Extensible Binary Meta Language
draft-ietf-cellar-ebml-02
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| Document | Type | This is an older version of an Internet-Draft that was ultimately published as an RFC. | |
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| Authors | Steve Lhomme <>, Dave Rice <>, Moritz Bunkus <> | ||
| Last updated | 2017-06-07 (Latest revision 2017-02-26) | ||
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draft-ietf-cellar-ebml-02
cellar S. Lhomme
Internet-Draft
Intended status: Standards Track D. Rice
Expires: August 30, 2017
M. Bunkus
February 26, 2017
Extensible Binary Meta Language
draft-ietf-cellar-ebml-02
Abstract
This document defines the Extensible Binary Meta Language (EBML)
format as a generalized file format for any type of data in a
hierarchical form. 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 30, 2017.
Copyright Notice
Copyright (c) 2017 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
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notation and Conventions . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Variable Size Integer . . . . . . . . . . . . . . . . . . . . 7
5.1. VINT_WIDTH . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. VINT_MARKER . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. VINT_DATA . . . . . . . . . . . . . . . . . . . . . . . . 8
5.4. VINT Examples . . . . . . . . . . . . . . . . . . . . . . 8
6. Element ID . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Element Data Size . . . . . . . . . . . . . . . . . . . . . . 10
8. EBML Element Types . . . . . . . . . . . . . . . . . . . . . 12
8.1. Signed Integer Element . . . . . . . . . . . . . . . . . 13
8.2. Unsigned Integer Element . . . . . . . . . . . . . . . . 13
8.3. Float Element . . . . . . . . . . . . . . . . . . . . . . 13
8.4. String Element . . . . . . . . . . . . . . . . . . . . . 13
8.5. UTF-8 Element . . . . . . . . . . . . . . . . . . . . . . 14
8.6. Date Element . . . . . . . . . . . . . . . . . . . . . . 14
8.7. Master Element . . . . . . . . . . . . . . . . . . . . . 14
8.8. Binary Element . . . . . . . . . . . . . . . . . . . . . 15
9. EBML Document . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. EBML Header . . . . . . . . . . . . . . . . . . . . . . . 15
9.2. EBML Body . . . . . . . . . . . . . . . . . . . . . . . . 16
10. EBML Stream . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. Elements semantic . . . . . . . . . . . . . . . . . . . . . . 16
11.1. EBML Schema . . . . . . . . . . . . . . . . . . . . . . 16
11.1.1. Element . . . . . . . . . . . . . . . . . . . . . . 17
11.1.2. Attributes . . . . . . . . . . . . . . . . . . . . . 17
11.1.3. Element . . . . . . . . . . . . . . . . . . . . . . 17
11.1.4. Attributes . . . . . . . . . . . . . . . . . . . . . 18
11.1.5. Element . . . . . . . . . . . . . . . . . . . . . . 22
11.1.6. Attributes . . . . . . . . . . . . . . . . . . . . . 22
11.1.7. Element . . . . . . . . . . . . . . . . . . . . . . 23
11.1.8. Element . . . . . . . . . . . . . . . . . . . . . . 23
11.1.9. Attributes . . . . . . . . . . . . . . . . . . . . . 23
11.1.10. XML Schema for EBML Schema . . . . . . . . . . . . . 23
11.1.11. EBML Schema Example . . . . . . . . . . . . . . . . 25
11.1.12. Identically Recurring Elements . . . . . . . . . . . 25
11.1.13. Expression of range . . . . . . . . . . . . . . . . 26
11.1.14. Textual expression of Floats . . . . . . . . . . . . 26
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11.1.15. Note on the Use of default attributes to define
Mandatory EBML Elements . . . . . . . . . . . . . . 27
11.2. EBML Header Elements . . . . . . . . . . . . . . . . . . 28
11.2.1. EBML Element . . . . . . . . . . . . . . . . . . . . 28
11.2.2. EBMLVersion Element . . . . . . . . . . . . . . . . 28
11.2.3. EBMLReadVersion Element . . . . . . . . . . . . . . 29
11.2.4. EBMLMaxIDLength Element . . . . . . . . . . . . . . 29
11.2.5. EBMLMaxSizeLength Element . . . . . . . . . . . . . 30
11.2.6. DocType Element . . . . . . . . . . . . . . . . . . 30
11.2.7. DocTypeVersion Element . . . . . . . . . . . . . . . 31
11.2.8. DocTypeReadVersion Element . . . . . . . . . . . . . 31
11.3. Global elements (used everywhere in the format) . . . . 31
11.3.1. Void Element . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.1. Normative References . . . . . . . . . . . . . . . . . . 32
12.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
"EBML", short for Extensible Binary Meta Language, specifies a binary
and octet (byte) aligned format inspired by the principle of XML (a
framework for structuring data).
The goal of this document is to define a generic, binary, space-
efficient format that can be used to define more complex formats
(such as containers for multimedia content) using an "EBML Schema".
The definition of the "EBML" format recognizes the idea behind HTML
and XML as a good one: separate structure and semantics allowing the
same structural layer to be used with multiple, possibly widely
differing semantic layers. Except for the "EBML Header" and a few
global elements this specification does not define particular "EBML"
format semantics; however this specification is intended to define
how other "EBML"-based formats can be defined.
"EBML" uses a simple approach of building "Elements" upon three
pieces of data (tag, length, and value) as this approach is well
known, easy to parse, and allows selective data parsing. The "EBML"
structure additionally allows for hierarchical arrangement to support
complex structural formats in an efficient manner.
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 [RFC2119].
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This document defines specific terms in order to define the format
and application of "EBML". Specific terms are defined below:
"Child Element": A "Child Element" is a relative term to describe the
"EBML Elements" immediately contained within a "Master Element".
"Descendant Element": A "Descendant Element" is 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.
"EBML": Extensible Binary Meta Language
"Element Data": The value(s) of the "EBML Element" which is
identified by its "Element ID" and "Element Data Size". The form of
the "Element Data" is defined by this document and the corresponding
"EBML Schema" of the Element's "EBML Document Type".
"Element Data Size": An expression, encoded as a "Variable Size
Integer", of the length in octets of "Element Data".
"EBML Body": All data of an "EBML Document" following the "EBML
Header" may be considered the "EBML Body".
"EBML Class": A representation of the octet length of an "Element
ID".
