<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">
<rfc version="3" ipr="trust200902" docName="draft-ietf-cellar-ebml-20" sortRefs="true" submissionType="IETF" category="std" xml:lang="en" xmlns:xi="http://www.w3.org/2001/XInclude" tocInclude="true" symRefs="true" consensus="true" number="8794">
<front>
<title abbrev="EBML">Extensible Binary Meta Language</title><seriesInfo value="8794" status="standard" name="RFC"></seriesInfo>
<author initials="S." surname="Lhomme" fullname="Steve Lhomme"><address><postal>
</postal><email>slhomme@matroska.org</email>
</address></author>
<author initials="D." surname="Rice" fullname="Dave Rice"><address><postal>
</postal><email>dave@dericed.com</email>
</address></author>
<author initials="M." surname="Bunkus" fullname="Moritz Bunkus"><address><postal>
</postal><email>moritz@bunkus.org</email>
</address></author>
<date year="2020" month="July"></date>
<area>art</area>
<workgroup>cellar</workgroup>
<keyword>binary</keyword>
<keyword>storage</keyword>
<keyword>xml</keyword>
<keyword>matroska</keyword>
<keyword>webm</keyword>
<abstract>
<t>This document defines the Extensible Binary Meta Language (EBML) format as a binary container format designed for audio/video storage. EBML is designed as a binary equivalent to XML and uses a storage-efficient approach to build nested Elements with identifiers, lengths, and values. Similar to how an XML Schema defines the structure and semantics of an XML Document, this document defines how EBML Schemas are created to convey the semantics of an EBML Document.</t>
</abstract>
</front>
<middle>
<section anchor="introduction"><name>Introduction</name>
<t>EBML, short for Extensible Binary Meta Language, specifies a binary format
aligned with octets (bytes) and inspired by the principle of XML (a framework for
structuring data).</t>
<t>The goal of this document is to define a generic, binary, space-efficient
format that can be used to define more complex formats using an EBML
Schema. EBML is used by the multimedia container, Matroska
<xref target="I-D.ietf-cellar-matroska"></xref>. The applicability of EBML for other
use cases is beyond the scope of this document.</t>
<t>The definition of the EBML format recognizes the idea behind HTML and XML
as a good one: separate structure and semantics allowing the same structural
layer to be used with multiple, possibly widely differing, semantic
layers. Except for the EBML Header and a few Global Elements, this
specification does not define particular EBML format semantics; however, this
specification is intended to define how other EBML-based formats can be
defined, such as the audio/video container formats Matroska and WebM <xref target="WebM"></xref>.</t>
<t>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.</t>
<t>A typical EBML file has the following structure:</t>
<artwork>EBML Header (master)
+ DocType (string)
+ DocTypeVersion (unsigned integer)
EBML Body Root (master)
+ ElementA (utf-8)
+ Parent (master)
+ ElementB (integer)
+ Parent (master)
+ ElementB (integer)
</artwork>
</section>
<section anchor="notation-and-conventions"><name>Notation and Conventions</name>
<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
"<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
described in BCP 14 <xref target="RFC2119"></xref> <xref target="RFC8174"></xref>
when, and only when, they appear in all capitals, as shown here.</t>
<t>This document defines specific terms in order to define the format and
application of <tt>EBML</tt>. Specific terms are defined below:</t>
<dl>
<dt><tt>EBML</tt>:</dt>
<dd>Extensible Binary Meta Language</dd>
<dt><tt>EBML Document Type</tt>:</dt>
<dd>A name provided by an <tt>EBML Schema</tt> to designate a particular
implementation of <tt>EBML</tt> for a data format (e.g., Matroska and WebM).</dd>
<dt><tt>EBML Schema</tt>:</dt>
<dd>A standardized definition for the structure of an <tt>EBML Document Type</tt>.</dd>
<dt><tt>EBML Document</tt>:</dt>
<dd>A datastream comprised of only two components, an <tt>EBML Header</tt> and
an <tt>EBML Body</tt>.</dd>
<dt><tt>EBML Reader</tt>:</dt>
<dd>A data parser that interprets the semantics of an <tt>EBML Document</tt>
and creates a way for programs to use <tt>EBML</tt>.</dd>
<dt><tt>EBML Stream</tt>:</dt>
<dd>A file that consists of one or more <tt>EBML Documents</tt> that are
concatenated together.</dd>
<dt><tt>EBML Header</tt>:</dt>
<dd>A declaration that provides processing instructions and identification of
the <tt>EBML Body</tt>. The <tt>EBML Header</tt> is analogous to an XML Declaration
<xref target="XML"></xref> (see Section 2.8 on "Prolog and Document Type Declaration").</dd>
<dt><tt>EBML Body</tt>:</dt>
<dd>All data of an <tt>EBML Document</tt> following the <tt>EBML Header</tt>.</dd>
<dt><tt>Variable-Size Integer</tt>:</dt>
<dd>A compact variable-length binary value that defines its own length.</dd>
<dt><tt>VINT</tt>:</dt>
<dd>Also known as <tt>Variable-Size Integer</tt>.</dd>
<dt><tt>EBML Element</tt>:</dt>
<dd>A foundation block of data that contains three parts: an <tt>Element ID</tt>,
an <tt>Element Data Size</tt>, and <tt>Element Data</tt>.</dd>
<dt><tt>Element ID</tt>:</dt>
<dd>A binary value, encoded as a <tt>Variable-Size Integer</tt>,
used to uniquely identify a defined <tt>EBML Element</tt> within a specific
<tt>EBML Schema</tt>.</dd>
<dt><tt>Element Data Size</tt>:</dt>
<dd>An expression, encoded as a <tt>Variable-Size Integer</tt>, of the length
in octets of <tt>Element Data</tt>.</dd>
<dt><tt>VINTMAX</tt>:</dt>
<dd>The maximum possible value that can be stored as <tt>Element Data Size</tt>.</dd>
<dt><tt>Unknown-Sized Element</tt>:</dt>
<dd>An <tt>Element</tt> with an unknown <tt>Element Data Size</tt>.</dd>
<dt><tt>Element Data</tt>:</dt>
<dd>The value(s) of the <tt>EBML Element</tt>, which is identified by its
<tt>Element ID</tt> and <tt>Element Data Size</tt>. The form of the <tt>Element Data</tt> is
defined by this document and the corresponding <tt>EBML Schema</tt> of the
Element's <tt>EBML Document Type</tt>.</dd>
<dt><tt>Root Level</tt>:</dt>
<dd>The starting level in the hierarchy of an <tt>EBML Document</tt>.</dd>
<dt><tt>Root Element</tt>:</dt>
<dd>A mandatory, nonrepeating <tt>EBML Element</tt> that occurs at the top
level of the path hierarchy within an <tt>EBML Body</tt> and contains all other
<tt>EBML Elements</tt> of the <tt>EBML Body</tt>, excepting optional <tt>Void Elements</tt>.</dd>
<dt><tt>Top-Level Element</tt>:</dt>
<dd>An <tt>EBML Element</tt> defined to only occur as a <tt>Child Element</tt>
of the <tt>Root Element</tt>.</dd>
<dt><tt>Master Element</tt>:</dt>
<dd>The <tt>Master Element</tt> contains zero, one, or many other <tt>EBML Elements</tt>.</dd>
<dt><tt>Child Element</tt>:</dt>
<dd>A <tt>Child Element</tt> is a relative term to describe the <tt>EBML Elements</tt>
immediately contained within a <tt>Master Element</tt>.</dd>
<dt><tt>Parent Element</tt>:</dt>
<dd>A relative term to describe the <tt>Master Element</tt> that contains a
specified element. For any specified <tt>EBML Element</tt> that is not at
<tt>Root Level</tt>, the <tt>Parent Element</tt> refers to the <tt>Master Element</tt>
in which that <tt>EBML Element</tt> is directly contained.</dd>
<dt><tt>Descendant Element</tt>:</dt>
<dd>A relative term to describe any <tt>EBML Elements</tt> contained within a
<tt>Master Element</tt>, including any of the <tt>Child Elements</tt> of its
<tt>Child Elements</tt>, and so on.</dd>
<dt><tt>Void Element</tt>:</dt>
<dd>An <tt>Element</tt> used to overwrite data or
reserve space within a <tt>Master Element</tt> for later use.</dd>
<dt><tt>Element Name</tt>:</dt>
<dd>The human-readable name of the <tt>EBML Element</tt>.</dd>
<dt><tt>Element Path</tt>:</dt>
<dd>The hierarchy of <tt>Parent Element</tt> where the <tt>EBML Element</tt>
is expected to be found in the <tt>EBML Body</tt>.</dd>
<dt><tt>Empty Element</tt>:</dt>
<dd>An <tt>EBML Element</tt> that has an <tt>Element Data Size</tt> with all
<tt>VINT_DATA</tt> bits set to zero, which indicates
that the <tt>Element Data</tt> of the <tt>Element</tt> is zero octets in
length.</dd>
</dl>
</section>
<section anchor="structure"><name>Structure</name>
<t>EBML uses a system of Elements to compose an EBML Document. EBML Elements incorporate three parts: an Element ID, an Element Data Size, and Element Data. The Element Data, which is described by the Element ID, includes either binary data, one or more other EBML Elements, or both.</t>
</section>
<section anchor="variable-size-integer"><name>Variable-Size Integer</name>
<t>The Element ID and Element Data Size are both encoded as a Variable-Size
Integer. The Variable-Size Integer is composed of a VINT_WIDTH, VINT_MARKER,
and VINT_DATA, in that order. Variable-Size Integers <bcp14>MUST</bcp14>
