notes"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="noteType">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="note_attribute" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="minOccurs"/>
<xs:enumeration value="maxOccurs"/>
<xs:enumeration value="range"/>
<xs:enumeration value="length"/>
<xs:enumeration value="default"/>
<xs:enumeration value="minver"/>
<xs:enumeration value="maxver"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
11.1.17. Identically Recurring Elements
An Identically Recurring Element is an EBML Element that MAY occur
within its Parent Element more than once, but each recurrence of it
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 datastream so that an EBML Reader that 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 that 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 that 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.
11.1.18. 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 [ISO9899] (see Section 6.4.4.2 on Floating Constants).
Table 9 provides examples of expressions of float ranges.
+===================+=========================================+
| as decimal | as Hexadecimal Floating-Point Constants |
+===================+=========================================+
| 0.0 | 0x0p+1 |
+-------------------+-----------------------------------------+
| 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 |
+-------------------+-----------------------------------------+
Table 9: Example of Floating-Point values and ranges as
decimal and Hexadecimal Floating-Point Constants
Within an expression of a float range, as in an integer range, the -
(hyphen) character is the separator between the minimum and maximum
values 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 that 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 minimum and
maximum values permitted by the range.
11.1.19. 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 read 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.
Table 10 clarifies whether a Mandatory EBML Element MUST be written,
according to whether the default value is declared, the value of the
EBML Element is equal to the declared default value, and/or the
Parent Element is used.
+=================+=============+===============+==================+
| Is the default | Is the | Is the Parent | Then is storing |
| value declared? | value equal | Element | the EBML Element |
| | to default? | present? | 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 |
+-----------------+-------------+---------------+------------------+
Table 10: Demonstration of the conditional requirements of VINT
Storage
11.2. EBML Header Elements
This document contains definitions of all EBML Elements of the EBML
Header.
11.2.1. EBML Element
name: EBML
path: "\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.
11.2.2. EBMLVersion Element
name: EBMLVersion
path: "\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.
11.2.3. EBMLReadVersion Element
name: EBMLReadVersion
path: "\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.
11.2.4. EBMLMaxIDLength Element
name: EBMLMaxIDLength
path: "\EBML\EBMLMaxIDLength"
id: 0x42F2
minOccurs: 1
maxOccurs: 1
range: >=4
default: 4
type: Unsigned Integer
description: The EBMLMaxIDLength Element stores the maximum
permitted 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.
11.2.5. EBMLMaxSizeLength Element
name: EBMLMaxSizeLength
path: "\EBML\EBMLMaxSizeLength"
id: 0x42F3
minOccurs: 1
maxOccurs: 1
range: not 0
default: 8
type: Unsigned Integer
description: The EBMLMaxSizeLength Element stores the maximum
permitted length in octets of the expressions of all Element Data
Sizes to be found within the EBML Body. The EBMLMaxSizeLength
Element documents an upper bound for the "length" of all Element
Data Size expressions within the EBML Body and not an upper bound
for the "value" of all Element Data Size expressions within the
EBML Body. EBML Elements that have an Element Data Size
expression that is larger in octets than what is expressed by
EBMLMaxSizeLength Element are invalid.
11.2.6. DocType Element
name: DocType
path: "\EBML\DocType"
id: 0x4282
minOccurs: 1
maxOccurs: 1
length: >0
type: String
description: A string that describes and identifies the content of
the EBML Body that follows this EBML Header.
11.2.7. DocTypeVersion Element
name: DocTypeVersion
path: "\EBML\DocTypeVersion"
id: 0x4287
minOccurs: 1
maxOccurs: 1
range: not 0
default: 1
type: Unsigned Integer
description: The version of DocType interpreter used to create the
EBML Document.
11.2.8. DocTypeReadVersion Element
name: DocTypeReadVersion
path: "\EBML\DocTypeReadVersion"
id: 0x4285
minOccurs: 1
maxOccurs: 1
range: not 0
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.
11.2.9. DocTypeExtension Element
name: DocTypeExtension
path: "\EBML\DocTypeExtension"
id: 0x4281
minOccurs: 0
type: Master Element
description: A DocTypeExtension adds extra Elements to the main
DocType+DocTypeVersion tuple it's attached to. An EBML Reader MAY
know these extra Elements and how to use them. A DocTypeExtension
MAY be used to iterate between experimental Elements before they
are integrated into a regular DocTypeVersion. Reading one
DocTypeExtension version of a DocType+DocTypeVersion tuple doesn't
imply one should be able to read upper versions of this
DocTypeExtension.
