Extensible Binary Meta Language
draft-ietf-cellar-ebml-00
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| Authors | Steve Lhomme <>, Dave Rice <>, Moritz Bunkus <> | ||
| Last updated | 2016-09-23 | ||
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draft-ietf-cellar-ebml-00
cellar S. Lhomme
Internet-Draft
Intended status: Standards Track D. Rice
Expires: March 27, 2017
M. Bunkus
September 23, 2016
Extensible Binary Meta Language
draft-ietf-cellar-ebml-00
Abstract
This document defines the Extensible Binary Meta Language (EBML)
format as a genearlized file format for any type of data in a
hierarchical form. EBML is designed as a binary equivalent to XML
and utilizes a storage-efficient approach to building 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 an EBML Schema to convey the semantics of an EBML Document.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 27, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. EBML specifications . . . . . . . . . . . . . . . . . . . . . 2
1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Notation and Conventions . . . . . . . . . . . . . . . . 3
1.3. Security Considerations . . . . . . . . . . . . . . . . . 3
1.4. Structure . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5. Variable Size Integer . . . . . . . . . . . . . . . . . . 4
1.5.1. VINT_WIDTH . . . . . . . . . . . . . . . . . . . . . 4
1.5.2. VINT_MARKER . . . . . . . . . . . . . . . . . . . . . 4
1.5.3. VINT_DATA . . . . . . . . . . . . . . . . . . . . . . 4
1.5.4. VINT Examples . . . . . . . . . . . . . . . . . . . . 5
1.6. Element ID . . . . . . . . . . . . . . . . . . . . . . . 5
1.7. Element Data Size . . . . . . . . . . . . . . . . . . . . 6
1.8. EBML Element Types . . . . . . . . . . . . . . . . . . . 8
1.9. EBML Document . . . . . . . . . . . . . . . . . . . . . . 11
1.9.1. EBML Header . . . . . . . . . . . . . . . . . . . . . 12
1.9.2. EBML Body . . . . . . . . . . . . . . . . . . . . . . 12
1.10. EBML Stream . . . . . . . . . . . . . . . . . . . . . . . 13
1.11. Elements semantic . . . . . . . . . . . . . . . . . . . . 13
1.11.1. EBML Schema . . . . . . . . . . . . . . . . . . . . 13
1.11.2. EBML Header Elements . . . . . . . . . . . . . . . . 22
1.11.3. Global elements (used everywhere in the format) . . 25
2.1. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. EBML specifications
1.1. Introduction
EBML, short for Extensible Binary Meta Language, specifies a binary
and octet (byte) aligned format inspired by the principle of XML.
The goal of the EBML Specification is to define a generic, binary,
space-efficient format that may be utilized to define more complex
formats (such as containers for multimedia content) using an EBML
Schema. The definition of the EBML format recognizes the idea behind
HTML and XML as a good one: separate structure and semantics allowing
the same structural layer to be used with multiple, possibly widely
differing semantic layers. Except for the EBML Header and a few
global elements this specification does not define particular EBML
format semantics; however this specification is intended to define
how other EBML-based formats may be defined.
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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.
1.2. Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
1.3. Security Considerations
EBML itself does not offer any kind of security. It has nothing to
do with authentication, it does not provide confidentiality, only
marginally useful and effective data integrity options (CRC
elements).
EBML does not provide any kind of authorization.
Even if the semantic layer offers any kind of encryption, EBML itself
may leak information at both the semantic layer (as declared via the
DocType element) and within the EBML structure (you can derive the
presence of EBML elements even with an unknown semantic layer with a
heuristic approach; not without errors, of course, but with a certain
degree of confidence).
Attacks on an EBML reader may include: - Invalid Element IDs that are
longer than the limit stated in the EBMLMaxIDLength Element of the
EBML Header. - Invalid Element IDs that are not encoded in the
shortest-possible way. - Invalid Element IDs comprised of reserved
values. - Invalid Element Data Size values that are longer than the
limit stated in the EBMLMaxSizeLength Element of the EBML Header. -
Invalid Element Data Size values (e.g. extending the length of the
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 Elements that are mandatory and
have no declared default value. - Usage of 0x00 octets in EBML
Elements with a string type. - Usage of invalid UTF-8 encoding in
EBML Elements of UTF-8 type (e.g. in order to trigger access-out-of-
bounds or buffer overflow issues). - Usage of invalid data in EBML
Elements with a date type.
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1.4. Structure
EBML uses a system of Elements to compose an EBML Document. Elements
incorporate three parts: an Element ID, an Element Data Size, and
Element Data. The Element Data, which is described by the Element
ID, may include either binary data or one or many other Elements.
1.5. Variable Size Integer
The Element ID and Element Data Size are both encoded as a Variable
Size Integer, developed according to a UTF-8 like system. The
Variable Size Integer is composed of a VINT_WIDTH, VINT_MARKER, and
VINT_DATA, in that order. Variable Size Integers shall be referred
to as VINT for shorthand.