left-pad the VINT_DATA value with zero bits so that the whole Variable-Size
Integer is octet aligned. The Variable-Size Integer will be referred to as
VINT for shorthand.</t>
<section anchor="vint-width"><name>VINT_WIDTH</name>
<t>Each Variable-Size Integer starts with a VINT_WIDTH followed by a
VINT_MARKER. VINT_WIDTH is a sequence of zero or more bits of value <tt>0</tt>
and is terminated by the VINT_MARKER, which is a single bit of value
<tt>1</tt>. The total length in bits of both VINT_WIDTH and VINT_MARKER is the
total length in octets in of the Variable-Size Integer.</t>
<t>The single bit <tt>1</tt> starts a Variable-Size Integer with a length of
one octet. The sequence of bits <tt>01</tt> starts a Variable-Size Integer
with a length of two octets. <tt>001</tt> starts a Variable-Size Integer with
a length of three octets, and so on, with each additional <tt>0</tt> bit adding one
octet to the length of the Variable-Size Integer.</t>
</section>
<section anchor="vint-marker"><name>VINT_MARKER</name>
<t>The VINT_MARKER serves as a separator between the VINT_WIDTH and VINT_DATA. Each Variable-Size Integer <bcp14>MUST</bcp14> contain exactly one VINT_MARKER. The VINT_MARKER is one bit in length and contain a bit with a value of one. The first bit with a value of one within the Variable-Size Integer is the VINT_MARKER.</t>
</section>
<section anchor="vint-data"><name>VINT_DATA</name>
<t>The VINT_DATA portion of the Variable-Size Integer includes all data following
(but not including) the VINT_MARKER until end of the Variable-Size
Integer whose length is derived from the VINT_WIDTH. The bits required for the
VINT_WIDTH and the VINT_MARKER use one out of every eight bits of the total
length of the Variable-Size Integer. Thus, a Variable-Size Integer of 1-octet
length supplies 7 bits for VINT_DATA, a 2-octet length supplies 14 bits for
VINT_DATA, and a 3-octet length supplies 21 bits for VINT_DATA. If the number
of bits required for VINT_DATA is less than the bit size of VINT_DATA, then
VINT_DATA <bcp14>MUST</bcp14> be zero-padded to the left to a size that
fits. The VINT_DATA value <bcp14>MUST</bcp14> be expressed as a big-endian
unsigned integer.</t>
</section>
<section anchor="vint-examples"><name>VINT Examples</name>
<t><xref target="tableUsableBits"></xref> shows examples of Variable-Size
Integers with lengths from 1 to 5 octets. The "Usable Bits" column refers to the
number of bits that can be used in the VINT_DATA. The "Representation" column
depicts a binary expression of Variable-Size Integers where VINT_WIDTH is
depicted by <tt>0</tt>, the VINT_MARKER as <tt>1</tt>, and the VINT_DATA as
<tt>x</tt>.</t>
<table anchor="tableUsableBits"><name>VINT examples depicting usable bits</name>
<thead>
<tr>
<th>Octet Length</th>
<th>Usable Bits</th>
<th align="left">Representation</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>7</td>
<td align="left">1xxx xxxx</td>
</tr>
<tr>
<td>2</td>
<td>14</td>
<td align="left">01xx xxxx xxxx xxxx</td>
</tr>
<tr>
<td>3</td>
<td>21</td>
<td align="left">001x xxxx xxxx xxxx xxxx xxxx</td>
</tr>
<tr>
<td>4</td>
<td>28</td>
<td align="left">0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx</td>
</tr>
<tr>
<td>5</td>
<td>35</td>
<td align="left">0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx</td>
</tr>
</tbody>
</table><t>A Variable-Size Integer may be rendered at octet lengths larger
than needed to store the data in order to facilitate overwriting it at a later
date -- e.g., when its final size isn't known in advance. In
<xref target="tableVariousSizes"></xref>, an integer <tt>2</tt> (with a
corresponding binary value of 0b10) is shown encoded as different Variable-Size
Integers with lengths from one octet to four octets. All four encoded
examples have identical semantic meaning, though the VINT_WIDTH and the padding
of the VINT_DATA vary.</t>
<table anchor="tableVariousSizes"><name>VINT examples depicting the same
integer value rendered at different VINT lengths</name>
<thead>
<tr>
<th>Integer</th>
<th>Octet Length</th>
<th align="right">As Represented in VINT (binary)</th>
<th align="right">As Represented in VINT (hexadecimal)</th>
</tr>
</thead>
<tbody>
<tr>
<td>2</td>
<td>1</td>
<td align="right">1000 0010</td>
<td align="right">0x82</td>
</tr>
<tr>
<td>2</td>
<td>2</td>
<td align="right">0100 0000 0000 0010</td>
<td align="right">0x4002</td>
</tr>
<tr>
<td>2</td>
<td>3</td>
<td align="right">0010 0000 0000 0000 0000 0010</td>
<td align="right">0x200002</td>
</tr>
<tr>
<td>2</td>
<td>4</td>
<td align="right">0001 0000 0000 0000 0000 0000 0000 0010</td>
<td align="right">0x10000002</td>
</tr>
</tbody>
</table></section>
</section>
<section anchor="element-id"><name>Element ID</name>
<t>An Element ID is a Variable-Size Integer. By default, Element IDs are from
one octet to four octets in length, although Element IDs of greater lengths
<bcp14>MAY</bcp14> be used if the EBMLMaxIDLength Element of the EBML Header
is set to a value greater than four (see
<xref target="ebmlmaxidlength-element"></xref>). The bits of the VINT_DATA component
of the Element ID <bcp14>MUST NOT</bcp14> be all <tt>0</tt> values or all
<tt>1</tt> values. The VINT_DATA component of the Element ID
<bcp14>MUST</bcp14> be encoded at the shortest valid length. For example, an
Element ID with binary encoding of <tt>1011 1111</tt> is valid, whereas an
Element ID with binary encoding of <tt>0100 0000 0011 1111</tt> stores a
semantically equal VINT_DATA but is invalid, because a shorter VINT encoding is
possible. Additionally, an Element ID with binary encoding of <tt>1111 1111</tt>
is invalid, since the VINT_DATA section is set to all one values,
whereas an Element ID with binary encoding of <tt>0100 0000 0111 1111</tt>
stores a semantically equal VINT_DATA and is the shortest-possible VINT
encoding.</t>
<t><xref target="tableElementIDValidity"></xref> details these specific examples further:</t>
<table anchor="tableElementIDValidity"><name>Examples of valid and invalid Element IDs</name>
<thead>
<tr>
<th align="right">VINT_WIDTH</th>
<th align="right">VINT_MARKER</th>
<th align="right">VINT_DATA</th>
<th align="left">Element ID Status</th>
</tr>
</thead>
<tbody>
<tr>
<td align="right"></td>
<td align="right">1</td>
<td align="right">0000000</td>
<td align="left">Invalid: VINT_DATA <bcp14>MUST NOT</bcp14> be set to all 0</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">00000000000000</td>
<td align="left">Invalid: VINT_DATA <bcp14>MUST NOT</bcp14> be set to all 0</td>
</tr>
<tr>
<td align="right"></td>
<td align="right">1</td>
<td align="right">0000001</td>
<td align="left">Valid</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">00000000000001</td>
<td align="left">Invalid: A shorter VINT_DATA encoding is available.</td>
</tr>
<tr>
<td align="right"></td>
<td align="right">1</td>
<td align="right">0111111</td>
<td align="left">Valid</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">00000000111111</td>
<td align="left">Invalid: A shorter VINT_DATA encoding is available.</td>
</tr>
<tr>
<td align="right"></td>
<td align="right">1</td>
<td align="right">1111111</td>
<td align="left">Invalid: VINT_DATA <bcp14>MUST NOT</bcp14> be set to all 1</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">00000001111111</td>
<td align="left">Valid</td>
</tr>
</tbody>
</table><t>The range and count of possible Element IDs are determined by their
octet length. Examples of this are provided in
<xref target="tableElementIDRanges"></xref>.</t>
<table anchor="tableElementIDRanges"><name>Examples of count and range for
Element IDs at various octet lengths</name>
<thead>
<tr>
<th align="center">Element ID Octet Length</th>
<th align="center">Range of Valid Element IDs</th>
<th align="right">Number of Valid Element IDs</th>
</tr>
</thead>
<tbody>
<tr>
<td align="center">1</td>
<td align="center">0x81 - 0xFE</td>
<td align="right">126</td>
</tr>
<tr>
<td align="center">2</td>
<td align="center">0x407F - 0x7FFE</td>
<td align="right">16,256</td>
</tr>
<tr>
<td align="center">3</td>
<td align="center">0x203FFF - 0x3FFFFE</td>
<td align="right">2,080,768</td>
</tr>
<tr>
<td align="center">4</td>
<td align="center">0x101FFFFF - 0x1FFFFFFE</td>
<td align="right">268,338,304</td>
</tr>
</tbody>
</table></section>
<section anchor="element-data-size"><name>Element Data Size</name>
<section anchor="data-size-format"><name>Data Size Format</name>
<t>The Element Data Size expresses the length in octets of Element Data. The
Element Data Size itself is encoded as a Variable-Size Integer. By default,
Element Data Sizes can be encoded in lengths from one octet to eight octets,
although Element Data Sizes of greater lengths <bcp14>MAY</bcp14> 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