"EBML Document": An "EBML Document" is a datastream comprised of only
two components, an "EBML Header" and an "EBML Body".
"EBML Document Type": An "EBML Document Type" is a name provided by
an "EBML Schema" for a particular implementation of "EBML" for a data
format (examples: matroska and webm).
"EBML Element": A foundation block of data that contains three parts:
an "Element ID", an "Element Data Size", and "Element Data".
"EBML Header": The "EBML Header" is a declaration that provides
processing instructions and identification of the "EBML Body". The
"EBML Header" may be considered as analogous to an XML Declaration
[W3C.REC-xml-20081126] (see section 2.8 on Prolog and Document Type
Declaration).
"EBML Reader": An "EBML Reader" is a data parser that interprets the
semantics of an "EBML Document" and creates a way for programs to use
"EBML".
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"EBML Schema": A standardized definition for the structure of an
"EBML Document Type".
"EBML Stream": An "EBML Stream" is a file that consists of one or
more "EBML Documents" that are concatenated together.
"Element ID": The "Element ID" is a binary value, encoded as a
"Variable Size Integer", used to uniquely identify a defined "EBML
Element" within a specific "EBML Schema".
"Element Name": The official human-readable name of the "EBML
Element".
"Element Path": The hierarchy of "Parent Element" where the "EBML
Element" is expected to be found in the "EBML Body".
"Empty Element": An "Empty Element" is an "EBML Element" that has an
"Element Data Size" with all "VINT_DATA" bits set to zero which
indicates that the "Element Data" of the Element is zero octets in
length.
"Master Element": The "Master Element" contains zero, one, or many
other "EBML Elements".
"Parent Element": A relative term to describe the "Master Element"
which contains a specified element. For any specified "EBML Element"
that is not at "Root Level", the "Parent Element" refers to the
"Master Element" in which that "EBML Element" is contained.
"Root Element": A mandatory, non-repeating "EBML Element" which
occurs at the top level of the path hierarchy within an "EBML Body"
and contains all other "EBML Elements" of the "EBML Body", excepting
optional "Void Elements".
"Root Level": The starting level in the hierarchy of an "EBML
Document".
"Top-Level Element": An "EBML Element" defined to only occur as a
"Child Element" of the "Root Element".
"Unknown-Sized Element": An Element with an unknown "Element Data
Size".
"Variable Size Integer": A compact variable-length binary value which
defines its own length.
"VINT": Also known as "Variable Size Integer".
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"VINTMAX": The maximum possible value that can be stored as "Element
Data Size".
3. Security Considerations
"EBML" itself does not offer any kind of security and does not
provide confidentiality. "EBML" does not provide any kind of
authorization. "EBML" only offers marginally useful and effective
data integrity options, such as CRC elements.
Even if the semantic layer offers any kind of encryption, "EBML"
itself could leak information at both the semantic layer (as declared
via the DocType element) and within the "EBML" structure (you can
derive the presence of "EBML Elements" even with an unknown semantic
layer with a heuristic approach; not without errors, of course, but
with a certain degree of confidence).
Attacks on an "EBML Reader" could include:
o Invalid "Element IDs" that are longer than the limit stated in the
"EBMLMaxIDLength Element" of the "EBML Header".
o Invalid "Element IDs" that are not encoded in the shortest-
possible way.
o Invalid "Element IDs" comprised of reserved values.
o Invalid "Element Data Size" values that are longer than the limit
stated in the "EBMLMaxSizeLength Element" of the "EBML Header".
o 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).
o Very high lengths in order to force out-of-memory situations
resulting in a denial of service, access-out-of-bounds issues etc.
o Missing "EBML Elements" that are mandatory and have no declared
default value.
o Usage of "0x00" octets in "EBML Elements" with a string type.
o 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).
o Usage of invalid data in "EBML Elements" with a date type.
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Side channel attacks could exploit:
o The semantic equivalence of the same string stored in a "String
Element" or "UTF-8 Element" with and without zero-bit padding.
o The semantic equivalence of "VINT_DATA" within "Element Data Size"
with to different lengths due to left-padding zero bits.
o Data contained within a "Master Element" which is not itself part
of an "EBML Element".
o Extraneous copies of "Identically Recurring Element".
o Copies of "Identically Recurring Element" within a "Parent
Element" that contain invalid "CRC-32 Elements".
o Use of "Void Elements".
4. Structure
"EBML" uses a system of Elements to compose an "EBML Document".
"EBML Elements" incorporate three parts: an "Element ID", an "Element
Data Size", and "Element Data". The "Element Data", which is
described by the "Element ID", includes either binary data, one or
many other "EBML Elements", or both.
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 left-pad the "VINT_DATA" value with zero bits so that
the whole "Variable Size Integer" is octet-aligned. "Variable Size
Integers" SHALL be referred to as "VINT" for shorthand.
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.
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Within the "EBML Header" the "VINT_WIDTH" MUST NOT exceed three bits
in length (meaning that the "Variable Size Integer" MUST NOT exceed
four octets in length). Within the "EBML Body", when "VINTs" are
used to express an "Element ID", the maximum length allowed for the
"VINT_WIDTH" is one less than the value set in the "EBMLMaxIDLength
Element". Within the "EBML Body", when "VINTs" are used to express
an "Element Data Size", the maximum length allowed for the
"VINT_WIDTH" is one less than the value set in the "EBMLMaxSizeLength
Element".
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".
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 out of eight bits of the total length
of the "Variable Size Integer". Thus a "Variable Size Integer" of 1
octet length supplies 7 bits for "VINT_DATA", a 2 octet length
supplies 14 bits for "VINT_DATA", and a 3 octet length supplies 21
bits for "VINT_DATA". If the number of bits required for "VINT_DATA"
are less than the bit size of "VINT_DATA", then "VINT_DATA" SHOULD be
zero-padded to the left to a size that fits. The "VINT_DATA" value
MUST be expressed as a big-endian unsigned integer.
5.4. VINT Examples
This table shows examples of "Variable Size Integers" with lengths
from 1 to 5 octets. The Size column refers to the size of the
"VINT_DATA" in bits. The Representation column depicts a binary
expression of "Variable Size Integers" where "VINT_WIDTH" is depicted
by '0', the "VINT_MARKER" as '1', and the "VINT_DATA" as 'x'.