1.5.1. VINT_WIDTH
Each Variable Size Integer begins with a VINT_WIDTH which consists of
zero or many zero-value bits. The count of consecutive zero-values
of the VINT_WIDTH plus one equals the length in octets of the
Variable Size Integer. For example, a Variable Size Integer that
starts with a VINT_WIDTH which contains zero consecutive zero-value
bits is one octet in length and a Variable Size Integer that starts
with one consecutive zero-value bit is two octets in length. The
VINT_WIDTH MUST only contain zero-value bits or be empty.
1.5.2. 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 MUST be one bit in length and contain a
bit with a value of one. The first bit with a value of one within
the Variable Size Integer is the VINT_MARKER.
1.5.3. VINT_DATA
The VINT_DATA portion of the Variable Size Integer includes all data
that follows (but not including) the VINT_MARKER until end of the
Variable Size Integer whose length is derived from the VINT_WIDTH.
The bits required for the VINT_WIDTH and the VINT_MARKER combined use
one bit per octet of the total length of the Variable Size Integer.
Thus a Variable Size Integer of 1 octet length supplies 7 bits for
VINT_DATA, a 2 octet length supplies 14 bits for VINT_DATA, and a 3
octet length supplies 21 bits for VINT_DATA. If the number of bits
required for VINT_DATA are less than the bit size of VINT_DATA, then
VINT_DATA may be zero-padded to the left to a size that fits. The
VINT_DATA value MUST be expressed as a big-endian unsigned integer.
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1.5.4. VINT Examples
This table shows examples of Variable Size Integers at widths of 1 to
5 octets. 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 Width | Size | Representation |
+-------------+------+----------------------------------------------+
| 1 | 2^7 | 1xxx xxxx |
| 2 | 2^14 | 01xx xxxx xxxx xxxx |
| 3 | 2^21 | 001x xxxx xxxx xxxx xxxx xxxx |
| 4 | 2^28 | 0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx |
| 5 | 2^35 | 0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx |
| | | xxxx |
+-------------+------+----------------------------------------------+
Note that data encoded as a Variable Size Integer may be rendered at
octet widths larger than needed to store the data. In this table a
binary value of 0b10 is shown encoded as different Variable Size
Integers with widths from one octet to four octet. All four encoded
examples have identical semantic meaning though the VINT_WIDTH and
the padding of the VINT_DATA vary.
+--------------+-------------+--------------------------------------+
| Binary Value | Octet Width | As Represented in Variable Size |
| | | Integer |
+--------------+-------------+--------------------------------------+
| 10 | 1 | 1000 0010 |
| 10 | 2 | 0100 0000 0000 0010 |
| 10 | 3 | 0010 0000 0000 0000 0000 0010 |
| 10 | 4 | 0001 0000 0000 0000 0000 0000 0000 |
| | | 0010 |
+--------------+-------------+--------------------------------------+
1.6. Element ID
The Element ID MUST be encoded as a Variable Size Integer. By
default, EBML Element IDs may be encoded in lengths from one octet to
four octets, although Element IDs of greater lengths may be used if
the octet length of the EBML Document's longest Element ID is
declared in the EBMLMaxIDLength Element of the EBML Header. The
VINT_DATA component of the Element ID MUST NOT be set to either all
zero values or all one values. The VINT_DATA component of the
Element ID MUST be encoded at the shortest valid length. For
example, an Element ID with binary encoding of 1011 1111 is valid,
whereas an Element ID with binary encoding of 0100 0000 0011 1111
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stores a semantically equal VINT_DATA but is invalid because a
shorter VINT encoding is possible. The following table details this
specific example further:
+------------+-------------+----------------+-------------------+
| VINT_WIDTH | VINT_MARKER | VINT_DATA | Element ID Status |
+------------+-------------+----------------+-------------------+
| | 1 | 0111111 | Valid |
| 0 | 1 | 00000000111111 | Invalid |
+------------+-------------+----------------+-------------------+
The octet length of an Element ID determines its EBML Class.
+------------+--------------+--------------------------------+
| EBML Class | Octet Length | Number of Possible Element IDs |
+------------+--------------+--------------------------------+
| Class A | 1 | 2^7 - 2 = 126 |
| Class B | 2 | 2^14 - 2^7 - 1 = 16,255 |
| Class C | 3 | 2^21 - 2^14 - 1 = 2,080,767 |
| Class D | 4 | 2^28 - 2^21 - 1 = 266,388,303 |
+------------+--------------+--------------------------------+
1.7. Element Data Size
The Element Data Size expresses the length in octets of Element Data.
The Element Data Size itself MUST be encoded as a Variable Size
Integer. By default, EBML 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 EBML
Document's longest Element Data Size is declared in the
EBMLMaxSizeLength Element of the EBML Header. Unlike the VINT_DATA
of the Element ID, the VINT_DATA component of the Element Data Size
is not required 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.
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 Data Types which do not mandate a non-zero length.
An Element Data Size with all VINT_DATA bits set to zero indicates
that the Element Data of the Element is zero octets in length. Such
an Element is referred to as an Empty Element. If an Empty Element
has a "default" value declared then that default value MUST be
interpreted as the value of the Empty Element. If an Empty Element
has no "default" value declared then the semantic meaning of Empty
Element is defined as part of the definition of the EBML Element
Types.