<xref target="ebmlmaxsizelength-element"></xref>). 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).</t>
<t>Although an Element ID with all VINT_DATA bits set to zero is invalid, an
Element Data Size with all VINT_DATA bits set to zero is allowed for EBML
Element Types that do not mandate a nonzero length (see
<xref target="ebml-element-types"></xref>). 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 <bcp14>MUST</bcp14>
interpret the value of the Empty Element as the default value. If an Empty
Element has no default value declared, then the EBML Reader <bcp14>MUST</bcp14>
use the value of the Empty Element for the corresponding EBML Element Type of
the Element ID, 0 for numbers and an empty string for strings.</t>
</section>
<section anchor="unknown-data-size"><name>Unknown Data Size</name>
<t>An Element Data Size with all VINT_DATA bits set to one is reserved as an
indicator that the size of the EBML Element is unknown. The only reserved
value for the VINT_DATA of Element Data Size is all bits set to one. An EBML
Element with an unknown Element Data Size is referred to as an Unknown-Sized
Element. Only a Master Element is allowed to be of unknown size, and it can
only be so if the <tt>unknownsizeallowed</tt> attribute of its EBML Schema is
set to true (see <xref target="unknownsizeallowed"></xref>).</t>
<t>The use of Unknown-Sized Elements allows an EBML Element to be written and read before the size of the EBML Element is known. Unknown-Sized Elements <bcp14>MUST</bcp14> only be used if the Element Data Size is not known before the Element Data is written, such as in some cases of datastreaming. The end of an Unknown-Sized Element is determined by whichever comes first:</t>
<ul>
<li><t>Any EBML Element that is a valid Parent Element of the Unknown-Sized Element according to the EBML Schema, Global Elements excluded.</t>
</li>
<li><t>Any valid EBML Element according to the EBML Schema, Global Elements
excluded, that is not a Descendant Element of the Unknown-Sized Element but
shares a common direct parent, such as a Top-Level Element.</t>
</li>
<li><t>Any EBML Element that is a valid Root Element according to the EBML Schema, Global Elements excluded.</t>
</li>
<li><t>The end of the Parent Element with a known size has been reached.</t>
</li>
<li><t>The end of the EBML Document, either when reaching the end of the file or because a new EBML Header started.</t>
</li>
</ul>
<t>Consider an Unknown-Sized Element whose EBML path is
<tt>\root\level1\level2\elt</tt>. When reading a new Element ID, assuming the
EBML Path of that new Element is valid, here are some possible and impossible
ways that this new Element is ending <tt>elt</tt>:</t>
<table><name>Examples of determining the end of an Unknown-Sized Element
</name>
<thead>
<tr>
<th align="left">EBML Path of new element</th>
<th align="left">Status</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left"><tt>\root\level1\level2</tt></td>
<td align="left">Ends the Unknown-Sized Element, as it is a new Parent Element</td>
</tr>
<tr>
<td align="left"><tt>\root\level1</tt></td>
<td align="left">Ends the Unknown-Sized Element, as it is a new Parent Element</td>
</tr>
<tr>
<td align="left"><tt>\root</tt></td>
<td align="left">Ends the Unknown-Sized Element, as it is a new Root Element</td>
</tr>
<tr>
<td align="left"><tt>\root2</tt></td>
<td align="left">Ends the Unknown-Sized Element, as it is a new Root Element</td>
</tr>
<tr>
<td align="left"><tt>\root\level1\level2\other</tt></td>
<td align="left">Ends the Unknown-Sized Element, as they share the same parent</td>
</tr>
<tr>
<td align="left"><tt>\root\level1\level2\elt</tt></td>
<td align="left">Ends the Unknown-Sized Element, as they share the same parent</td>
</tr>
<tr>
<td align="left"><tt>\root\level1\level2\elt\inside</tt></td>
<td align="left">Doesn't end the Unknown-Sized Element; it's a child of <tt>elt</tt></td>
</tr>
<tr>
<td align="left"><tt>\root\level1\level2\elt\<global></tt></td>
<td align="left">Global Element is valid; it's a child of <tt>elt</tt></td>
</tr>
<tr>
<td align="left"><tt>\root\level1\level2\<global></tt></td>
<td align="left">Global Element cannot be interpreted with this path; while parsing <tt>elt</tt>, a Global Element can only be a child of <tt>elt</tt></td>
</tr>
</tbody>
</table></section>
<section anchor="data-size-values"><name>Data Size Values</name>
<t>For Element Data Sizes encoded at octet lengths from one to eight,
<xref target="tableVintRangePerLength"></xref> 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<sup>56</sup>-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.</t>
<table anchor="tableVintRangePerLength"><name>Possible range of values that
can be stored in VINTs, by octet length</name>
<thead>
<tr>
<th>Octet Length</th>
<th>Possible Value Range</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>0 to 2<sup>7</sup> - 2</td>
</tr>
<tr>
<td>2</td>
<td>0 to 2<sup>14</sup> - 2</td>
</tr>
<tr>
<td>3</td>
<td>0 to 2<sup>21</sup> - 2</td>
</tr>
<tr>
<td>4</td>
<td>0 to 2<sup>28</sup> - 2</td>
</tr>
<tr>
<td>5</td>
<td>0 to 2<sup>35</sup> - 2</td>
</tr>
<tr>
<td>6</td>
<td>0 to 2<sup>42</sup> - 2</td>
</tr>
<tr>
<td>7</td>
<td>0 to 2<sup>49</sup> - 2</td>
</tr>
<tr>
<td>8</td>
<td>0 to 2<sup>56</sup> - 2</td>
</tr>
</tbody>
</table><t>If the length of Element Data equals 2<sup>n*7</sup>-1, then the octet
length of the Element Data Size <bcp14>MUST</bcp14> be at least n+1. This rule
prevents an Element Data Size from being expressed as the unknown-size
value. <xref target="tableVintReservation"></xref> 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<sup>1*7</sup>-1) and 16,383 (which is equal to
2<sup>2*7</sup>-1).</t>
<table anchor="tableVintReservation"><name>Demonstration of VINT_DATA
reservation for VINTs of unknown size</name>
<thead>
<tr>
<th align="right">VINT_WIDTH</th>
<th align="right">VINT_MARKER</th>
<th align="right">VINT_DATA</th>
<th>Element Data Size Status</th>
</tr>
</thead>
<tbody>
<tr>
<td align="right"></td>
<td align="right">1</td>
<td align="right">1111111</td>
<td>Reserved (meaning Unknown)</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">00000001111111</td>
<td>Valid (meaning 127 octets)</td>
</tr>
<tr>
<td align="right">00</td>
<td align="right">1</td>
<td align="right">000000000000001111111</td>
<td>Valid (meaning 127 octets)</td>
</tr>
<tr>
<td align="right">0</td>
<td align="right">1</td>
<td align="right">11111111111111</td>
<td>Reserved (meaning Unknown)</td>
</tr>
<tr>
<td align="right">00</td>
<td align="right">1</td>
<td align="right">000000011111111111111</td>
<td>Valid (16,383 octets)</td>
</tr>
</tbody>
</table></section>
</section>
<section anchor="ebml-element-types"><name>EBML Element Types</name>
<t>EBML Elements are defined by an EBML Schema (see
<xref target="ebml-schema"></xref>), which <bcp14>MUST</bcp14> 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.</t>
<t>EBML Elements that are defined as a Signed Integer Element, Unsigned
Integer Element, Float Element, or Date Element use big-endian storage.</t>
<section anchor="signed-integer-element"><name>Signed Integer Element</name>
<t>A Signed Integer Element <bcp14>MUST</bcp14> 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.</t>
<t>A Signed Integer Element stores an integer (meaning that it can be written
without a fractional component) that could be negative, positive, or
zero. Signed Integers are stored with two's complement notation with the
leftmost bit being the sign bit. Because EBML limits Signed Integers to 8
octets in length, a Signed Integer Element stores a number from
-9,223,372,036,854,775,808 to +9,223,372,036,854,775,807.</t>
</section>
<section anchor="unsigned-integer-element"><name>Unsigned Integer Element</name>
<t>An Unsigned Integer Element <bcp14>MUST</bcp14> 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.</t>
<t>An Unsigned Integer Element stores an integer (meaning that it can be
written without a fractional component) that could be positive or
zero. Because EBML limits Unsigned Integers to 8 octets in length, an Unsigned
Integer Element stores a number from 0 to 18,446,744,073,709,551,615.</t>
</section>
<section anchor="float-element"><name>Float Element</name>
<t>A Float Element <bcp14>MUST</bcp14> 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.</t>
<t>A Float Element stores a floating-point number in the 32-bit and 64-bit
binary interchange format, as defined in <xref target="IEEE.754"></xref>.</t>
</section>
<section anchor="string-element"><name>String Element</name>
<t>A String Element <bcp14>MUST</bcp14> 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.</t>