11.2.10. DocTypeExtensionName Element
name: DocTypeExtensionName
path: "\EBML\DocTypeExtension\DocTypeExtensionName"
id: 0x4283
minOccurs: 1
maxOccurs: 1
length: >0
type: String
description: The name of the DocTypeExtension to differentiate it
from other DocTypeExtensions of the same DocType+DocTypeVersion
tuple. A DocTypeExtensionName value MUST be unique within the
EBML Header.
11.2.11. DocTypeExtensionVersion Element
name: DocTypeExtensionVersion
path: "\EBML\DocTypeExtension\DocTypeExtensionVersion"
id: 0x4284
minOccurs: 1
maxOccurs: 1
range: not 0
type: Unsigned Integer
description: The version of the DocTypeExtension. Different
DocTypeExtensionVersion values of the same DocType +
DocTypeVersion + DocTypeExtensionName tuple MAY contain completely
different sets of extra Elements. An EBML Reader MAY support
multiple versions of the same tuple, only one version of the
tuple, or not support the tuple at all.
11.3. Global Elements
EBML allows some special Elements to be found within more than one
parent in an EBML Document or optionally at the Root Level of an EBML
Body. These Elements are called Global Elements. There are two
Global Elements that can be found in any EBML Document: the CRC-32
Element and the Void Element. An EBML Schema MAY add other Global
Elements to the format it defines. These extra elements apply only
to the EBML Body, not the EBML Header.
Global Elements are EBML Elements whose EBMLLastParent part of the
path has a GlobalPlaceholder. Because it is the last Parent part of
the path, a Global Element might also have EBMLParentPath parts in
its path. In this case, the Global Element can only be found within
this EBMLParentPath path -- i.e., it's not fully "global".
A Global Element can be found in many Parent Elements, allowing the
same number of occurrences in each Parent where this Element is
found.
11.3.1. CRC-32 Element
name: CRC-32
path: "\(1-\)CRC-32"
id: 0xBF
minOccurs: 0
maxOccurs: 1
length: 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 [ISO3309] standard
and in Section 8.1.1.6.2 of [ITU.V42], with initial value of
0xFFFFFFFF. The CRC value MUST be computed on a little-endian
bytestream and MUST use little-endian storage.
11.3.2. Void Element
name: Void
path: "\(-\)Void"
id: 0xEC
minOccurs: 0
type: Binary
description: Used to void data or to avoid unexpected behaviors when
using damaged data. The content is discarded. Also used to
reserve space in a subelement for later use.
12. Considerations for Reading EBML Data
The following scenarios describe events to consider when reading EBML
Documents, as well as the recommended design of an EBML Reader.
If a Master Element contains a CRC-32 Element that doesn't validate,
then the EBML Reader MAY ignore all contained data except for
Descendant Elements that contain their own valid CRC-32 Element.
In the following XML representation of a simple, hypothetical EBML
fragment, a Master Element called CONTACT contains two Child
Elements, NAME and ADDRESS. In this example, some data within the
NAME Element had been altered so that the CRC-32 of the NAME Element
does not validate, and thus any Ancestor Element with a CRC-32 would
therefore also no longer validate. However, even though the CONTACT
Element has a CRC-32 that does not validate (because of the changed
data within the NAME Element), the CRC-32 of the ADDRESS Element does
validate, and thus the contents and semantics of the ADDRESS Element
MAY be used.
<CONTACT>
<CRC-32>c119a69b</CRC-32><!-- does not validate -->
<NAME>
<CRC-32>1f59ee2b</CRC-32><!-- does not validate -->
<FIRST-NAME>invalid data</FIRST-NAME>
<LAST-NAME>invalid data</LAST-NAME>
</NAME>
<ADDRESS>
<CRC-32>df941cc9</CRC-32><!-- validates -->
<STREET>valid data</STREET>
<CITY>valid data</CITY>
</ADDRESS>
</CONTACT>
If a Master Element contains more occurrences of a Child Master
Element than permitted according to the "maxOccurs" and "recurring"
attributes of the definition of that Element, then the occurrences in
addition to "maxOccurs" MAY be ignored.