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+-------------+------+----------------------------------------------+
| Octet | Size | Representation |
| Length | | |
+-------------+------+----------------------------------------------+
| 1 | 2^7 | 1xxx xxxx |
| 2 | 2^14 | 01xx xxxx xxxx xxxx |
| 3 | 2^21 | 001x xxxx xxxx xxxx xxxx xxxx |
| 4 | 2^28 | 0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx |
| 5 | 2^35 | 0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx |
| | | xxxx |
+-------------+------+----------------------------------------------+
Data encoded as a "Variable Size Integer" MAY be rendered at octet
lengths larger than needed to store the data. In this table a binary
value of "0b10" is shown encoded as different "Variable Size
Integers" with lengths from one octet to four octet. All four
encoded examples have identical semantic meaning though the
"VINT_WIDTH" and the padding of the "VINT_DATA" vary.
+--------------+--------------+-------------------------------------+
| Binary Value | Octet Length | As Represented in Variable Size |
| | | Integer |
+--------------+--------------+-------------------------------------+
| 10 | 1 | 1000 0010 |
| 10 | 2 | 0100 0000 0000 0010 |
| 10 | 3 | 0010 0000 0000 0000 0000 0010 |
| 10 | 4 | 0001 0000 0000 0000 0000 0000 0000 |
| | | 0010 |
+--------------+--------------+-------------------------------------+
6. Element ID
The "Element ID" MUST be encoded as a "Variable Size Integer". By
default, "Element IDs" are encoded in lengths from one octet to four
octets, although "Element IDs" of greater lengths are used if the
octet length of the longest "Element ID" of the "EBML Document" is
declared in the "EBMLMaxIDLength Element" of the "EBML Header" (see
Section 11.2.4). The "VINT_DATA" component of the "Element ID" MUST
NOT be set to either all zero values or all one values. The
"VINT_DATA" component of the "Element ID" MUST be encoded at the
shortest valid length. For example, an "Element ID" with binary
encoding of "1011 1111" is valid, whereas an "Element ID" with binary
encoding of "0100 0000 0011 1111" stores a semantically equal
"VINT_DATA" but is invalid because a shorter "VINT" encoding is
possible. Additionally, an "Element ID" with binary encoding of
"1111 1111" is invalid since the "VINT_DATA" section is set to all
one values, whereas an "Element ID" with binary encoding of "0100
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0000 0111 1111" stores a semantically equal "VINT_DATA" and is the
shortest "VINT" encoding is possible.
The following table details these specific examples further:
+------------+-------------+----------------+-----------------------+
| VINT_WIDTH | VINT_MARKER | VINT_DATA | Element ID Status |
+------------+-------------+----------------+-----------------------+
| | 1 | 0000000 | Invalid: "VINT_DATA" |
| | | | MUST NOT be set to |
| | | | all 0 |
| 0 | 1 | 00000000000000 | Invalid: "VINT_DATA" |
| | | | MUST NOT be set to |
| | | | all 0 |
| | 1 | 0000001 | Valid |
| 0 | 1 | 00000000000001 | Invalid: A shorter |
| | | | "VINT_DATA" encoding |
| | | | is available. |
| | 1 | 0111111 | Valid |
| 0 | 1 | 00000000111111 | Invalid: A shorter |
| | | | "VINT_DATA" encoding |
| | | | is available. |
| | 1 | 1111111 | Invalid: "VINT_DATA" |
| | | | MUST NOT be set to |
| | | | all 1 |
| 0 | 1 | 00000001111111 | Valid |
+------------+-------------+----------------+-----------------------+
The octet length of an "Element ID" determines its "EBML Class".
+------------+--------------+--------------------------------+
| EBML Class | Octet Length | Number of Possible Element IDs |
+------------+--------------+--------------------------------+
| Class A | 1 | 2^7 - 2 = 126 |
| Class B | 2 | 2^14 - 2^7 - 1 = 16,255 |
| Class C | 3 | 2^21 - 2^14 - 1 = 2,080,767 |
| Class D | 4 | 2^28 - 2^21 - 1 = 266,338,303 |
+------------+--------------+--------------------------------+
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, "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
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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" which do not mandate a non-zero
length (see Section 8). An "Element Data Size" with all "VINT_DATA"
bits set to zero indicates that the "Element Data" is zero octets in
length. Such an "EBML Element" is referred to as an "Empty Element".
If an "Empty Element" has a "default" value declared then the "EBML
Reader" MUST interpret the value of the "Empty Element" as the
"default" value. If an "Empty Element" has no "default" value
declared then the "EBML Reader" MUST interpret the value of the
"Empty Element" as defined as part of the definition of the
corresponding "EBML Element Type" associated with the "Element ID".
An "Element Data Size" with all "VINT_DATA" bits set to one is
reserved as an indicator that the size of the "EBML Element" is
unknown. The only reserved value for the "VINT_DATA" of "Element
Data Size" is all bits set to one. An "EBML Element" with an unknown
"Element Data Size" is referred to as an "Unknown-Sized Element".
Only "Master Elements" SHALL be "Unknown-Sized Elements". "Master
Elements" MUST NOT use an unknown size unless the
"unknownsizeallowed" attribute of their "EBML Schema" is set to true
(see Section 11.1.4.10). The use of "Unknown-Sized Elements" allows
for an "EBML Element" to be written and read before the size of the
"EBML Element" is known. "Unknown-Sized Element" MUST NOT be used or
defined unnecessarily; however if the "Element Data Size" is not
known before the "Element Data" is written, such as in some cases of
data streaming, then "Unknown-Sized Elements" MAY be used. The end
of an "Unknown-Sized Element" is determined by whichever comes first:
the end of the file or the beginning of the next "EBML Element",
defined by this document or the corresponding "EBML Schema", that is
not independently valid as "Descendant Element" of the "Unknown-Sized
Element".
For "Element Data Sizes" encoded at octet lengths from one to eight,
this table depicts the range of possible values that can be encoded
as an "Element Data Size". An "Element Data Size" with an octet
length of 8 is able to express a size of 2^56-2 or
72,057,594,037,927,934 octets (or about 72 petabytes). The maximum
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possible value that can be stored as "Element Data Size" is referred
to as "VINTMAX".