<t>A String Element <bcp14>MUST</bcp14> either be empty (zero-length) or contain printable ASCII characters <xref target="RFC0020"></xref> in the range of 0x20 to 0x7E, with an exception made for termination (see <xref target="terminating-elements"></xref>).</t>
</section>
<section anchor="utf-8-element"><name>UTF-8 Element</name>
<t>A UTF-8 Element <bcp14>MUST</bcp14> 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.</t>
<t>A UTF-8 Element contains only a valid Unicode string as defined in <xref target="RFC3629"></xref>, with an exception made for termination (see <xref target="terminating-elements"></xref>).</t>
</section>
<section anchor="date-element"><name>Date Element</name>
<t>A Date Element <bcp14>MUST</bcp14> 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 <xref target="RFC3339"></xref>.</t>
<t>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.</t>
</section>
<section anchor="master-element"><name>Master Element</name>
<t>A Master Element <bcp14>MUST</bcp14> declare a length in octets from zero to VINTMAX or be of unknown length. See <xref target="element-data-size"></xref> for rules that apply to elements of unknown length.</t>
<t>The Master Element contains zero or more other elements. EBML Elements contained within a Master Element <bcp14>MUST</bcp14> have the EBMLParentPath of their Element Path equal to the EBMLFullPath of the Master Element Element Path (see <xref target="path"></xref>). Element Data stored within Master Elements <bcp14>SHOULD</bcp14> only consist of EBML Elements and <bcp14>SHOULD NOT</bcp14> contain any data that is not part of an EBML Element. The EBML Schema identifies what Element IDs are valid within the Master Elements for that version of the EBML Document Type. Any data contained within a Master Element that is not part of a Child Element <bcp14>MUST</bcp14> be ignored.</t>
</section>
<section anchor="binary-element"><name>Binary Element</name>
<t>A Binary Element <bcp14>MUST</bcp14> declare a length in octets from zero to VINTMAX.</t>
<t>The contents of a Binary Element should not be interpreted by the EBML Reader.</t>
</section>
</section>
<section anchor="ebml-document"><name>EBML Document</name>
<t>An EBML Document is composed of only two components, an EBML Header and an EBML Body. An EBML Document <bcp14>MUST</bcp14> start with an EBML Header that declares significant characteristics of the entire EBML Body. An EBML Document consists of EBML Elements and <bcp14>MUST NOT</bcp14> contain any data that is not part of an EBML Element.</t>
<section anchor="ebml-header"><name>EBML Header</name>
<t>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.</t>
<t>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.</t>
<t>The EBML Header <bcp14>MUST</bcp14> contain a single Master Element with an
Element Name of <tt>EBML</tt> and Element ID of <tt>0x1A45DFA3</tt> (see
<xref target="ebml-element"></xref>); the Master Element may have any number of
additional EBML Elements within it. The EBML Header of an EBML Document that
uses an EBMLVersion of 1 <bcp14>MUST</bcp14> only contain EBML Elements that
are defined as part of this document.</t>
<t>Elements within an EBML Header can be at most 4 octets long, except for the
EBML Element with Element Name <tt>EBML</tt> and Element ID
<tt>0x1A45DFA3</tt> (see <xref target="ebml-element"></xref>); this Element can be up
to 8 octets long.</t>
</section>
<section anchor="ebml-body"><name>EBML Body</name>
<t>All data of an EBML Document following the EBML Header is the EBML Body. The end of the EBML Body, as well as the end of the EBML Document that contains the EBML Body, is reached at whichever comes first: the beginning of a new EBML Header at the Root Level or the end of the file. This document defines precisely which EBML Elements are to be used within the EBML Header but does not name or define which EBML Elements are to be used within the EBML Body. The definition of which EBML Elements are to be used within the EBML Body is defined by an EBML Schema.</t>
<t>Within the EBML Body, the maximum octet length allowed for any Element ID
is set by the EBMLMaxIDLength Element of the EBML Header, and the maximum octet
length allowed for any Element Data Size is set by the EBMLMaxSizeLength
Element of the EBML Header.</t>
</section>
</section>
<section anchor="ebml-stream"><name>EBML Stream</name>
<t>An EBML Stream is a file that consists of one or more EBML Documents that are concatenated together. An occurrence of an EBML Header at the Root Level marks the beginning of an EBML Document.</t>
</section>
<section anchor="ebml-versioning"><name>EBML Versioning</name>
<t>An EBML Document handles 2 different versions: the version of the EBML Header and the version of the EBML Body. Both versions are meant to be backward compatible.</t>
<section anchor="ebml-header-version"><name>EBML Header Version</name>
<t>The version of the EBML Header is found in EBMLVersion. An EBML parser can read an EBML Header if it can read either the EBMLVersion version or a version equal or higher than the one found in EBMLReadVersion.</t>
</section>
<section anchor="ebml-document-version"><name>EBML Document Version</name>
<t>The version of the EBML Body is found in EBMLDocTypeVersion. A parser for the particular DocType format can read the EBML Document if it can read either the EBMLDocTypeVersion version of that format or a version equal or higher than the one found in EBMLDocTypeReadVersion.</t>
</section>
</section>
<section anchor="elements-semantics"><name>Elements semantics</name>
<section anchor="ebml-schema"><name>EBML Schema</name>
<t>An EBML Schema is a well-formed XML Document
<xref target="XML"></xref> that defines the properties,
arrangement, and usage of EBML Elements that compose a specific EBML Document
Type. The relationship of an EBML Schema to an EBML Document is analogous to
the relationship of an XML Schema <xref target="XML-SCHEMA"></xref> to an XML
Document <xref target="XML"></xref>. An EBML Schema
<bcp14>MUST</bcp14> be clearly associated with one or more EBML Document
Types. An EBML Document Type is identified by a string stored within the EBML
Header in the DocType Element -- for example, Matroska or WebM (see
<xref target="doctype-element"></xref>). The DocType value for an EBML Document Type
<bcp14>MUST</bcp14> be unique, persistent, and described in the IANA registry
(see <xref target="ebml-doctypes-registry"></xref>).</t>
<t>An EBML Schema <bcp14>MUST</bcp14> declare exactly one EBML Element at Root Level (referred to as the Root Element) that occurs exactly once within an EBML Document. The Void Element <bcp14>MAY</bcp14> also occur at Root Level but is not a Root Element (see <xref target="void-element"></xref>).</t>
<t>The EBML Schema <bcp14>MUST</bcp14> document all Elements of the EBML
Body. The EBML Schema does not document Global Elements that are defined by
this document (namely, the Void Element and the CRC-32 Element).</t>
<t>The EBML Schema <bcp14>MUST NOT</bcp14> use the Element ID
<tt>0x1A45DFA3</tt>, which is reserved for the EBML Header for the purpose of
resynchronization.</t>
<t>An EBML Schema <bcp14>MAY</bcp14> constrain the use of EBML Header Elements
(see <xref target="ebml-header-elements"></xref>) by adding or constraining
that Element's <tt>range</tt> attribute. For example, an EBML Schema
<bcp14>MAY</bcp14> constrain the EBMLMaxSizeLength to a maximum value of
<tt>8</tt> or <bcp14>MAY</bcp14> constrain the EBMLVersion to only support a
value of <tt>1</tt>. If an EBML Schema adopts the EBML Header Element as is,
then it is not required to document that Element within the EBML Schema. If an
EBML Schema constrains the range of an EBML Header Element, then that Element
<bcp14>MUST</bcp14> be documented within an <tt><element></tt> node of
the EBML Schema. This document provides an example of an EBML Schema; see
<xref target="ebml-schema-example"></xref>.</t>
<section anchor="ebml-schema-example"><name>EBML Schema Example</name>
<sourcecode type="xml"><?xml version="1.0" encoding="utf-8"?>
<EBMLSchema xmlns="urn:ietf:rfc:8794"
docType="files-in-ebml-demo" version="1">
<!-- constraints to the range of two EBML Header Elements -->
<element name="EBMLReadVersion" path="\EBML\EBMLReadVersion"
id="0x42F7" minOccurs="1" maxOccurs="1" range="1" default="1"
type="uinteger"/>
<element name="EBMLMaxSizeLength"
path="\EBML\EBMLMaxSizeLength" id="0x42F3" minOccurs="1"
maxOccurs="1" range="8" default="8" type="uinteger"/>
<!-- Root Element-->
<element name="Files" path="\Files" id="0x1946696C"
type="master">
<documentation lang="en"
purpose="definition">Container of data and
attributes representing one or many files.</documentation>
</element>
<element name="File" path="\Files\File" id="0x6146"
type="master" minOccurs="1">
<documentation lang="en" purpose="definition">
An attached file.