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An Element Data Size with all VINT_DATA bits set to one is reserved
as an indicator that the size of the Element is unknown. The only
reserved value for the VINT_DATA of Element Data Size is all bits set
to one. This rule allows for an Element to be written and read
before the size of the Element is known; however unknown Element Data
Size values SHOULD NOT be used unnecessarily. An Element with an
unknown Element Data Size MUST be a Master-element in that it
contains other EBML Elements as sub-elements. Master-elements MAY
only use an unknown size if the "unknownsizeallowed" attribute of the
EBML Schema is set to true. The end of a Master-element with unknown
size is determined by the beginning of the next element that is not a
valid sub-element of that Master-element. An Element with an unknown
Element Data Size is referred to as an "Unknown-Sized Element".
For Element Data Sizes encoded at octet lengths from one to eight,
this table depicts the range of possible values that can be encoded
as an Element Data Size. An Element Data Size with an octet length
of 8 is able to express a size of 2^56-2 or 72,057,594,037,927,934
octets (or about 72 petabytes). The maximum possible value that can
be stored as Element Data Size is referred to as "VINTMAX".
+--------------+----------------------+
| Octet Length | Possible Value Range |
+--------------+----------------------+
| 1 | 0 to 2^7-2 |
| 2 | 0 to 2^14-2 |
| 3 | 0 to 2^21-2 |
| 4 | 0 to 2^28-2 |
| 5 | 0 to 2^35-2 |
| 6 | 0 to 2^42-2 |
| 7 | 0 to 2^49-2 |
| 8 | 0 to 2^56-2 |
+--------------+----------------------+
If the length of Element Data equals 2^(n*7)-1 then the octet length
of the Element Data Size MUST be at least n+1. This rule prevents an
Element Data Size from being expressed as a reserved value. For
example, an Element with an octet length of 127 MUST NOT be encoded
in an Element Data Size encoding with a one octet length. The
following table clarifies this rule by showing a valid and invalid
expression of an Element Data Size with a VINT_DATA of 127 (which is
equal to 2^(1*7)-1).
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+------------+-------------+----------------+-----------------------+
| VINT_WIDTH | VINT_MARKER | VINT_DATA | Element Data Size |
| | | | Status |
+------------+-------------+----------------+-----------------------+
| | 1 | 1111111 | Reserved (meaning |
| | | | Unknown) |
| 0 | 1 | 00000001111111 | Valid (meaning 127 |
| | | | octets) |
+------------+-------------+----------------+-----------------------+
1.8. EBML Element Types
Each defined EBML Element MUST have a declared Element Type. The
Element Type defines a concept for storing data that may be
constrained by length, endianness, and purpose.
+------------+------------------------------------------------------+
| Element | Signed Integer |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | Big-endian |
| Length | A Signed Integer Element MUST declare a length that |
| | is no greater than 8 octets. An Signed Integer |
| | Element with a zero-octet length represents an |
| | integer value of zero. |
| Definition | A Signed Integer stores an integer (meaning that it |
| | can be written without a fractional component) which |
| | may be negative, positive, or zero. Because EBML |
| | limits Signed Integers to 8 octets in length a |
| | Signed Element may store a number from |
| | -9,223,372,036,854,775,808 to |
| | +9,223,372,036,854,775,807. |
+------------+------------------------------------------------------+
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+------------+------------------------------------------------------+
| Element | Unsigned Integer |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | Big-endian |
| Length | A Unsigned Integer Element MUST declare a length |
| | that is no greater than 8 octets. An Unsigned |
| | Integer Element with a zero-octet length represents |
| | an integer value of zero. |
| Definition | An Unsigned Integer stores an integer (meaning that |
| | it can be written without a fractional component) |
| | which may be positive or zero. Because EBML limits |
| | Unsigned Integers to 8 octets in length an unsigned |
| | Element may store a number from 0 to |
| | 18,446,744,073,709,551,615. |
+------------+------------------------------------------------------+
+------------+------------------------------------------------------+
| Element | Float |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | Big-endian |
| Length | A Float Element MUST declare of length of either 0 |
| | octets (0 bit), 4 octets (32 bit) or 8 octets (64 |
| | bit). A Float Element with a zero-octet length |
| | represents a numerical value of zero. |
| Definition | A Float Elements stores a floating-point number as |
| | defined in IEEE 754. |
+------------+------------------------------------------------------+
+------------+------------------------------------------------------+
| Element | String |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | None |
| Length | A String Element may declare any length from zero to |
| | "VINTMAX". |
| Definition | A String Element may either be empty (zero-length) |
| | or contain Printable ASCII characters in the range |
| | of 0x20 to 0x7E. Octets with all bits set to zero |
| | may follow the string value when needed. |
+------------+------------------------------------------------------+
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+------------+------------------------------------------------------+
| Element | UTF-8 |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | None |
| Length | A UTF-8 Element may declare any length from zero to |
| | "VINTMAX". |
| Definition | A UTF-8 Element shall contain only a valid Unicode |
| | string as defined in RFC 2279 [2]. Octets with all |
| | bits set to zero may follow the UTF-8 value when |
| | needed. |
+------------+------------------------------------------------------+
+------------+------------------------------------------------------+
| Element | Date |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | None |
| Length | A Date Element MUST declare a length of either 0 |
| | octets or 8 octets. A Date Element with a zero-octet |
| | length represents a timestamp of |