+--------------+----------------------+
| Octet Length | Possible Value Range |
+--------------+----------------------+
| 1 | 0 to 2^7-2 |
| 2 | 0 to 2^14-2 |
| 3 | 0 to 2^21-2 |
| 4 | 0 to 2^28-2 |
| 5 | 0 to 2^35-2 |
| 6 | 0 to 2^42-2 |
| 7 | 0 to 2^49-2 |
| 8 | 0 to 2^56-2 |
+--------------+----------------------+
If the length of "Element Data" equals "2^(n*7)-1" then the octet
length of the "Element Data Size" MUST be at least "n+1". This rule
prevents an "Element Data Size" from being expressed as a reserved
value. For example, an "EBML Element" with an octet length of 127
MUST NOT be encoded in an "Element Data Size" encoding with a one
octet length. The following table clarifies this rule by showing a
valid and invalid expression of an "Element Data Size" with a
"VINT_DATA" of 127 (which is equal to 2^(1*7)-1).
+------------+-------------+----------------+-----------------------+
| VINT_WIDTH | VINT_MARKER | VINT_DATA | Element Data Size |
| | | | Status |
+------------+-------------+----------------+-----------------------+
| | 1 | 1111111 | Reserved (meaning |
| | | | Unknown) |
| 0 | 1 | 00000001111111 | Valid (meaning 127 |
| | | | octets) |
+------------+-------------+----------------+-----------------------+
8. EBML Element Types
"EBML Elements" are defined by an "EBML Schema" which MUST declare
one of the following "EBML Element Types" for each "EBML Element".
An "EBML Element Type" defines a concept of storing data within an
"EBML Element" that describes such characteristics as length,
endianness, and definition.
"EBML Elements" which are defined as a "Signed Integer Element",
"Unsigned Integer Element", "Float Element", or "Date Element" use
big endian storage.
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8.1. 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) which could be negative,
positive, or zero. Signed Integers MUST be stored with two's
complement notation with the leftmost bit being the sign bit.
Because "EBML" limits Signed Integers to 8 octets in length a "Signed
Integer Element" stores a number from -9,223,372,036,854,775,808 to
+9,223,372,036,854,775,807.
8.2. 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) which could be positive or
zero. Because "EBML" limits Unsigned Integers to 8 octets in length
an "Unsigned Integer Element" stores a number from 0 to
18,446,744,073,709,551,615.
8.3. 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 as defined in
[IEEE.754.1985].
8.4. 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
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"0x7E". Octets with all bits set to zero MAY follow the string value
when needed, such as reducing the length of a stored string while
maintaining the same "Element Data Size". A string with one or more
octets with all bits set to zero and a string without one or more
octets with all bits set to zero are semantically equal.
8.5. 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
[RFC2279]. Octets with all bits set to zero MAY follow the string
value when needed, such as reducing the length of a stored UTF-8 data
while maintaining the same "Element Data Size". A UTF-8 value with
one or more octets with all bits set to zero and a UTF-8 value
without one or more octets with all bits set to zero are semantically
equal.
8.6. 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.
8.7. Master Element
A "Master Element" MUST declare a length in octets from zero to
"VINTMAX". The "Master Element" MAY also use an unknown length. See
Section 7 for rules that apply to elements of unknown length.
The "Master Element" contains zero, one, or many other elements.
"EBML Elements" contained within a "Master Element" MUST have the
"EBMLParentPath" of their "Element Path" equals to the
"EBMLReferencePath" of the "Master Element" "Element Path" (see
Section 11.1.4.2). "Element Data" stored within "Master Elements"
SHOULD only consist of "EBML Elements" and SHOULD NOT contain any
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data that is not part of an "EBML Element". When "EBML" is used in
transmission or streaming, data that is not part of an "EBML Element"
is permitted to be present within a "Master Element" if
"unknownsizeallowed" is enabled within the definition for that
"Master Element". In this case, the "EBML Reader" should skip data
until a valid "Element ID" of the same "EBMLParentPath" or the next
upper level "Element Path" of the "Master Element" is found. What
"Element IDs" are considered valid within a "Master Element" is
identified by the "EBML Schema" for that version of the "EBML
Document Type". Any data contained within a "Master Element" that is
not part of a "Child Element" MUST be ignored.
8.8. 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".
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" 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".
9.1. 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" consists of a single "Master Element" with an
"Element Name" of "EBML" and "Element ID" of "0x1A45DFA3" (see
Section 11.2.1). The "EBML Header" MUST only contain "EBML Elements"
that are defined as part of this document.
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All "EBML Elements" within the "EBML Header" MUST NOT use any
"Element ID" with a length greater than 4 octets. All "EBML
Elements" within the "EBML Header" MUST NOT use any "Element Data
Size" with a length greater than 4 octets.
9.2. 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 considered as
whichever comes first: the beginning of a new "EBML Header" at the
"Root Level" or the end of the file. The "EBML Body" MUST consist
only of "EBML Elements" and MUST NOT contain any data that is not
part of an "EBML Element". This document defines precisely what
"EBML Elements" are to be used within the "EBML Header", but does not
name or define what "EBML Elements" are to be used within the "EBML
Body". The definition of what "EBML Elements" are to be used within
the "EBML Body" is defined by an "EBML Schema".
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 "EBML
Header" at the "Root Level" marks the beginning of an "EBML
Document".
11. Elements semantic
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 [W3C.REC-xmlschema-0-20010502] to an XML Document
[W3C.REC-xml-20081126]. An "EBML Schema" MUST be clearly associated
with one or many "EBML Document Types". An "EBML Schema" must be
expressed as well-formed XML. An "EBML Document Type" is identified
by a string stored within the "EBML Header" in the "DocType Element";
for example "matroska" or "webm" (see Section 11.2.6). The "DocType"
value for an "EBML Document Type" SHOULD be unique and persistent.
An "EBML Schema" MUST declare exactly one "EBML Element" at "Root
Level" (referred to as the "Root Element") that MUST occur exactly
once within an "EBML Document". The "Void Element" MAY also occur at
"Root Level" but is not considered to be "Root Elements" (see
Section 11.3.1).
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The "EBML Schema" does not itself document the "EBML Header", but
documents all data of the "EBML Document" that follows the "EBML
Header". The "EBML Header" itself is documented by this
specification in the "EBML Header Elements" (see Section 11.2). The
"EBML Schema" also does not document "Global Elements" that are
defined by this document (namely the "Void Element" and the "CRC-32
Element").