</documentation>
</element>
<element name="FileName" path="\Files\File\FileName"
id="0x614E" type="utf-8"
minOccurs="1">
<documentation lang="en" purpose="definition">
Filename of the attached file.
</documentation>
</element>
<element name="MimeType" path="\Files\File\MimeType"
id="0x464D" type="string"
minOccurs="1">
<documentation lang="en" purpose="definition">
MIME type of the file.
</documentation>
</element>
<element name="ModificationTimestamp"
path="\Files\File\ModificationTimestamp" id="0x4654"
type="date" minOccurs="1">
<documentation lang="en" purpose="definition">
Modification timestamp of the file.
</documentation>
</element>
<element name="Data" path="\Files\File\Data" id="0x4664"
type="binary" minOccurs="1">
<documentation lang="en" purpose="definition">
The data of the file.
</documentation>
</element>
</EBMLSchema>
</sourcecode>
</section>
<section anchor="ebmlschema-element"><name><tt><EBMLSchema></tt> Element</name>
<t>Within an EBML Schema, the XPath <xref target="XPath"></xref> of the
<tt><EBMLSchema></tt> element is <tt>/EBMLSchema</tt>.</t>
<t>When used as an XML Document, the EBML Schema <bcp14>MUST</bcp14> use
<tt><EBMLSchema></tt> as the top-level element. The
<tt><EBMLSchema></tt> element can contain <tt><element></tt>
subelements.</t>
</section>
<section anchor="ebmlschema-namespace"><name><tt><EBMLSchema></tt> Namespace</name>
<t>The namespace URI for elements of the EBML Schema is a URN as defined by
<xref target="RFC2141"></xref> that uses the namespace identifier 'ietf' defined by <xref target="RFC2648"></xref>
and extended by <xref target="RFC3688"></xref>. This URN is <tt>urn:ietf:rfc:8794</tt>.</t>
</section>
<section anchor="ebmlschema-attributes"><name><tt><EBMLSchema></tt> Attributes</name>
<t>Within an EBML Schema, the <tt><EBMLSchema></tt> element uses the
following attributes to define an EBML Element:</t>
<section anchor="doctype"><name>docType</name>
<t>Within an EBML Schema, the XPath of the <tt>@docType</tt> attribute is
<tt>/EBMLSchema/@docType</tt>.</t>
<t>The docType lists the official name of the EBML Document Type that is
defined by the EBML Schema; for example,
<tt><EBMLSchema docType="matroska"></tt>.</t>
<t>The <tt>docType</tt> attribute is <bcp14>REQUIRED</bcp14> within the
<tt><EBMLSchema></tt> Element.</t>
</section>
<section anchor="version"><name>version</name>
<t>Within an EBML Schema, the XPath of the <tt>@version</tt> attribute is
<tt>/EBMLSchema/@version</tt>.</t>
<t>The version lists a nonnegative integer that specifies the version of the
docType documented by the EBML Schema. Unlike XML Schemas, an EBML Schema
documents all versions of a docType's definition rather than using separate
EBML Schemas for each version of a docType. EBML Elements may be introduced
and deprecated by using the <tt>minver</tt> and <tt>maxver</tt> attributes of
<tt><element></tt>.</t>
<t>The <tt>version</tt> attribute is <bcp14>REQUIRED</bcp14> within the
<tt><EBMLSchema></tt> Element.</t>
</section>
<section anchor="ebml"><name>ebml</name>
<t>Within an EBML Schema, the XPath of the <tt>@ebml</tt> attribute is <tt>/EBMLSchema/@ebml</tt>.</t>
<t>The <tt>ebml</tt> attribute is a positive integer that specifies the
version of the EBML Header (see <xref target="ebmlversion-element"></xref>)
used by the EBML Schema. If the attribute is omitted, the EBML Header version
is 1.</t>
</section>
</section>
<section anchor="element-element"><name><tt><element></tt> Element</name>
<t>Within an EBML Schema, the XPath of the <tt><element></tt> element is
<tt>/EBMLSchema/element</tt>.</t>
<t>Each <tt><element></tt> defines one EBML Element through the use of
several attributes that are defined in
<xref target="element-attributes"></xref>. EBML Schemas <bcp14>MAY</bcp14> contain
additional attributes to extend the semantics but <bcp14>MUST NOT</bcp14>
conflict with the definitions of the <tt><element></tt> attributes
defined within this document.</t>
<t>The <tt><element></tt> nodes contain a description of the meaning and use of the EBML Element stored within one or more <tt><documentation></tt> subelements, followed by optional <tt><implementation_note></tt> subelements, followed by zero or one <tt><restriction></tt> subelement, followed by optional <tt><extension></tt> subelements. All <tt><element></tt> nodes <bcp14>MUST</bcp14> be subelements of the <tt><EBMLSchema></tt>.</t>
</section>
<section anchor="element-attributes"><name><tt><element></tt> Attributes</name>
<t>Within an EBML Schema, the <tt><element></tt> uses the following
attributes to define an EBML Element:</t>
<section anchor="name"><name>name</name>
<t>Within an EBML Schema, the XPath of the <tt>@name</tt> attribute is
<tt>/EBMLSchema/element/@name</tt>.</t>
<t>The name provides the human-readable name of the EBML Element. The value of
the name <bcp14>MUST</bcp14> be in the form of characters "A" to
"Z", "a" to "z", "0" to "9",
"-", and ".". The first character of the name
<bcp14>MUST</bcp14> be in the form of an "A" to "Z",
"a" to "z", or "0" to "9" character.</t>
<t>The <tt>name</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
<section anchor="path"><name>path</name>
<t>Within an EBML Schema, the XPath of the <tt>@path</tt> attribute is
<tt>/EBMLSchema/element/@path</tt>.</t>
<t>The path defines the allowed storage locations of the EBML Element within
an EBML Document. This path <bcp14>MUST</bcp14> be defined with the full
hierarchy of EBML Elements separated with a <tt>\</tt>. The top EBML Element
in the path hierarchy is the first in the value. The syntax of the
<tt>path</tt> attribute is defined using this Augmented Backus-Naur Form
(ABNF) <xref target="RFC5234"></xref> with the case-sensitive update
<xref target="RFC7405"></xref> notation:</t>
<t>The <tt>path</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
<sourcecode type="abnf">EBMLFullPath = EBMLParentPath EBMLElement
EBMLParentPath = PathDelimiter [EBMLParents]
EBMLParents = 0*IntermediatePathAtom EBMLLastParent
IntermediatePathAtom = EBMLPathAtom / GlobalPlaceholder
EBMLLastParent = EBMLPathAtom / GlobalPlaceholder
EBMLPathAtom = [IsRecursive] EBMLAtomName PathDelimiter
EBMLElement = [IsRecursive] EBMLAtomName
PathDelimiter = "\"
IsRecursive = "+"
EBMLAtomName = ALPHA / DIGIT 0*EBMLNameChar
EBMLNameChar = ALPHA / DIGIT / "-" / "."
GlobalPlaceholder = "(" GlobalParentOccurrence "\)"
GlobalParentOccurrence = [PathMinOccurrence] "-" [PathMaxOccurrence]
PathMinOccurrence = 1*DIGIT ; no upper limit
PathMaxOccurrence = 1*DIGIT ; no upper limit
</sourcecode>
<t>The <tt>*</tt>, <tt>(</tt>, and <tt>)</tt> symbols are interpreted as defined in <xref target="RFC5234"></xref>.</t>
<t>The EBMLAtomName of the EBMLElement part <bcp14>MUST</bcp14> be equal to the <tt>@name</tt> attribute of the EBML Schema.