| | 2001-01-01T00:00:00.000000000 UTC. |
| Definition | The Date Element MUST contain a Signed Integer 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. |
+------------+------------------------------------------------------+
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+------------+------------------------------------------------------+
| Element | Master-element |
| Data Type | |
+------------+------------------------------------------------------+
| Endianness | None |
| Length | A Master-element may declare any length from zero to |
| | "VINTMAX". The Master-element may also use an |
| | unknown length. See the section on Element Data Size |
| | for rules that apply to elements of unknown length. |
| Definition | The Master-element contains zero, one, or many other |
| | elements. Elements contained within a Master-element |
| | must be defined for use at levels greater than the |
| | level of the Master-element. For instance is a |
| | Master-element occurs on level 2 then all contained |
| | Elements must be valid at level 3. Element Data |
| | stored within Master-elements SHOULD only consist of |
| | EBML Elements and SHOULD NOT contain any data that |
| | is not part of an EBML Element. When EBML is used in |
| | transmission or streaming, data that is not part of |
| | an EBML Element is permitted to be present within a |
| | Master-element if "unknownsizeallowed" is enabled |
| | within that Master-element's definition. In this |
| | case, the reader should skip data until a valid |
| | Element ID of the same level or the next greater |
| | level of the Master-element is found. What Element |
| | IDs are considered valid within a Master-element is |
| | identified by the EBML Schema for that version of |
| | the EBML Document Type. Any data contained with a |
| | Master-element that is not part of an Element SHOULD |
| | be ignored. |
+------------+------------------------------------------------------+
+--------------+----------------------------------------------------+
| Element Data | Binary |
| Type | |
+--------------+----------------------------------------------------+
| Endianness | None |
| Length | A binary element may declare any length from zero |
| | to "VINTMAX". |
| Definition | The contents of a Binary element should not be |
| | interpreted by the EBML parser. |
+--------------+----------------------------------------------------+
1.9. EBML Document
An EBML Document is comprised of only two components, an EBML Header
and an EBML Body. An EBML Document MUST start with an EBML Header
which declares significant characteristics of the entire EBML Body.
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An EBML Document MAY only consist of EBML Elements and MUST NOT
contain any data that is not part of an EBML Element. The initial
EBML Element of an EBML Document and the Elements that follow it are
considered Level 0 Elements. If an EBML Master-element is considered
to be at level N and it contains one or many other EBML Elements then
the contained Elements shall be considered at Level N+1. Thus a
Level 2 Element would have to be contained by a Master-element (at
Level 1) that is contained by another Master-element (at Level 0).
1.9.1. EBML Header
The EBML Header is a declaration that provides processing
instructions and identification of the EBML Body. The EBML Header
may be considered as analogous to an XML Declaration. All EBML
Documents MUST begin with a valid EBML Header.
The EBML Header documents the EBML Schema (also known as the EBML
DocType) that may be used to semantically interpret the structure and
meaning of the EBML Document. Additionally the EBML Header documents
the versions of both EBML and the EBML Schema that were used to write
the EBML Document and the versions required to read the EBML
Document.
The EBML Header consists of a single Master-element with an Element
ID of 'EBML'. The EBML Header MUST ONLY contain EBML Elements that
are defined as part of the EBML Specification.
All EBML Elements within the EBML Header MUST NOT utilize any Element
ID with a length greater than 4 octets. All EBML Elements within the
EBML Header MUST NOT utilize any Element Data Size with a length
greater than 4 octets.
1.9.2. EBML Body
All data of an EBML Document following the EBML Header may be
considered the EBML Body. The end of the EBML Body, as well as the
end of the EBML Document that contains the EBML Body, is considered
as whichever comes first: the beginning of a new level 0 EBML Header
or the end of the file. The EBML Body MAY only consist of EBML
Elements and MUST NOT contain any data that is not part of an EBML
Element. Although the EBML specification itself defines precisely
what EBML Elements are to be used within the EBML Header, the EBML
specification does not name or define what EBML Elements are to be
used within the EBML Body. The definition of what EBML Elements are
to be used within the EBML Body is defined by an EBML Schema.
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1.10. EBML Stream
An EBML Stream is a file that consists of one or many EBML Documents
that are concatenated together. An occurrence of a Level 0 EBML
Header marks the beginning of an EBML Document.
1.11. Elements semantic
1.11.1. EBML Schema
An EBML Schema is an XML Document that defines the properties,
arrangement, and usage of EBML Elements that compose a specific EBML
Document Type. The relationship of an EBML Schema to an EBML
Document may be considered analogous to the relationship of an XML
Schema [3] to an XML Document [4]. An EBML Schema MUST be clearly
associated with one or many EBML Document Types. An EBML Schema must
be expressed as well-formed XML. An EBML Document Type is identified
by a unique string stored within the EBML Header element called
DocType; for example "matroska" or "webm".
As an XML Document, the EBML Schema MUST use "<EBMLSchema>" as the
top level element. The "<EBMLSchema>" element MAY contain
"<element>" sub-elements. Each "<element>" defines one EBML Element
through the use of several attributes which are defined in the
section on Section 1.11.1.1. EBML Schemas MAY contain additional
attributes to extend the semantics but MUST NOT conflict is the
definitions of the "<element>" attributes defined within this
specification.