If a Master Element contains more occurrences of a Child Element than
permitted according to the "maxOccurs" attribute of the definition of
that Element, then all instances of that Element after the first
"maxOccurs" occurrences from the beginning of its Parent Element
SHOULD be ignored.
13. Terminating Elements
Null Octets, which are octets with all bits set to zero, MAY follow
the value of a String Element or UTF-8 Element to serve as a
terminator. An EBML Writer MAY terminate a String Element or UTF-8
Element with Null Octets in order to overwrite a stored value with a
new value of lesser length while maintaining the same Element Data
Size; this can prevent the need to rewrite large portions of an EBML
Document. Otherwise, the use of Null Octets within a String Element
or UTF-8 Element is NOT RECOMMENDED. The Element Data of a UTF-8
Element MUST be a valid UTF-8 string up to whichever comes first: the
end of the Element or the first occurring Null octet. Within the
Element Data of a String or UTF-8 Element, any Null octet itself and
any following data within that Element SHOULD be ignored. A string
value and a copy of that string value terminated by one or more Null
Octets are semantically equal.
Table 11 shows examples of semantics and validation for the use of
Null Octets. Values to represent Stored Values and the Semantic
Meaning as represented as hexadecimal values.
+=====================+=====================+
| Stored Value | Semantic Meaning |
+=====================+=====================+
| 0x65 0x62 0x6D 0x6C | 0x65 0x62 0x6D 0x6C |
+---------------------+---------------------+
| 0x65 0x62 0x00 0x6C | 0x65 0x62 |
+---------------------+---------------------+
| 0x65 0x62 0x00 0x00 | 0x65 0x62 |
+---------------------+---------------------+
| 0x65 0x62 | 0x65 0x62 |
+---------------------+---------------------+
Table 11: Examples of semantics for Null
Octets in VINT_DATA
14. Guidelines for Updating Elements
An EBML Document can be updated without requiring that the entire
EBML Document be rewritten. These recommendations describe
strategies for changing the Element Data of a written EBML Element
with minimal disruption to the rest of the EBML Document.
14.1. Reducing Element Data in Size
There are three methods to reduce the size of Element Data of a
written EBML Element.
14.1.1. Adding a Void Element
When an EBML Element is changed to reduce its total length by more
than one octet, an EBML Writer SHOULD fill the freed space with a
Void Element.
14.1.2. Extending the Element Data Size
The same value for Element Data Size MAY be written in various
lengths, so for minor reductions of the Element Data, the Element
Size MAY be written to a longer octet length to fill the freed space.
For example, the first row of Table 12 depicts a String Element that
stores an Element ID (3 octets), Element Data Size (1 octet), and
Element Data (4 octets). If the Element Data is changed to reduce
the length by one octet, and if the current length of the Element
Data Size is less than its maximum permitted length, then the Element
Data Size of that Element MAY be rewritten to increase its length by
one octet. Thus, before and after the change, the EBML Element
maintains the same length of 8 octets, and data around the Element
does not need to be moved.
+=============+============+===================+==============+
| Status | Element ID | Element Data Size | Element Data |
+=============+============+===================+==============+
| Before edit | 0x3B4040 | 0x84 | 0x65626D6C |
+-------------+------------+-------------------+--------------+
| After edit | 0x3B4040 | 0x4003 | 0x6D6B76 |
+-------------+------------+-------------------+--------------+
Table 12: Example of editing a VINT to reduce VINT_DATA
length by one octet
This method is RECOMMENDED when the Element Data is reduced by a
single octet; for reductions by two or more octets, it is RECOMMENDED
to fill the freed space with a Void Element.
Note that if the Element Data length needs to be rewritten as
shortened by one octet and the Element Data Size could be rewritten
as a shorter VINT, then it is RECOMMENDED to rewrite the Element Data
Size as one octet shorter, shorten the Element Data by one octet, and
follow that Element with a Void Element. For example, Table 13
depicts a String Element that stores an Element ID (3 octets),
Element Data Size (2 octets, but could be rewritten in one octet),
and Element Data (3 octets). If the Element Data is to be rewritten
to a two-octet length, then another octet can be taken from Element
Data Size so that there is enough space to add a two-octet Void
Element.