If the EBMLElement part contains an IsRecursive part, the EBML Element can occur within itself recursively (see <xref target="recursive"></xref>).</t>
<t>The starting PathDelimiter of EBMLParentPath corresponds to the root of the EBML Document.</t>
<t>The <tt>@path</tt> value <bcp14>MUST</bcp14> be unique within the EBML Schema. The <tt>@id</tt> value corresponding to this <tt>@path</tt> <bcp14>MUST NOT</bcp14> be defined for use within another EBML Element with the same EBMLParentPath as this <tt>@path</tt>.</t>
<t>A path with a GlobalPlaceholder as the EBMLLastParent defines a Global Element; see <xref target="global-elements"></xref>.
If the element has no EBMLLastParent part, or the EBMLLastParent part is not a
GlobalPlaceholder, then the Element is not a Global Element.</t>
<t>The GlobalParentOccurrence part is interpreted as the number of valid
EBMLPathAtom parts that can replace the GlobalPlaceholder in the path.
PathMinOccurrence represents the minimum number of EBMLPathAtoms required to
replace the GlobalPlaceholder. PathMaxOccurrence represents the maximum number
of EBMLPathAtoms possible to replace the GlobalPlaceholder.</t>
<t>If PathMinOccurrence is not present, then that GlobalParentOccurrence has a
PathMinOccurrence value of 0.
If PathMaxOccurrence is not present, then there is no upper bound for the
permitted number of EBMLPathAtoms possible to replace the GlobalPlaceholder.
PathMaxOccurrence <bcp14>MUST NOT</bcp14> have the value 0, as it would mean
no EBMLPathAtom can replace the GlobalPlaceholder, and the EBMLFullPath would
be the same without that GlobalPlaceholder part.
PathMaxOccurrence <bcp14>MUST</bcp14> be bigger than, or equal to,
PathMinOccurrence.</t>
<t>For example, in <tt>\a\(0-1\)global</tt>, the Element path
<tt>\a\x\global</tt> corresponds to an EBMLPathAtom occurrence of 1. The
Element <tt>\a\x\y\global</tt> corresponds to an EBMLPathAtom occurrence of 2,
etc. In those cases, <tt>\a\x</tt> or <tt>\a\x\y</tt> <bcp14>MUST</bcp14> be valid
paths to be able to contain the element <tt>global</tt>.</t>
<t>Consider another EBML Path, <tt>\a\(1-\)global</tt>. There has to be at
least one EBMLPathAtom between the <tt>\a\</tt> part and <tt>global</tt>.
So the <tt>global</tt> EBML Element cannot be found inside the <tt>\a</tt>
EBML Element, as it means the resulting path <tt>\a\global</tt> has no
EBMLPathAtom between the <tt>\a\</tt> and <tt>global</tt>. However, the
<tt>global</tt> EBML Element can be found inside the <tt>\a\b</tt> EBML
Element, because the resulting path, <tt>\a\b\global</tt>, has one EBMLPathAtom
between the <tt>\a\</tt> and <tt>global</tt>. Alternatively, it can be found
inside the <tt>\a\b\c</tt> EBML Element (two EBMLPathAtom), or inside the
<tt>\a\b\c\d</tt> EBML Element (three EBMLPathAtom), etc.</t>
<t>Consider another EBML Path, <tt>\a\(0-1\)global</tt>. There has to be at
most one EBMLPathAtom between the <tt>\a\</tt> part and <tt>global</tt>.
So the <tt>global</tt> EBML Element can be found inside either the <tt>\a</tt>
EBML Element (0 EBMLPathAtom replacing GlobalPlaceholder) or the
<tt>\a\b</tt> EBML Element (one replacement EBMLPathAtom).
But it cannot be found inside the <tt>\a\b\c</tt> EBML Element, because the
resulting path, <tt>\a\b\c\global</tt>, has two EBMLPathAtom between
<tt>\a\</tt> and <tt>global</tt>.</t>
</section>
<section anchor="id"><name>id</name>
<t>Within an EBML Schema, the XPath of the <tt>@id</tt> attribute is
<tt>/EBMLSchema/element/@id</tt>.</t>
<t>The Element ID is encoded as a Variable-Size Integer. It is read and stored in big-endian
order. In the EBML Schema, it is expressed in
hexadecimal notation prefixed by a 0x. To reduce the risk of false positives while parsing EBML Streams, the
Element IDs of the Root Element and Top-Level Elements <bcp14>SHOULD</bcp14>
be at least 4 octets in length. Element IDs defined for use at Root Level or
directly under the Root Level <bcp14>MAY</bcp14> use shorter octet lengths to
facilitate padding and optimize edits to EBML Documents; for instance, the
Void Element uses an Element ID with a length of one octet to allow its usage
in more writing and editing scenarios.</t>
<t>The Element ID of any Element found within an EBML Document <bcp14>MUST</bcp14> only match a single <tt>@path</tt> value of its corresponding EBML Schema, but a separate instance of that Element ID value defined by the EBML Schema <bcp14>MAY</bcp14> occur within a different <tt>@path</tt>. If more than one Element is defined to use the same <tt>@id</tt> value, then the <tt>@path</tt> values of those Elements <bcp14>MUST NOT</bcp14> share the same EBMLParentPath. Elements <bcp14>MUST NOT</bcp14> be defined to use the same <tt>@id</tt> value if one of their common Parent Elements could be an Unknown-Sized Element.</t>
<t>The <tt>id</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
<section anchor="minoccurs"><name>minOccurs</name>
<t>Within an EBML Schema, the XPath of the <tt>@minOccurs</tt> attribute is
<tt>/EBMLSchema/element/@minOccurs</tt>.</t>
<t><tt>minOccurs</tt> is a nonnegative integer expressing the minimum permitted number
of occurrences of this EBML Element within its Parent Element.</t>
<t>Each instance of the Parent Element <bcp14>MUST</bcp14> contain at least this many instances of this EBML Element.
If the EBML Element has an empty EBMLParentPath, then <tt>minOccurs</tt> refers to
constraints on the occurrence of the EBML Element within the EBML Document.
EBML Elements with <tt>minOccurs</tt> set to "1" that also have a default
value (see <xref target="default"></xref>) declared are not
<bcp14>REQUIRED</bcp14> to be stored but are <bcp14>REQUIRED</bcp14> to be
interpreted; see
<xref target="note-on-the-use-of-default-attributes-to-define-mandatory-ebml-elements"></xref>.</t>
<t>An EBML Element defined with a <tt>minOccurs</tt> value greater than zero is called
a Mandatory EBML Element.</t>
<t>The <tt>minOccurs</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>minOccurs</tt>
attribute is not present, then that EBML Element has a <tt>minOccurs</tt> value of
0.</t>
<t>The semantic meaning of <tt>minOccurs</tt> within an EBML Schema is analogous to the meaning of <tt>minOccurs</tt> within an XML Schema.</t>
</section>
<section anchor="maxoccurs"><name>maxOccurs</name>
<t>Within an EBML Schema, the XPath of the <tt>@maxOccurs</tt> attribute is
<tt>/EBMLSchema/element/@maxOccurs</tt>.</t>
<t><tt>maxOccurs</tt> is a nonnegative integer expressing the maximum permitted number
of occurrences of this EBML Element within its Parent Element.</t>
<t>Each instance of the Parent Element <bcp14>MUST</bcp14> contain at most
this many instances of this EBML Element, including the unwritten mandatory
element with a default value; see
<xref target="note-on-the-use-of-default-attributes-to-define-mandatory-ebml-elements"></xref>.