11.1.1. Element
As an XML Document, the "EBML Schema" MUST use "<EBMLSchema>" as the
top level element. The "<EBMLSchema>" element MAY contain
"<element>" sub-elements.
11.1.2. Attributes
Within an "EBML Schema" the "<EBMLSchema>" element uses the following
attributes:
11.1.2.1. 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.
11.1.2.2. version
The "version" lists an incremental non-negative integer that
specifies the version of the docType documented by the "EBML Schema".
Unlike XML Schemas, an "EBML Schema" documents all versions of a
docType's definition rather than using separate "EBML Schemas" for
each version of a "docType". "EBML Elements" may be introduced and
deprecated by using the "minver" and "maxver" attributes of
"<element>".
The "version" attribute is REQUIRED within the "<EBMLSchema>"
Element.
11.1.3. Element
Each "<element>" defines one "EBML Element" through the use of
several attributes that are defined in Section 11.1.2. "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.
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The "<element>" nodes contain a description of the meaning and use of
the "EBML Element" stored within one or many "<documentation>" sub-
elements and zero or one "<restriction>" sub-element. All
"<element>" nodes MUST be sub-elements of the "<EBMLSchema>".
11.1.4. Attributes
Within an "EBML Schema" the "<element>" uses the following attributes
to define an "EBML Element":
11.1.4.1. name
The "name" provides the official human-readable name of the "EBML
Element". The value of the name MUST be in the form of characters
"A" to "Z", "a" to "z", "0" to "9", "-" and ".".
The "name" attribute is REQUIRED.
11.1.4.2. path
The path defines the allowed storage locations of the "EBML Element"
within an "EBML Document". This path MUST be defined with the full
hierarchy of "EBML Elements" separated with a "/". The top "EBML
Element" in the path hierarchy being the first in the value. The
syntax of the "path" attribute is defined using this Augmented
Backus-Naur Form (ABNF) [RFC5234] with the case sensitive update
[RFC7405] notation:
The "path" attribute is REQUIRED.
EBMLFullPath = EBMLElementOccurrence "(" EBMLReferencePath ")"
EBMLReferencePath = [EBMLParentPath] EBMLElementPath
EBMLParentPath = EBMLFixedParent EBMLLastParent
EBMLFixedParent = *(EBMLPathAtom)
EBMLElementPath = EBMLPathAtom / EBMLPathAtomRecursive
EBMLPathAtom = PathDelimiter EBMLAtomName
EBMLPathAtomRecursive = "(1*(" EBMLPathAtom "))"
EBMLLastParent = EBMLPathAtom / EBMLVariableParent
EBMLVariableParent = "(" VariableParentOccurrence "\)"
EBMLAtomName = 1*(EBMLNameChar)
EBMLNameChar = ALPHA / DIGIT / "-" / "."
PathDelimiter = "\"
EBMLElementOccurrence = [EBMLMinOccurrence] "*" [EBMLMaxOccurrence]
EBMLMinOccurrence = 1*DIGIT
EBMLMaxOccurrence = 1*DIGIT
VariableParentOccurrence = [PathMinOccurrence] "*" [PathMaxOccurrence]
PathMinOccurrence = 1*DIGIT
PathMaxOccurrence = 1*DIGIT
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The ""*"", ""("" and "")"" symbols MUST be interpreted as they are
defined in the ABNF.
The "EBMLPathAtom" part of the "EBMLElementPath" MUST be equal to the
"name" attribute of the "EBML Schema".
The starting "PathDelimiter" of the path corresponds to the root of
the "EBML Document".
The "EBMLElementOccurrence" part is interpreted as an ABNF Variable
Repetition. The repetition amounts correspond to how many times the
"EBML Element" can be found in its "Parent Element".
The "EBMLMinOccurrence" represents the minimum number of occurrences
of this "EBML Element" within its "Parent Element". Each instance of
the "Parent Element" MUST contain at least this many instances of
this "EBML Element". If the "EBML Element" has an empty
"EBMLParentPath" then "EBMLMinOccurrence" refers to constraints on
the occurrence of the "EBML Element" within the "EBML Document". If
"EBMLMinOccurrence" is not present then that "EBML Element" is
considered to have a "EBMLMinOccurrence" value of 0. The semantic
meaning of "EBMLMinOccurrence" within an "EBML Schema" is considered
analogous to the meaning of "minOccurs" within an "XML Schema".
"EBML Elements" with "EBMLMinOccurrence" set to "1" that also have a
"default" value (see Section 11.1.4.8) declared are not REQUIRED to
be stored but are REQUIRED to be interpreted, see Section 11.1.15.
An "EBML Element" defined with a "EBMLMinOccurrence" value greater
than zero is called a "Mandatory EBML Element".
The "EBMLMaxOccurrence" represents the maximum number of occurrences
of this "EBML Element" within its "Parent Element". Each instance of
the "Parent Element" MUST contain at most this many instances of this
"EBML Element". If the "EBML Element" has an empty "EBMLParentPath"
then "EBMLMaxOccurrence" refers to constraints on the occurrence of
the "EBML Element" within the "EBML Document". If
"EBMLMaxOccurrence" is not present then that "EBML Element" is
considered to have no maximum occurrence. The semantic meaning of
"EBMLMaxOccurrence" within an "EBML Schema path" is considered
analogous to the meaning of "maxOccurs" within an "XML Schema".
The "VariableParentOccurrence" part is interpreted as an ABNF
Variable Repetition. The repetition amounts correspond to the amount
of unspecified "Parent Element" levels there can be between the
"EBMLFixedParent" and the actual "EBMLElementPath".
If the path contains a "EBMLPathAtomRecursive" part, the "EBML
Element" can occur within itself recursively (see the
Section 11.1.4.11).
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11.1.4.3. id
The "Element ID" encoded as a "Variable Size Integer" expressed in
hexadecimal notation prefixed by a "0x" that is read and stored in
big-endian order. To reduce the risk of false positives while
parsing "EBML Streams", the "Element IDs" of the "Root Element" and
"Top-Level Elements" SHOULD be at least 4 octets in length. "Element
IDs" defined for use at "Root Level" or directly under the "Root
Level" MAY use shorter octet lengths to facilitate padding and
optimize edits to "EBML Documents"; for instance, the "Void Element"
uses an "Element ID" with a one octet length to allow its usage in
more writing and editing scenarios.