+========+============+===================+==============+=========+
| Status | Element ID | Element Data Size | Element Data | Void |
| | | | | Element |
+========+============+===================+==============+=========+
| Before | 0x3B4040 | 0x4003 | 0x6D6B76 | |
+--------+------------+-------------------+--------------+---------+
| After | 0x3B4040 | 0x82 | 0x6869 | 0xEC80 |
+--------+------------+-------------------+--------------+---------+
Table 13: Example of editing a VINT to reduce VINT_DATA length
by more than one octet
14.1.3. Terminating Element Data
For String Elements and UTF-8 Elements, the length of Element Data
could be reduced by adding Null Octets to terminate the Element Data
(see Section 13).
In Table 14, Element Data four octets long is changed to a value
three octets long, followed by a Null Octet; the Element Data Size
includes any Null Octets used to terminate Element Data and therefore
remains unchanged.
+=============+============+===================+==============+
| Status | Element ID | Element Data Size | Element Data |
+=============+============+===================+==============+
| Before edit | 0x3B4040 | 0x84 | 0x65626D6C |
+-------------+------------+-------------------+--------------+
| After edit | 0x3B4040 | 0x84 | 0x6D6B7600 |
+-------------+------------+-------------------+--------------+
Table 14: Example of terminating VINT_DATA with a Null
Octet when reducing VINT length during an edit
Note that this method is NOT RECOMMENDED. For reductions of one
octet, the method for Extending the Element Data Size SHOULD be used.
For reduction by more than one octet, the method for Adding a Void
Element SHOULD be used.
14.2. Considerations when Updating Elements with Cyclic Redundancy
Check (CRC)
If the Element to be changed is a Descendant Element of any Master
Element that contains a CRC-32 Element (see Section 11.3.1), then the
CRC-32 Element MUST be verified before permitting the change.
Additionally, the CRC-32 Element value MUST be subsequently updated
to reflect the changed data.
15. Backward and Forward Compatibility
Elements of an EBML format SHOULD be designed with backward and
forward compatibility in mind.
15.1. Backward Compatibility
Backward compatibility of new EBML Elements can be achieved by using
default values for mandatory elements. The default value MUST
represent the state that was assumed for previous versions of the
EBML Schema, without this new EBML Element. If such a state doesn't
make sense for previous versions, then the new EBML Element SHOULD
NOT be mandatory.
Non-mandatory EBML Elements can be added in a new EBMLDocTypeVersion.
Since they are not mandatory, they won't be found in older versions
of the EBMLDocTypeVersion, just as they might not be found in newer
versions. This causes no compatibility issue.
15.2. Forward Compatibility
EBML Elements MAY be marked as deprecated in a new EBMLDocTypeVersion
using the "maxver" attribute of the EBML Schema. If such an Element
is found in an EBML Document with a newer version of the
EBMLDocTypeVersion, it SHOULD be discarded.
16. 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 (the presence of
EBML Elements can be derived even with an unknown semantic layer
using a heuristic approach -- not without errors, of course, but with
a certain degree of confidence).
An EBML Document that has the following issues may still be handled
by the EBML Reader and the data accepted as such, depending on how
strict the EBML Reader wants to be:
* 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 Data Size values that are longer than the limit
stated in the EBMLMaxSizeLength Element of the EBML Header.
Element IDs that are unknown to the EBML Reader MAY be accepted as
valid EBML IDs in order to skip such elements.
EBML Elements with a string type may contain extra data after the
first 0x00. These data MUST be discarded according to the Section 13
rules.
An EBML Reader may discard some or all data if the following errors
are found in the EBML Document:
* 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 in a Master Element and
have no declared default value, making the semantic invalid at
that Master Element level.
* 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,
triggering bogus date accesses.
* The CRC-32 Element (see Section 11.3.1) of a Master Element
doesn't match the rest of the content of that Master Element.
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, making
comparison at the semantic level invalid.
* The semantic equivalence of VINT_DATA within Element Data Size
with two different lengths due to left-padding zero bits, making
comparison at the semantic level invalid.
* Data contained within a Master Element that is not itself part of
a Child Element, which can trigger incorrect parsing behavior in
EBML Readers.
* Extraneous copies of Identically Recurring Element, making parsing
unnecessarily slow to the point of not being usable.
* Copies of Identically Recurring Element within a Parent Element
that contain invalid CRC-32 Elements. EBML Readers not checking
the CRC-32 might use the version of the element with mismatching
CRC-32s.