If the EBML Element has an empty EBMLParentPath, then <tt>maxOccurs</tt> refers to
constraints on the occurrence of the EBML Element within the EBML
Document.</t>
<t>The <tt>maxOccurs</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>maxOccurs</tt>
attribute is not present, then there is no upper bound for the permitted
number of occurrences of this EBML Element within its Parent Element or within
the EBML Document, depending on whether or not the EBMLParentPath of the EBML Element
is empty.</t>
<t>The semantic meaning of <tt>maxOccurs</tt> within an EBML Schema is analogous to the
meaning of <tt>maxOccurs</tt> within an XML Schema; when it is not present, it's
similar to xml:maxOccurs="unbounded" in an XML Schema.</t>
</section>
<section anchor="range"><name>range</name>
<t>Within an EBML Schema, the XPath of the <tt>@range</tt> attribute is
<tt>/EBMLSchema/element/@range</tt>.</t>
<t>A numerical range for EBML Elements that are of numerical types (Unsigned
Integer, Signed Integer, Float, and Date). If specified, the value of the EBML
Element <bcp14>MUST</bcp14> be within the defined range. See
<xref target="expression-of-range"></xref> for rules applied to expression of range
values.</t>
<t>The <tt>range</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the
<tt>range</tt> attribute is
not present, then any value legal for the <tt>type</tt> attribute is valid.</t>
<section anchor="expression-of-range"><name>Expression of range</name>
<t>The <tt>range</tt> attribute <bcp14>MUST</bcp14> only be used with EBML Elements
that are either signed integer, unsigned integer, float, or date. The
expression defines the upper, lower, exact, or excluded value of the EBML
Element and optionally an upper boundary value combined with a lower
boundary. The range expression may contain whitespace (using the ASCII 0x20
character) for readability, but whitespace within a range expression
<bcp14>MUST NOT</bcp14> convey meaning.</t>
<t>To set a fixed value for the range, the value is used as the attribute
value. For example, <tt>1234</tt> means the EBML element always has the value
1234. The value can be prefixed with <tt>not</tt> to indicate that the fixed
value <bcp14>MUST NOT</bcp14> be used for that Element. For example,
<tt>not 1234</tt> means the Element can use all values of its type except 1234.</t>
<t>The <tt>></tt> sign is used for an exclusive lower boundary, and the
<tt>>=</tt> sign is used for an inclusive lower boundary. For example,
<tt>>3</tt> means the Element value <bcp14>MUST</bcp14> be greater than 3,
and <tt>>=0x1p+0</tt> means the Element value <bcp14>MUST</bcp14> be
greater than or equal to the floating value 1.0; see
<xref target="textual-expression-of-floats"></xref>.</t>
<t>The <tt><</tt> sign is used for an exclusive upper boundary, and the
<tt><=</tt> sign is used for an inclusive upper boundary. For example,
<tt><-2</tt> means the Element value <bcp14>MUST</bcp14> be less than -2,
and <tt><=10</tt> means the Element value <bcp14>MUST</bcp14> be less than
or equal to 10.</t>
<t>The lower and upper bounds can be combined into an expression to form a
closed boundary. The lower boundary comes first, followed by the upper
boundary, separated by a comma. For example, <tt>>3,<= 20</tt> means the
Element value <bcp14>MUST</bcp14> be greater than 3 and less than or equal to
20.</t>
<t>A special form of lower and upper bounds using the <tt>-</tt> separator is
possible, meaning the Element value <bcp14>MUST</bcp14> be greater than, or equal to,
the first value and <bcp14>MUST</bcp14> be less than or equal to the
second value. For example, <tt>1-10</tt> is equivalent to
<tt>>=1,<=10</tt>. If the upper boundary is negative, the <tt>range</tt> attribute
<bcp14>MUST</bcp14> only use the latter form.</t>
</section>
</section>
<section anchor="length"><name>length</name>
<t>Within an EBML Schema, the XPath of the <tt>@length</tt> attribute is
<tt>/EBMLSchema/element/@length</tt>.</t>
<t>The <tt>length</tt> attribute is a value to express the valid length of the Element
Data as written, measured in octets. The length provides a constraint in
addition to the Length value of the definition of the corresponding EBML
Element Type. This length <bcp14>MUST</bcp14> be expressed as either a
nonnegative integer or a range (see
<xref target="expression-of-range"></xref>) that consists of only nonnegative
integers and valid operators.</t>
<t>The <tt>length</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the
<tt>length</tt> attribute is
not present for that EBML Element, then that EBML Element is only limited in
length by the definition of the associated EBML Element Type.</t>
</section>
<section anchor="default"><name>default</name>
<t>Within an EBML Schema, the XPath of the <tt>@default</tt> attribute is
<tt>/EBMLSchema/element/@default</tt>.</t>
<t>If an Element is mandatory (has a <tt>minOccurs</tt> 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 <bcp14>MUST</bcp14> semantically interpret the EBML Element as present with this specified default value for the EBML Element.
An unwritten mandatory Element with a declared default value is semantically equivalent to that Element if written with the default value stored as the Element Data.
EBML Elements that are Master Elements <bcp14>MUST NOT</bcp14> declare a default value.
EBML Elements with a <tt>minOccurs</tt> value greater than 1 <bcp14>MUST NOT</bcp14> declare a default value.</t>
<t>The default attribute is <bcp14>OPTIONAL</bcp14>.</t>
</section>
<section anchor="type"><name>type</name>
<t>Within an EBML Schema, the XPath of the <tt>@type</tt> attribute is
<tt>/EBMLSchema/element/@type</tt>.</t>
<t>The type <bcp14>MUST</bcp14> be set to one of the following values:
<tt>integer</tt> (signed integer), <tt>uinteger</tt> (unsigned integer),
<tt>float</tt>, <tt>string</tt>, <tt>date</tt>, <tt>utf-8</tt>,
<tt>master</tt>, or <tt>binary</tt>. The content of each type is defined
in <xref target="ebml-element-types"></xref>.</t>
<t>The <tt>type</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
<section anchor="unknownsizeallowed"><name>unknownsizeallowed</name>
<t>Within an EBML Schema, the XPath of the <tt>@unknownsizeallowed</tt>
attribute is <tt>/EBMLSchema/element/@unknownsizeallowed</tt>.</t>
<t>This attribute is a boolean to express whether an EBML Element is permitted to
be an Unknown-Sized Element (having all VINT_DATA bits of Element Data Size set
to 1). EBML Elements that are not Master Elements <bcp14>MUST NOT</bcp14> set
<tt>unknownsizeallowed</tt> to true. An EBML Element that is defined with an
<tt>unknownsizeallowed</tt> attribute set to 1 <bcp14>MUST</bcp14> also have the
<tt>unknownsizeallowed</tt> attribute of its Parent Element set to 1.</t>
<t>An EBML Element with the <tt>unknownsizeallowed</tt> attribute set to 1
<bcp14>MUST NOT</bcp14> have its <tt>recursive</tt> attribute set to 1.</t>
<t>The <tt>unknownsizeallowed</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the
<tt>unknownsizeallowed</tt> attribute is not used, then that EBML Element is not
allowed to use an unknown Element Data Size.</t>
</section>
<section anchor="recursive"><name>recursive</name>
<t>Within an EBML Schema, the XPath of the <tt>@recursive</tt> attribute is
<tt>/EBMLSchema/element/@recursive</tt>.</t>
<t>This attribute is a boolean to express whether an EBML Element is permitted to
be stored recursively. If it is allowed, the EBML Element <bcp14>MAY</bcp14> 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 <bcp14>MUST NOT</bcp14> set recursive to
true.</t>
<t>If the EBMLElement part of the <tt>@path</tt> contains an IsRecursive part,
then the <tt>recursive</tt> value <bcp14>MUST</bcp14> be true; otherwise, it
<bcp14>MUST</bcp14> be false.</t>
<t>An EBML Element with the <tt>recursive</tt> attribute set to 1 <bcp14>MUST NOT</bcp14>
have its <tt>unknownsizeallowed</tt> attribute set to 1.</t>
<t>The <tt>recursive</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>recursive</tt>
attribute is not present, then the EBML Element <bcp14>MUST NOT</bcp14> be
used recursively.</t>
</section>
<section anchor="recurring"><name>recurring</name>
<t>Within an EBML Schema, the XPath of the <tt>@recurring</tt> attribute is
<tt>/EBMLSchema/element/@recurring</tt>.</t>
<t>This attribute is a boolean to express whether or not an EBML Element is defined as an
Identically Recurring Element; see
<xref target="identically-recurring-elements"></xref>.</t>
<t>The <tt>recurring</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>recurring</tt>
attribute is not present, then the EBML Element is not an Identically
Recurring Element.</t>
</section>
<section anchor="minver"><name>minver</name>
<t>Within an EBML Schema, the XPath of the <tt>@minver</tt> attribute is
<tt>/EBMLSchema/element/@minver</tt>.</t>
<t>The <tt>minver</tt> (minimum version) attribute stores a nonnegative integer that
represents the first version of the docType to support the EBML Element.</t>
<t>The <tt>minver</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>minver</tt>
attribute is not present, then the EBML Element has a minimum version of
"1".</t>
</section>
<section anchor="maxver"><name>maxver</name>
<t>Within an EBML Schema, the XPath of the <tt>@maxver</tt> attribute is
<tt>/EBMLSchema/element/@maxver</tt>.</t>
<t>The <tt>maxver</tt> (maximum version) attribute stores a nonnegative integer that