Within the EBML Schema each EBML Element is defined to occur at a
specific level. For any specificied EBML Element that is not at
level 0, the Parent EBML Element refers to the EBML Master-element
that that EBML Element is contained within. For any specifiied EBML
Master-element the Child EBML Element refers to the EBML Elements
that may be immediately contained within that Master-element. For
any EBML Element that is not defined at level 0, the Parent EBML
Element may be identified by the preceding "<element>" node which has
a lower value as the defined "level" attribute. The only exception
for this rule are Global EBML Elements which may occur within any
Parent EBML Element within the restriction of the Global EBML
Element's range declaration.
An EBML Schema MUST declare exactly one Element at Level 0 (referred
to as the Root Element) that MUST occur exactly once within an EBML
Document. The Root Element MUST be mandatory (with minOccurs set to
1) and MUST be defined to occur exactly once (maxOccurs set to 1).
Note that the EBML and Void Elements may also occur at Level 0 but
are not considered to be Root Elements.
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Elements defined to only occur at Level 1 are known as Top-Level
Elements.
The EBML Schema does not itself document the EBML Header, but
documents all data of the EBML Document that follows the EBML Header.
The EBML Header itself is documented by this specification in the
Section 1.11.2 section. The EBML Schema also does not document
Global Elements that are defined by the EBML Specification (namely
Void and CRC-32).
1.11.1.1. EBML Schema Element Attributes
Within an EBML Schema the "<EBMLSchema>" uses the following
attributes to define the EBML Schema:
+-----------+----------+--------------------------------------------+
| attribute | required | definition |
| name | | |
+-----------+----------+--------------------------------------------+
| docType | Yes | The "docType" lists the official name of |
| | | the EBML Document Type that is defined by |
| | | the EBML Schema; for example, "<EBMLSchema |
| | | docType="matroska">". |
| version | Yes | The "version" lists an incremental non- |
| | | negative integer that specifies the |
| | | version of the docType documented by the |
| | | EBML Schema. Unlike XML Schemas, an EBML |
| | | Schema documents all versions of a |
| | | docType's definition rather than using |
| | | separate EBML Schemas for each version of |
| | | a docType. Elements may be introduced and |
| | | deprecated by using the "minver" and |
| | | "maxver" attributes of . |
+-----------+----------+--------------------------------------------+
Within an EBML Schema the "<element>" uses the following attributes
to define an EBML Element.
+--------------------+----------+-----------------------------------+
| attribute name | required | definition |
+--------------------+----------+-----------------------------------+
| name | Yes | The official human-readable name |
| | | of the EBML Element. The value of |
| | | the name MUST be in the form of |
| | | an NCName as defined by the XML |
| | | Schema specification [5]. |
| level | Yes | The level notes at what |
| | | hierarchical depth the EBML |
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| | | Element may occur within an EBML |
| | | Document. The Root Element of an |
| | | EBML Document is at level 0 and |
| | | the Elements that it may contain |
| | | are at level 1. The level MUST be |
| | | expressed as an integer. Note |
| | | that Elements defined as "global" |
| | | and "recursive" MAY occur at a |
| | | level greater than or equal to |
| | | the defined "level". |
| global | No | A boolean to express if an EBML |
| | | Element MUST occur at its defined |
| | | level or may occur within any |
| | | Parent EBML Element. If the |
| | | "global" attribute is not |
| | | expressed for that Element then |
| | | that element is to be considered |
| | | not global. |
| id | Yes | The Element ID expressed in |
| | | hexadecimal notation prefixed by |
| | | a '0x'. To reduce the risk of |
| | | false positives while parsing |
| | | EBML Streams, the IDs of the Root |
| | | Element and Top-Level Elements |
| | | SHOULD be at least 4 octets in |
| | | length. Element IDs defined for |
| | | use at Level 0 or Level 1 MAY use |
| | | shorter octet lengths to |
| | | facilitate padding and optimize |
| | | edits to EBML Documents; for |
| | | instance, the EBML Void Element |
| | | uses an Element ID with a one |
| | | octet length to allow its usage |
| | | in more writing and editing |
| | | scenarios. |
| minOccurs | No | An integer to express the minimal |
| | | number of occurrences that the |
| | | EBML Element MUST occur within |
| | | its Parent Element if its Parent |
| | | Element is used. If the Element |
| | | has no Parent Level (as is the |
| | | case with Elements at Level 0), |
| | | then minOccurs refers to |
| | | constaints on the Element's |
| | | occurrence within the EBML |
| | | Document. If the minOccurs |
| | | attribute is not expressed for |
| | | that Element then that Element |
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| | | shall be considered to have a |
| | | minOccurs value of 0. This value |
| | | of minOccurs MUST be a positive |
| | | integer. The semantic meaning of |
| | | minOccurs within an EBML Schema |
| | | is considered analogous to the |
| | | meaning of minOccurs within an |
| | | XML Schema [6]. Note that |
| | | Elements with minOccurs set to |