The "id" attribute is REQUIRED.
11.1.4.4. minOccurs
An integer expressing the minimum number of occurrences of this "EBML
Element" within its "Parent Element". The "minOccurs" value MUST be
equal to the "EBMLMinOccurrence" value of the "path".
The "minOccurs" attribute is OPTIONAL. If the "minOccurs" attribute
is not present then that "EBML Element" is considered to have a
"minOccurs" value of 0.
11.1.4.5. maxOccurs
An integer expressing the maximum number of occurrences of this "EBML
Element" within its "Parent Element". The "maxOccurs" value MUST be
equal to the "EBMLMaxOccurrence" value of the "path".
The "maxOccurs" attribute is OPTIONAL. If the "maxOccurs" attribute
is not present then that "EBML Element" is considered to have no
maximum occurrence, similar to "unbounded" in the XML world.
11.1.4.6. range
A numerical range for "EBML Elements" which are of numerical types
(Unsigned Integer, Signed Integer, Float, and Date). If specified
the value of the "EBML Element" MUST be within the defined range.
See Section 11.1.13 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.
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11.1.4.7. size
A value to express the valid length of the "Element Data" as written
measured in octets. The "size" provides a constraint in addition to
the Length value of the definition of the corresponding "EBML Element
Type". This "size" MUST be expressed as either a non-negative
integer or a range (see Section 11.1.13) that consists of only non-
negative integers and valid operators.
The "size" attribute is OPTIONAL. If the "size" attribute is not
present for that "EBML Element" then that "EBML Element" is only
limited in size by the definition of the associated "EBML Element
Type".
11.1.4.8. default
If an Element is mandatory (has a "EBMLMinOccurrence" value greater
than zero) but not written within its "Parent Element" or stored as
an "Empty Element", then the "EBML Reader" of the "EBML Document"
MUST semantically interpret the "EBML Element" as present with this
specified default value for the "EBML Element". "EBML Elements" that
are "Master Elements" MUST NOT declare a "default" value. "EBML
Elements" with a "minOccurs" value greater than 1 MUST NOT declare a
"default" value.
The "default" attribute is OPTIONAL.
11.1.4.9. type
The "type" MUST be set to one of the following values: 'integer'
(signed integer), 'uinteger' (unsigned integer), 'float', 'string',
'date', 'utf-8', 'master', or 'binary'. The content of each "type"
is defined within Section 8.
The "type" attribute is REQUIRED.
11.1.4.10. unknownsizeallowed
A boolean to express if an "EBML Element" MAY be used as an "Unknown-
Sized Element" (having all "VINT_DATA" bits of "Element Data Size"
set to 1). "EBML Elements" that are not "Master Elements" MUST NOT
set "unknownsizeallowed" to true. An "EBML Element" that is defined
with an "unknownsizeallowed" attribute set to 1 MUST also have the
"unknownsizeallowed" attribute of its "Parent Element" set to 1.
The "unknownsizeallowed" attribute is OPTIONAL. If the
"unknownsizeallowed" attribute is not used then that "EBML Element"
is not allowed to use an unknown "Element Data Size".
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11.1.4.11. recursive
A boolean to express if an "EBML Element" MAY be stored recursively.
In this case the "EBML Element" MAY be stored within another "EBML
Element" that has the same "Element ID". Which itself can be stored
in an "EBML Element" that has the same "Element ID", and so on.
"EBML Elements" that are not "Master Elements" MUST NOT set
"recursive" to true.
If the "path" contains a "EBMLPathAtomRecursive" part then the
"recursive" value MUST be true and false otherwise.
The "recursive" attribute is OPTIONAL. If the "recursive" attribute
is not present then the "EBML Element" MUST NOT be used recursively.
11.1.4.12. minver
The "minver" (minimum version) attribute stores a non-negative
integer that represents the first version of the "docType" to support
the "EBML Element".
The "minver" attribute is OPTIONAL. If the "minver" attribute is not
present then the "EBML Element" has a minimum version of "1".
11.1.4.13. maxver
The "maxver" (maximum version) attribute stores a non-negative
integer that represents the last or most recent version of the
"docType" to support the element. "maxver" MUST be greater than or
equal to "minver".
The "maxver" attribute is OPTIONAL. If the "maxver" attribute is not
present then the "EBML Element" has a maximum version equal to the
value stored in the "version" attribute of "<EBMLSchema>".
11.1.5. Element
The "<documentation>" element provides additional information about
the "EBML Element".
11.1.6. Attributes
11.1.6.1. lang
A "lang" attribute which is set to the [RFC5646] value of the
language of the element's documentation.
The "lang" attribute is OPTIONAL.
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11.1.6.2. type
A "type" attribute distinguishes the meaning of the documentation.
Values for the "<documentation>" sub-element's "type" attribute MUST
include one of the following: "definition", "rationale", "usage
notes", and "references".
The "type" attribute is OPTIONAL.
11.1.7. Element
The "<restriction>" element provides information about restrictions
to the allowable values for the "EBML Element" which are listed in
"<enum>" elements.
11.1.8. Element
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">" can not be a valid value
for a "EBML Element" that is defined as an unsigned integer). An
"<enum>" element MAY also store "<documentation>" elements to further
describe the "<enum>".
11.1.9. Attributes
11.1.9.1. label
The "label" provides a concise expression for human consumption that
describes what the "value" of the "<enum>" represents.
The "label" attribute is OPTIONAL.
11.1.9.2. value
The "value" represents data that MAY be stored within the "EBML
Element".
The "value" attribute is REQUIRED.