* Use of Void Elements that could be used to hide content or create
bogus resynchronization points seen by some EBML Readers and not
others.
17. IANA Considerations
17.1. EBML Element IDs Registry
This document creates a new IANA registry called the "EBML Element
IDs" registry.
Element IDs are described in Section 5. Element IDs are encoded
using the VINT mechanism described in Section 4 and can be between
one and five octets long. Five-octet-long Element IDs are possible
only if declared in the header.
This IANA registry only applies to Elements that can be contained in
the EBML Header, thus including Global Elements. Elements only found
in the EBML Body have their own set of independent Element IDs and
are not part of this IANA registry.
One-octet Element IDs MUST be between 0x81 and 0xFE. These items are
valuable because they are short, and they need to be used for
commonly repeated elements. Element IDs are to be allocated within
this range according to the "RFC Required" policy [RFC8126].
The following one-octet Element IDs are RESERVED: 0xFF and 0x80.
Values in the one-octet range of 0x00 to 0x7F are not valid for use
as an Element ID.
Two-octet Element IDs MUST be between 0x407F and 0x7FFE. Element IDs
are to be allocated within this range according to the "Specification
Required" policy [RFC8126].
The following two-octet Element IDs are RESERVED: 0x7FFF and 0x4000.
Values in the two-octet ranges of 0x0000 to 0x3FFF and 0x8000 to
0xFFFF are not valid for use as an Element ID.
Three-octet Element IDs MUST be between 0x203FFF and 0x3FFFFE.
Element IDs are to be allocated within this range according to the
"First Come First Served" policy [RFC8126].
The following three-octet Element IDs are RESERVED: 0x3FFFFF and
0x200000.
Values in the three-octet ranges of 0x000000 to 0x1FFFFF and 0x400000
to 0xFFFFFF are not valid for use as an Element ID.
Four-octet Element IDs MUST be between 0x101FFFFF and 0x1FFFFFFE.
Four-octet Element IDs are somewhat special in that they are useful
for resynchronizing to major structures in the event of data
corruption or loss. As such, four-octet Element IDs are split into
two categories. Four-octet Element IDs whose lower three octets (as
encoded) would make printable 7-bit ASCII values (0x20 to 0x7E,
inclusive) MUST be allocated by the "Specification Required" policy.
Sequential allocation of values is not required: specifications
SHOULD include a specific request and are encouraged to do early
allocations.
To be clear about the above category: four-octet Element IDs always
start with hex 0x10 to 0x1F, and that octet may be chosen so that the
entire VINT has some desirable property, such as a specific CRC. The
other three octets, when ALL having values between 0x20 (32, ASCII
Space) and 0x7E (126, ASCII "~"), fall into this category.
Other four-octet Element IDs may be allocated by the "First Come
First Served" policy.
The following four-octet Element IDs are RESERVED: 0x1FFFFFFF and
0x10000000.
Values in the four-octet ranges of 0x00000000 to 0x0FFFFFFF and
0x20000000 to 0xFFFFFFFF are not valid for use as an Element ID.
Five-octet Element IDs (values from 0x080FFFFFFF to 0x0FFFFFFFFE) are
RESERVED according to the "Experimental Use" policy [RFC8126]: they
may be used by anyone at any time, but there is no coordination.
ID Values found in this document are assigned as initial values as
follows:
+============+=========================+=================+
| Element ID | Element Name | Reference |
+============+=========================+=================+
| 0x1A45DFA3 | EBML | Described in |
| | | Section 11.2.1 |
+------------+-------------------------+-----------------+
| 0x4286 | EBMLVersion | Described in |
| | | Section 11.2.2 |
+------------+-------------------------+-----------------+
| 0x42F7 | EBMLReadVersion | Described in |
| | | Section 11.2.3 |
+------------+-------------------------+-----------------+
| 0x42F2 | EBMLMaxIDLength | Described in |
| | | Section 11.2.4 |
+------------+-------------------------+-----------------+
| 0x42F3 | EBMLMaxSizeLength | Described in |
| | | Section 11.2.5 |
+------------+-------------------------+-----------------+
| 0x4282 | DocType | Described in |
| | | Section 11.2.6 |
+------------+-------------------------+-----------------+
| 0x4287 | DocTypeVersion | Described in |
| | | Section 11.2.7 |
+------------+-------------------------+-----------------+
| 0x4285 | DocTypeReadVersion | Described in |
| | | Section 11.2.8 |
+------------+-------------------------+-----------------+
| 0x4281 | DocTypeExtension | Described in |
| | | Section 11.2.9 |
+------------+-------------------------+-----------------+
| 0x4283 | DocTypeExtensionName | Described in |
| | | Section 11.2.10 |
+------------+-------------------------+-----------------+
| 0x4284 | DocTypeExtensionVersion | Described in |
| | | Section 11.2.11 |
+------------+-------------------------+-----------------+
| 0xBF | CRC-32 | Described in |
| | | Section 11.3.1 |
+------------+-------------------------+-----------------+
| 0xEC | Void | Described in |
| | | Section 11.3.2 |
+------------+-------------------------+-----------------+
Table 15: IDs and Names for EBML Elements assigned by
this document
17.2. EBML DocTypes Registry
This document creates a new IANA registry called the "EBML DocTypes"
registry.