| | | "1" that also have a default |
| | | value declared are not required |
| | | to be stored but are required to |
| | | be interpretted, see the Section |
| | | 1.11.1.6. |
| maxOccurs | No | A value to express the maximum |
| | | number of occurrences that the |
| | | EBML Element MAY occur within its |
| | | Parent Element if its Parent |
| | | Element is used. If the Element |
| | | has no Parent Level (as is the |
| | | case with Elements at Level 0), |
| | | then maxOccurs refers to |
| | | constaints on the Element's |
| | | occurrence within the EBML |
| | | Document. This value may be |
| | | either a positive integer or the |
| | | term "unbounded" to indicate |
| | | there is no maximum number of |
| | | occurrences or the term |
| | | "identical" to indicate that the |
| | | Element is an Section 1.11.1.3. |
| | | If the maxOccurs attribute is not |
| | | expressed for that Element then |
| | | that Element shall be considered |
| | | to have a maxOccurs value of 1. |
| | | The semantic meaning of maxOccurs |
| | | within an EBML Schema is |
| | | considered analogous to the |
| | | meaning of minOccurs within an |
| | | XML Schema [7], with EBML Schema |
| | | adding the concept of Identically |
| | | Recurring Elements. |
| range | No | For Elements which are of |
| | | numerical types (Unsigned |
| | | Integer, Signed Integer, Float, |
| | | and Date) a numerical range may |
| | | be specified. If specified that |
| | | the value of the EBML Element |
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| | | MUST be within the defined range |
| | | inclusively. See the Section |
| | | 1.11.1.4 for rules applied to |
| | | expression of range values. |
| default | No | A default value may be provided. |
| | | If an Element is mandatory but |
| | | not written within its Parent |
| | | EBML Element, then the parser of |
| | | the EBML Document MUST insert the |
| | | defined default value of the |
| | | Element. EBML Elements that are |
| | | Master-elements MUST NOT declare |
| | | a default value. |
| type | Yes | As defined within the Section |
| | | 1.8, 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'. |
| unknownsizeallowed | No | A boolean to express if an EBML |
| | | Element MAY be used as an |
| | | "Unknown-Sized Element" (having |
| | | all VINT_DATA bits of Element |
| | | Data Size set to 1). The |
| | | "unknownsizeallowed" attribute |
| | | only applies to Master-elements. |
| | | If the "unknownsizeallowed" |
| | | attribute is not used it is |
| | | assumed that the element is not |
| | | allowed to use an unknown Element |
| | | Data Size. |
| recursive | No | A boolean to express if an EBML |
| | | Element MAY be stored |
| | | recursively. In this case the |
| | | Element MAY be stored at levels |
| | | greater that defined in the |
| | | "level" attribute if the Element |
| | | is a Child Element of a Parent |
| | | Element with the same Element ID. |
| | | The "recursive" attribute only |
| | | applies to Master-elements. If |
| | | the "recursive" attribute is not |
| | | used it is assumed that the |
| | | element is not allowed to be used |
| | | recursively. |
| minver | No | The "minver" (minimum version) |
| | | attribute stores a non-negative |
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| | | integer that represents the first |
| | | version of the docType to support |
| | | the element. If the "minver" |
| | | attribute is not used it is |
| | | assumed that the element has a |
| | | minimum version of "1". |
| maxver | No | The "maxver" (maximum version) |
| | | attribute stores a non-negative |
| | | integer that represents the last |
| | | or most recent version of the |
| | | docType to support the element. |
| | | If the "maxver" attribute is not |
| | | used it is assumed that the |
| | | element has a maximum version |
| | | equal to the value stored in the |
| | | "version" attribute of . |
+--------------------+----------+-----------------------------------+
The "<element>" nodes shall contain a description of the meaning and
use of the EBML Element stored within one or many "<documentation>"
sub-elements. The "<documentation>" sub-element may use a "lang"
attribute which may be set to the RFC 5646 value of the language of
the element's documentation. The "<documentation>" sub-element may
use a "type" attribute to distinguish the meaning of the
documentation. Recommended values for the "<documentation>" sub-
element's "type" attribute include: "definition", "rationale", "usage
notes", and "references".
The "<element>" nodes MUST be arranged hierarchically according to
the permitted structure of the EBML Document Type. An "<element>"
node that defines an EBML Element which is a Child Element of another
Parent Element MUST be stored as an immediate sub-element of the
"<element>" node that defines the Parent Element. "<element>" nodes
that define Level 0 Elements and Global Elements should be sub-
elements of "<EBMLSchema>".
1.11.1.2. EBML Schema Example
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<?xml version="1.0" encoding="utf-8"?>
<EBMLSchema docType="files-in-ebml-demo" version="1">
<!-- Root Element-->
<element name="Files" level="0" id="0x1946696C" type="master">
<documentation lang="en" type="definition">Container of data and
attributes representing one or many files.</documentation>
<element name="File" level="1" id="0x6146" type="master" minOccurs="1"
maxOccurs="unbounded">
<documentation lang="en" type="definition">An attached file.