11.1.10. XML Schema for EBML Schema
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns="https://ietf.org/cellar/ebml" targetNamespace="https://ietf.org/cellar/ebml" xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" version="01">
<xsd:element name="EBMLSchema" type="EBMLSchemaType"/>
<xsd:complexType name="EBMLSchemaType">
<xsd:sequence>
<xsd:element name="element" type="elementType" minOccurs="0" maxOccurs="unbounded"/>
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</xsd:sequence>
<xsd:attribute name="docType" use="required"/>
<xsd:attribute name="version" use="required"/>
</xsd:complexType>
<xsd:complexType name="elementType">
<xsd:sequence>
<xsd:element name="documentation" type="documentationType" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element name="restriction" type="restrictionType" minOccurs="0" maxOccurs="1"/>
</xsd:sequence>
<xsd:attribute name="name" use="required"/>
<xsd:attribute name="path" use="required"/>
<xsd:attribute name="id" use="required"/>
<xsd:attribute name="minOccurs" default="0"/>
<xsd:attribute name="maxOccurs" default="1"/>
<xsd:attribute name="range"/>
<xsd:attribute name="size"/>
<xsd:attribute name="default"/>
<xsd:attribute name="type" use="required"/>
<xsd:attribute name="unknownsizeallowed"/>
<xsd:attribute name="recursive"/>
<xsd:attribute name="minver" default="1"/>
<xsd:attribute name="maxver"/>
</xsd:complexType>
<xsd:complexType name="restrictionType">
<xsd:sequence>
<xsd:element name="enum" type="enumType" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="enumType">
<xsd:sequence>
<xsd:element name="documentation" type="documentationType" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="label"/>
<xsd:attribute name="value" use="required"/>
</xsd:complexType>
<xsd:complexType name="documentationType" mixed="true">
<xsd:sequence>
<xsd:any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="lang"/>
<xsd:attribute name="type"/>
</xsd:complexType>
</xsd:schema>
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11.1.11. EBML Schema Example
<?xml version="1.0" encoding="utf-8"?>
<EBMLSchema xmlns="https://ietf.org/cellar/ebml" docType="files-in-ebml-demo" version="1">
<!-- Root Element-->
<element name="Files" path="*1(\Files)" id="0x1946696C" type="master">
<documentation lang="en" type="definition">Container of data and
attributes representing one or many files.</documentation>
</element>
<element name="File" path="1*(\Files\File)" id="0x6146" type="master" minOccurs="1">
<documentation lang="en" type="definition">An attached file.</documentation>
</element>
<element name="FileName" path="1*1(\Files\File\FileName)" id="0x614E" type="utf-8"
minOccurs="1">
<documentation lang="en" type="definition">Filename of the attached file.
</documentation>
</element>
<element name="MimeType" path="1*1(\Files\File\MimeType)" id="0x464D" type="string"
minOccurs="1">
<documentation lang="en" type="definition">MIME type of the file.</documentation>
</element>
<element name="ModificationTimestamp" path="1*1(\Files\File\ModificationTimestamp)"
id="0x4654" type="date" minOccurs="1">
<documentation lang="en" type="definition">Modification timestamp of the file.
</documentation>
</element>
<element name="Data" path="1*1(\Files\File\Data)" id="0x4664" type="binary"
minOccurs="1">
<documentation lang="en" type="definition">The data of the file.</documentation>
</element>
</EBMLSchema>
11.1.12. Identically Recurring Elements
An "Identically Recurring Element" is an "EBML Element" that MAY
occur within its "Parent Element" more than once but that each
recurrence within that "Parent Element" MUST be identical both in
storage and semantics. "Identically Recurring Elements" are
permitted to be stored multiple times within the same "Parent
Element" in order to increase data resilience and optimize the use of
"EBML" in transmission. For instance a pertinent "Top-Level Element"
could be periodically resent within a data stream so that an "EBML
Reader" which starts reading the stream from the middle could better
interpret the contents. "Identically Recurring Elements" SHOULD
include a "CRC-32 Element" as a "Child Element"; this is especially
recommended when "EBML" is used for long-term storage or
transmission. If a "Parent Element" contains more than one copy of
an "Identically Recurring Element" which includes a "CRC-32 Element"
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as a "Child Element" then the first instance of the "Identically
Recurring Element" with a valid CRC-32 value should be used for
interpretation. If a "Parent Element" contains more than one copy of
an "Identically Recurring Element" which does not contain a "CRC-32
Element" or if "CRC-32 Elements" are present but none are valid then
the first instance of the "Identically Recurring Element" should be
used for interpretation.
11.1.13. Expression of range
The "range" attribute MUST only be used with "EBML Elements" that are
either "signed integer", "unsigned integer", "float", or "date". The
"range" expression may contain whitespace for readability but
whitespace within a "range" expression MUST NOT convey meaning. The
expression of the "range" MUST adhere to one of the following forms:
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". "x" MUST be less than "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 greater than or equal to "x".
o "<x" where "x" is an integer or float, meaning that the value MUST
be less than "x".
o "<=x" where "x" is an integer or float, meaning that the value
MUST be less than or equal to "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 Section 11.1.14.
11.1.14. Textual expression of Floats
When a float value is represented textually in an "EBML Schema", such
as within a "default" or "range" value, the float values MUST be
expressed as Hexadecimal Floating-Point Constants as defined in the
C11 standard [ISO.9899.2011] (see section 6.4.4.2 on Floating
Constants). The following table provides examples of expressions of
float ranges.
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+-------------------+-----------------------------------------+
| as decimal | as Hexadecimal Floating-Point Constants |
+-------------------+-----------------------------------------+
| 0.0-1.0 | "0x0p+1-0x1p+0" |
| 1.0-256.0 | "0x1p+0-0x1p+8" |
| 0.857421875 | "0x1.b7p-1" |
| -1.0--0.857421875 | "-0x1p+0--0x1.b7p-1" |
+-------------------+-----------------------------------------+
Within an expression of a float range, as in an integer range, the
"-" (hyphen) character is the separator between the minimal and
maximum value permitted by the range. Hexadecimal Floating-Point
Constants also use a "-" (hyphen) when indicating a negative binary
power. Within a float range, when a "-" (hyphen) is immediately
preceded by a letter "p", then the "-" (hyphen) is a part of the
Hexadecimal Floating-Point Constant which notes negative binary
power. Within a float range, when a "-" (hyphen) is not immediately
preceded by a letter "p", then the "-" (hyphen) represents the
separator between the minimal and maximum value permitted by the
range.
11.1.15. Note on the Use of default attributes to define Mandatory EBML
Elements
If a "Mandatory EBML Element" has a default value declared by an
"EBML Schema" and the value of the "EBML Element" is equal to the
declared default value then that "EBML Element" is not required to be
present within the "EBML Document" if its "Parent Element" is
present. In this case, the default value of the "Mandatory EBML
Element" MUST be interpreted by the "EBML Reader" although the "EBML
Element" is not present within its "Parent Element".