To register a new DocType in this registry, one needs a DocType name,
a Description of the DocType, a Change Controller (IESG or email of
registrant), and an optional Reference to a document describing the
DocType.
DocType values are described in Section 11.1.4.1. DocTypes are ASCII
strings, defined in Section 7.4, which label the official name of the
EBML Document Type. The strings may be allocated according to the
"First Come First Served" policy.
The use of ASCII corresponds to the types and code already in use;
the value is not meant to be visible to the user.
DocType string values of "matroska" and "webm" are RESERVED to the
IETF for future use. These can be assigned via the "IESG Approval"
or "RFC Required" policies [RFC8126].
18. Normative References
[IEEE.754] IEEE, "IEEE Standard for Binary Floating-Point
Arithmetic", 13 June 2019,
<https://standards.ieee.org/standard/754-2019.html>.
[ISO3309] International Organization for Standardization, "Data
communication -- High-level data link control procedures
-- Frame structure", ISO 3309, 3rd Edition, October 1984,
<https://www.iso.org/standard/8558.html>.
[ISO9899] International Organization for Standardization,
"Information technology -- Programming languages -- C",
ISO/IEC 9899:2011, 2011,
<https://www.iso.org/standard/57853.html>.
[ITU.V42] International Telecommunications Union, "Error-correcting
procedures for DCEs using asynchronous-to-synchronous
conversion", ITU-T Recommendation V.42, March 2002,
<https://www.itu.int/rec/T-REC-V.42>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
DOI 10.17487/RFC2648, August 1999,
<https://www.rfc-editor.org/info/rfc2648>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XHTML] McCarron, S., "XHTML(tm) Basic 1.1 -- Second Edition",
Latest version available
at https://www.w3.org/TR/xhtml-basic, 27 March 2018,
<https://www.w3.org/TR/2018/SPSD-xhtml-basic-20180327/>.
[XML] Bray, T., Ed., Paoli, J., Ed., Sperberg-McQueen, C.M.,
Ed., Maler, E., Ed., and F. Yergeau, Ed., "Extensible
Markup Language (XML) 1.0 (Fifth Edition)", Latest version
available at https://www.w3.org/TR/xml/, 26 November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126/>.
[XML-SCHEMA]
Fallside, D.C. and P. Walmsley, "XML Schema Part 0: Primer
Second Edition", Latest version available at
http://www.w3.org/TR/xmlschema-0/, 28 October 2004,
<https://www.w3.org/TR/2004/REC-xmlschema-0-20041028/>.
19. Informative References
[Matroska] Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
Container Format Specifications", Work in Progress,
Internet-Draft, draft-ietf-cellar-matroska-05, 17 April
2020, <https://tools.ietf.org/html/draft-ietf-cellar-
matroska-05>.
[WebM] The WebM Project, "WebM Container Guidelines", 28 November
2017, <https://www.webmproject.org/docs/container/>.
[XPath] Clark, J., Ed. and S. DeRose, "XML Path Language (XPath)
Version 1.0", Latest version available
at https://www.w3.org/TR/xpath, 16 November 1999,
<https://www.w3.org/TR/1999/REC-xpath-19991116>.
Authors' Addresses
Steve Lhomme
Email: slhomme@matroska.org
Dave Rice
Email: dave@dericed.com
Moritz Bunkus
Email: moritz@bunkus.org