</documentation>
<element name="FileName" level="2" id="0x614E" type="utf-8"
minOccurs="1">
<documentation lang="en" type="definition">Filename of the attached
file.</documentation>
</element>
<element name="MimeType" level="2" id="0x464D" type="string"
minOccurs="1">
<documentation lang="en" type="definition">MIME type of the
file.</documentation>
</element>
<element name="ModificationTimestamp" level="2" id="0x4654"
type="date" minOccurs="1">
<documentation lang="en" type="definition">Modification timestamp of
the file.</documentation>
</element>
<element name="Data" level="2" id="0x4664" type="binary"
minOccurs="1">
<documentation lang="en" type="definition">The data of the
file.</documentation>
</element>
</element>
</element>
</EBMLSchema>
1.11.1.3. Identically Recurring Elements
An Identically Recurring Element is an Element that may occur within
its Parent Element more than once but that each recurrence within
that Parent Element MUST be identical both in storage and semantics.
Identically Recurring Elements are permitted to be stored multiple
times within the same Parent Element in order to increase data
resilience and optimize the use of EBML in transmission. Identically
Recurring Elements SHOULD include a CRC-32 Element as a Child
Element; this is especially recommended when EBML is used for long-
term storage or transmission. If a Parent Element contains more than
one copy of an Identically Recurring Element which includes a CRC-32
Child Element then the first instance of the Identically Recurring
Element with a valid CRC-32 value should be used for interpretation.
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If a Parent Element contains more than one copy of an Identically
Recurring Element which does not contain a CRC-32 Child Element or if
CRC-32 Child Elements are present but none are valid then the first
instance of the Identically Recurring Element should be used for
interpretation.
1.11.1.4. Expression of range
The "range" attribute MUST only be used with EBML Elements that are
either "signed integer", "unsigned integer", or "float". The "range"
attribute does not support date EBML Elements. The "range"
expression may contain whitespace for readability but whitespace
within a "range" expression MUST NOT convey meaning. The expression
of the "range" MUST adhere to one of the following forms:
o "x-y" where x and y are integers or floats and "y" must be greater
than "x", meaning that the value must be greater than or equal to
"x" and less than or equal to "y".
o ">x" where "x" is an integer or float, meaning that the value MUST
be greater than "x".
o "x" where "x" is an integer or float, meaning that the value MUST
be equal "x".
The "range" may use the prefix "not" to indicate that the expressed
range is negated. Please also see the section on Section 1.11.1.5.
1.11.1.5. 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 a Hexadecimal Floating-Point Constants as defined in the
C11 standard ISO/IEC 9899:2011 [8] (see section 6.4.4.2 on Floating
Constants). The following table provides examples of expressions of
float ranges.
+-------------------+-----------------------------------------+
| as decimal | as Hexadecimal Floating-Point Constants |
+-------------------+-----------------------------------------+
| 0.0-1.0 | 0x0p+1-0x1p+0 |
| 1.0-256.0 | 0x1p+0-0x1p+8 |
| 0.857421875 | 0x1.b7p-1 |
| -1.0--0.857421875 | -0x1p+0--0x1.b7p-1 |
+-------------------+-----------------------------------------+
Within an expression of a float range, as in an integer range, the
"-" (hyphen) character is the separator between the minimal and
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maximum value permitted by the range. Note that Hexadecimal
Floating-Point Constants also use a "-" (hyphen) when indicating a
negative binary power. Within a float range, when a "-" (hyphen) is
immediately preceded by a letter "p", then the "-" (hyphen) is a part
of the Hexadecimal Floating-Point Constant which notes negative
binary power. Within a float range, when a "-" (hyphen) is not
immediately preceded by a letter "p", then the "-" (hyphen)
represents the separator between the minimal and maximum value
permitted by the range.
1.11.1.6. 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 EBML Element's value is equal to the declared default
value then that Element is not required to be present within the EBML
Document if its Parent EBML Element is present. In this case, the
default value of the Mandatory EBML Element may be assumed although
the EBML Element is not present within its Parent EBML Element. Also
in this case the parser of the EBML Document MUST insert the defined
default value of the Element.
If a Mandatory EBML Element has no default value declared by an EBML
Schema and its Parent EBML Element is present than 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 EBML Element is
present and the EBML Element's value is NOT equal to the declared
default value then the EBML Element MUST be used.
This table clarifies if a Mandatory EBML Element MUST be written,
according to if the default value is declared, if the value of the
EBML Element is equal to the declared default value, and if the
Parent EBML Element is used.
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+---------------+----------------+--------------+-------------------+
| Is the | Is the value | Is the | Then is storing |
| default value | equal to | Parent | the EBML Element |
| declared? | default? | Element | required? |
| | | used? | |
+---------------+----------------+--------------+-------------------+
| 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 |
| No | n/a | Yes | Yes |
| No | n/a | No | No |
+---------------+----------------+--------------+-------------------+
1.11.1.7. Note on the Use of default attributes to define non-Mandatory
EBML Elements
If an EBML Element is not Mandatory, has a defined default value, and
is an Empty EBML Element then the EBML Element MUST be interpreted as
expressing the default value.
1.11.2. EBML Header Elements
This specification here contains definitions of all EBML Elements of
the EBML Header.