If a "Mandatory EBML Element" has no default value declared by an
"EBML Schema" and its "Parent Element" is present then the "EBML
Element" MUST be present as well. If a "Mandatory EBML Element" has
a default value declared by an "EBML Schema" and its "Parent Element"
is present and the value of the "EBML Element" is NOT equal to the
declared default value then the "EBML Element" MUST be present.
This table clarifies if a "Mandatory EBML Element" MUST be written,
according to if the "default" value is declared, if the value of the
"EBML Element" is equal to the declared "default" value, and if the
"Parent Element" is used.
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+---------------+---------------+---------------+-------------------+
| Is the | Is the value | Is the Parent | Then is storing |
| default value | equal to | Element | the EBML Element |
| declared? | 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 |
+---------------+---------------+---------------+-------------------+
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: "1*1(\EBML)"
id: "0x1A45DFA3"
minOccurs: 1
maxOccurs: 1
type: "Master Element"
description: Set the "EBML" characteristics of the data to follow.
Each "EBML Document" has to start with this.
11.2.2. EBMLVersion Element
name: "EBMLVersion"
path: "1*1(\EBML\EBMLVersion)"
id "0x4286"
minOccurs: 1
maxOccurs: 1
range: not 0
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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: "1*1(\EBML\EBMLReadVersion)"
id: "0x42F7"
minOccurs: 1
maxOccurs: 1
range: 1
default: 1
type: Unsigned Integer
description: The minimum "EBML" version an "EBML Reader" has to
support to read this "EBML Document". The "EBMLReadVersion Element"
MUST be less than or equal to "EBMLVersion".
11.2.4. EBMLMaxIDLength Element
name: "EBMLMaxIDLength"
path: "1*1(\EBML\EBMLMaxIDLength)"
id "0x42F2"
minOccurs: 1
maxOccurs: 1
range: >=4
default: 4
type: Unsigned Integer
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description: The "EBMLMaxIDLength Element" stores the maximum length
in octets of the "Element IDs" to be found within the "EBML Body".
An "EBMLMaxIDLength Element" value of four is RECOMMENDED, though
larger values are allowed.
11.2.5. EBMLMaxSizeLength Element
name: "EBMLMaxSizeLength"
path: "1*1(\EBML\EBMLMaxSizeLength)"
id "0x42F3"
minOccurs: 1
maxOccurs: 1
range: not 0
default: 8
type: Unsigned Integer
description: The "EBMLMaxSizeLength Element" stores the maximum
length in octets of the expression of all "Element Data Sizes" to be
found within the "EBML Body". To be clear the "EBMLMaxSizeLength
Element" documents the maximum 'length' of all "Element Data Size"
expressions within the "EBML Body" and not the maximum 'value' of all
"Element Data Size" expressions within the "EBML Body". "EBML
Elements" that have an "Element Data Size" expression which is larger
in octets than what is expressed by "EBMLMaxSizeLength ELEMENT" SHALL
be considered invalid.
11.2.6. DocType Element
name: "DocType"
path: "1*1(\EBML\DocType)"
id "0x4282"
minOccurs: 1
maxOccurs: 1
size: >0
type: String
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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: "1*1(\EBML\DocTypeVersion)"
id "0x4287"
minOccurs: 1
maxOccurs: 1
default: 1
type: Unsigned Integer
description: The version of "DocType" interpreter used to create the
"EBML Document".
11.2.8. DocTypeReadVersion Element
name: DocTypeReadVersion
path: "1*1(\EBML\DocTypeReadVersion)"
id "0x4285"
minOccurs: 1
maxOccurs: 1
default: 1
type: Unsigned Integer
description: The minimum "DocType" version an "EBML Reader" has to
support to read this "EBML Document". The value of the
"DocTypeReadVersion Element" MUST be less than or equal to the value
of the "DocTypeVersion Element".
11.3. Global elements (used everywhere in the format)
name: CRC-32
path: "*1((1*\)\CRC-32)"
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id: "0xBF"
minOccurs: 0
maxOccurs: 1
size: 4
type: Binary
description: The "CRC-32 Element" contains a 32-bit Cyclic Redundancy
Check value of all the "Element Data" of the "Parent Element" as
stored except for the "CRC-32 Element" itself. When the "CRC-32
Element" is present, the "CRC-32 Element" MUST be the first ordered
"EBML Element" within its "Parent Element" for easier reading. All
"Top-Level Elements" of an "EBML Document" that are "Master Elements"
SHOULD include a "CRC-32 Element" as a "Child Element". The CRC in
use is the IEEE-CRC-32 algorithm as used in the [ISO.3309.1979]
standard and in section 8.1.1.6.2 of [ITU.V42.1994], with initial
value of "0xFFFFFFFF". The CRC value MUST be computed on a little
endian bitstream and MUST use little endian storage.
11.3.1. Void Element
name: Void
path: "*((*\)\Void)"
id: "0xEC"
minOccurs: 0
type: Binary
description: Used to void damaged data, to avoid unexpected behaviors
when using damaged data. The content is discarded. Also used to
reserve space in a sub-element for later use.
12. References
12.1. Normative References
[IEEE.754.1985]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754, August 1985.
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[ISO.3309.1979]
International Organization for Standardization, "Data
communication - High-level data link control procedures -
Frame structure", ISO Standard 3309, 1979.
[ISO.9899.2011]
International Organization for Standardization,
"Programming languages - C", ISO Standard 9899, 2011.
[ITU.V42.1994]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 1994.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<http://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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, DOI 10.17487/RFC2279, January 1998,
<http://www.rfc-editor.org/info/rfc2279>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://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, <http://www.rfc-editor.org/info/rfc5646>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<http://www.rfc-editor.org/info/rfc7405>.
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[W3C.REC-xml-20081126]
Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
12.2. Informative References
[W3C.REC-xmlschema-0-20010502]
Fallside, D., "XML Schema Part 0: Primer", World Wide Web
Consortium Recommendation REC-xmlschema-0-20010502, May
2001,
<http://www.w3.org/TR/2001/REC-xmlschema-0-20010502>.
Authors' Addresses
Steve Lhomme
Dave Rice
Moritz Bunkus
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