+-------------+-----------------------------------------------------+
| Name | EBML |
+-------------+-----------------------------------------------------+
| Level | 0 |
| EBML ID | [1A][45][DF][A3] |
| Mandatory | Yes |
| Multiple | No |
| Range | - |
| Default | - |
| Type | Master-element |
| Description | Set the EBML characteristics of the data to follow. |
| | Each EBML Document has to start with this. |
+-------------+-----------------------------------------------------+
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+-------------+-----------------------------------------------------+
| Name | EBMLVersion |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][86] |
| Mandatory | Yes |
| Multiple | No |
| Range | 1 |
| Default | 1 |
| Type | Unsigned Integer |
| Description | The version of EBML parser used to create the EBML |
| | Document. |
+-------------+-----------------------------------------------------+
+-------------+-----------------------------------------------------+
| Name | EBMLReadVersion |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][F7] |
| Mandatory | Yes |
| Multiple | No |
| Range | 1 |
| Default | 1 |
| Type | Unsigned Integer |
| Description | The minimum EBML version a parser has to support to |
| | read this EBML Document. |
+-------------+-----------------------------------------------------+
+-------------+-----------------------------------------------------+
| Name | EBMLMaxIDLength |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][F2] |
| Mandatory | Yes |
| Multiple | No |
| Range | >3 |
| Default | 4 |
| Type | Unsigned Integer |
| Description | The EBMLMaxIDLength is the maximum length in octets |
| | of the Element IDs to be found within the EBML |
| | Body. An EBMLMaxIDLength value of four is |
| | recommended, though larger values are allowed. |
+-------------+-----------------------------------------------------+
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+-------------+-----------------------------------------------------+
| Name | EBMLMaxSizeLength |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][F3] |
| Mandatory | Yes |
| Multiple | No |
| Range | >0 |
| Default | 8 |
| Type | Unsigned Integer |
| Description | The EBMLMaxSizeLength is the maximum length in |
| | octets of the expression of all Element Data Sizes |
| | to be found within the EBML Body. To be clear |
| | EBMLMaxSizeLength documents the maximum 'length' of |
| | all Element Data Size expressions within the EBML |
| | Body and not the maximum 'value' of all Element |
| | Data Size expressions within the EBML Body. |
| | Elements that have a Element Data Size expression |
| | which is larger in octets than what is expressed by |
| | EBMLMaxSizeLength SHALL be considered invalid. |
+-------------+-----------------------------------------------------+
+-------------+-----------------------------------------------------+
| Name | DocType |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][82] |
| Mandatory | Yes |
| Multiple | No |
| Range | - |
| Default | matroska |
| Type | String |
| Description | A string that describes and identifies the content |
| | of the EBML Body that follows this EBML Header. |
+-------------+-----------------------------------------------------+
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+-------------+-----------------------------------------------------+
| Name | DocTypeVersion |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][87] |
| Mandatory | Yes |
| Multiple | No |
| Range | - |
| Default | 1 |
| Type | Unsigned Integer |
| Description | The version of DocType interpreter used to create |
| | the EBML Document. |
+-------------+-----------------------------------------------------+
+-------------+-----------------------------------------------------+
| Name | DocTypeReadVersion |
+-------------+-----------------------------------------------------+
| Level | 1 |
| EBML ID | [42][85] |
| Mandatory | Yes |
| Multiple | No |
| Range | - |
| Default | 1 |
| Type | Unsigned Integer |
| Description | The minimum DocType version an interpreter has to |
| | support to read this EBML Document. |
+-------------+-----------------------------------------------------+
1.11.3. Global elements (used everywhere in the format)
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+-------------+-----------------------------------------------------+
| Name | CRC-32 |
+-------------+-----------------------------------------------------+
| Level | 1+ |
| Global | Yes |
| EBML ID | [BF] |
| Mandatory | No |
| Range | - |
| Default | - |
| 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 |
| | Element within its Parent Element for easier |
| | reading. All Top-Level Elements of an EBML Document |
| | SHOULD include a CRC-32 Element as a Child Element. |
| | The CRC in use is the IEEE-CRC-32 algorithm as used |
| | in the ISO 3309 standard and in section 8.1.1.6.2 |
| | of ITU-T recommendation V.42, with initial value of |
| | 0xFFFFFFFF. The CRC value MUST be computed on a |
| | little endian bitstream and MUST use little endian |
| | storage. |
+-------------+-----------------------------------------------------+
+-------------+-----------------------------------------------------+
| Name | Void |
+-------------+-----------------------------------------------------+
| Level | 0+ |
| Global | Yes |
| EBML ID | [EC] |
| Mandatory | No |
| Multiple | Yes |
| Range | - |
| Default | - |
| Type | Binary |
| Description | Used to void damaged data, to avoid unexpected |
| | behaviors when using damaged data. The content is |
| | discarded. Also used to reserve space in a sub- |
| | element for later use. |
+-------------+-----------------------------------------------------+
2. References
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2.1. URIs
[1] https://tools.ietf.org/html/rfc2119
[2] http://www.faqs.org/rfcs/rfc2279.html
[3] http://www.w3.org/XML/Schema#dev
[4] http://www.w3.org/TR/xml/
[5] http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-decl
[6] https://www.w3.org/TR/xmlschema-0/#ref6
[7] https://www.w3.org/TR/xmlschema-0/#ref6
[8] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
Authors' Addresses
Steve Lhomme
Dave Rice
Moritz Bunkus
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