represents the last or most recent version of the docType to support the
element. <tt>maxver</tt> <bcp14>MUST</bcp14> be greater than or equal to <tt>minver</tt>.</t>
<t>The <tt>maxver</tt> attribute is <bcp14>OPTIONAL</bcp14>. If the <tt>maxver</tt> attribute is
not present, then the EBML Element has a maximum version equal to the value
stored in the <tt>version</tt> attribute of <tt><EBMLSchema></tt>.</t>
</section>
</section>
<section anchor="documentation-element"><name><tt><documentation></tt> Element</name>
<t>Within an EBML Schema, the XPaths of the <tt><documentation></tt>
elements are <tt>/EBMLSchema/element/documentation</tt> and <tt>/EBMLSchema/element/restriction/enum/documentation</tt>.</t>
<t>The <tt><documentation></tt> element provides additional information
about EBML Elements or enumeration values. Within the <tt><documentation></tt> element, the
following XHTML <xref target="XHTML"></xref> elements <bcp14>MAY</bcp14> be
used: <tt><a></tt>, <tt><br></tt>, and <tt><strong></tt>.</t>
</section>
<section anchor="documentation-attributes"><name><tt><documentation></tt> Attributes</name>
<section anchor="lang"><name>lang</name>
<t>Within an EBML Schema, the XPath of the <tt>@lang</tt> attribute is
<tt>/EBMLSchema/element/documentation/@lang</tt>.</t>
<t>The <tt>lang</tt> attribute is set to the value from <xref target="RFC5646"></xref> of
the language of the element's documentation.</t>
<t>The <tt>lang</tt> attribute is <bcp14>OPTIONAL</bcp14>.</t>
</section>
<section anchor="purpose"><name>purpose</name>
<t>Within an EBML Schema, the XPath of the <tt>@purpose</tt> attribute is
<tt>/EBMLSchema/element/documentation/@purpose</tt>.</t>
<t>A <tt>purpose</tt> attribute distinguishes the meaning of the documentation. Values
for the <tt><documentation></tt> subelement's <tt>purpose</tt> attribute
<bcp14>MUST</bcp14> include one of the values listed in
<xref target="tablePurposeDefinitions"></xref>.</t>
<table anchor="tablePurposeDefinitions"><name>Definitions of the permitted
values for the <tt>purpose</tt> attribute of the documentation Element</name>
<thead>
<tr>
<th align="left">value of <tt>purpose</tt> attribute</th>
<th align="left">definition</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">definition</td>
<td align="left">A "definition" is recommended for every defined EBML Element. This documentation explains the semantic meaning of the EBML Element.</td>
</tr>
<tr>
<td align="left">rationale</td>
<td align="left">An explanation about the reason or catalyst for the definition of the Element.</td>
</tr>
<tr>
<td align="left">usage notes</td>
<td align="left">Recommended practices or guidelines for both reading, writing, or interpreting the Element.</td>
</tr>
<tr>
<td align="left">references</td>
<td align="left">Informational references to support the contextualization and understanding of the value of the Element.</td>
</tr>
</tbody>
</table><t>The <tt>purpose</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
</section>
<section anchor="implementation-note-element"><name><tt><implementation_note></tt> Element</name>
<t>Within an EBML Schema, the XPath of the <tt><implementation_note></tt>
element is <tt>/EBMLSchema/element/implementation_note</tt>.</t>
<t>In some cases within an EBML Document Type, the attributes of the
<tt><element></tt> element are not sufficient to clearly communicate how
the defined EBML Element is intended to be implemented.
For instance, one EBML Element might only be mandatory if another EBML Element
is present. As another example, the default value of an EBML Element might
be derived from a related Element's content. In these cases where the Element's
definition is conditional or advanced implementation notes are needed, one or many
<tt><implementation_note></tt> elements can be used to store that
information.
The <tt><implementation_note></tt> refers to a specific attribute of the
parent <tt><element></tt> as expressed by the <tt>note_attribute</tt>
attribute (see <xref target="note-attribute"></xref>).</t>
</section>
<section anchor="implementation-note-attributes"><name><tt><implementation_note></tt> Attributes</name>
<section anchor="note-attribute"><name>note_attribute</name>
<t>Within an EBML Schema, the XPath of the <tt>@note_attribute</tt> attribute
is <tt>/EBMLSchema/element/implementation_note/@note_attribute</tt>.</t>
<t>The <tt>note_attribute</tt> attribute references which of the attributes of the
<tt><element></tt> the <tt><implementation_note></tt> relates to.
The <tt>note_attribute</tt> attribute <bcp14>MUST</bcp14> be set to one of the
following values (corresponding to that attribute of the parent
<tt><element></tt>): <tt>minOccurs</tt>, <tt>maxOccurs</tt>,
<tt>range</tt>, <tt>length</tt>, <tt>default</tt>, <tt>minver</tt>, or
<tt>maxver</tt>. The <tt><implementation_note></tt> <bcp14>SHALL</bcp14>
supersede the parent <tt><element></tt>'s attribute that is named in the
<tt>note_attribute</tt> attribute.
An <tt><element></tt> <bcp14>SHALL NOT</bcp14> have more than one <tt><implementation_note></tt> of the same <tt>note_attribute</tt>.</t>
<t>The <tt>note_attribute</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
<section anchor="implementation-note-example"><name><tt><implementation_note></tt> Example</name>
<t>The following fragment of an EBML Schema demonstrates how an
<tt><implementation_note></tt> is used. In this case, an EBML Schema
documents a list of items that are described with an optional cost. The
Currency Element uses an <tt><implementation_note></tt> to say that the
Currency Element is <bcp14>REQUIRED</bcp14> if the Cost Element is set,
otherwise not.</t>
<sourcecode type="xml"><element name="Items" path="\Items" id="0x4025" type="master"
minOccurs="1" maxOccurs="1">
<documentation lang="en" purpose="definition">
A set of items.
</documentation>
</element>
<element name="Item" path="\Items\Item" id="0x4026"
type="master">
<documentation lang="en" purpose="definition">
An item.
</documentation>
</element>
<element name="Cost" path="\Items\Item\Cost" id="0x4024"
type="float" maxOccurs="1">
<documentation lang="en" purpose="definition">
The cost of the item, if any.
</documentation>
</element>
<element name="Currency" path="\Items\Item\Currency" id="0x403F"
type="string" maxOccurs="1">
<documentation lang="en" purpose="definition">
The currency of the item's cost.
</documentation>
<implementation_note note_attribute="minOccurs">
Currency MUST be set (minOccurs=1) if the associated Item stores
a Cost, else Currency MAY be unset (minOccurs=0).
</implementation_note>
</element>
</sourcecode>
</section>
</section>
<section anchor="restriction-element"><name><tt><restriction></tt> Element</name>
<t>Within an EBML Schema, the XPath of the <tt><restriction></tt>
element is <tt>/EBMLSchema/element/restriction</tt>.</t>
<t>The <tt><restriction></tt> element provides information about
restrictions to the allowable values for the EBML Element, which are listed in
<tt><enum></tt> elements.</t>
</section>
<section anchor="enum-element"><name><tt><enum></tt> Element</name>
<t>Within an EBML Schema, the XPath of the <tt><enum></tt> element is
<tt>/EBMLSchema/element/restriction/enum</tt>.</t>
<t>The <tt><enum></tt> element stores a list of values allowed for
storage in the EBML Element. The values <bcp14>MUST</bcp14> match the type of
the EBML Element (for example, <tt><enum value="Yes"></tt>
cannot be a valid value for an EBML Element that is defined as an unsigned
integer). An <tt><enum></tt> element <bcp14>MAY</bcp14> also store
<tt><documentation></tt> elements to further describe the
<tt><enum></tt>.</t>
</section>
<section anchor="enum-attributes"><name><tt><enum></tt> Attributes</name>
<section anchor="label"><name>label</name>
<t>Within an EBML Schema, the XPath of the <tt>@label</tt> attribute is
<tt>/EBMLSchema/element/restriction/enum/@label</tt>.</t>
<t>The label provides a concise expression for human consumption that
describes what the value of <tt><enum></tt> represents.</t>
<t>The <tt>label</tt> attribute is <bcp14>OPTIONAL</bcp14>.</t>
</section>
<section anchor="value"><name>value</name>
<t>Within an EBML Schema, the XPath of the <tt>@value</tt> attribute is
<tt>/EBMLSchema/element/restriction/enum/@value</tt>.</t>
<t>The value represents data that <bcp14>MAY</bcp14> be stored within the EBML Element.</t>
<t>The <tt>value</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
</section>
<section anchor="extension-element"><name><tt><extension></tt> Element</name>
<t>Within an EBML Schema, the XPath of the <tt><extension></tt>
element is <tt>/EBMLSchema/element/extension</tt>.</t>
<t>The <tt><extension></tt> element provides an unconstrained element to
contain information about the associated EBML <tt><element></tt>, which
is undefined by this document but <bcp14>MAY</bcp14> be defined by the
associated EBML Document Type. The <tt><extension></tt> element
<bcp14>MUST</bcp14> contain a <tt>type</tt> attribute and also
<bcp14>MAY</bcp14> contain any other attribute or subelement as long as the
EBML Schema remains as a well-formed XML Document. All
<tt><extension></tt> elements <bcp14>MUST</bcp14> be subelements of the
<tt><element></tt>.</t>
</section>
<section anchor="extension-attributes"><name><tt><extension></tt> Attributes</name>
<section anchor="type-1"><name>type</name>
<t>Within an EBML Schema, the XPath of the <tt>@type</tt> attribute is
<tt>/EBMLSchema/element/extension/@type</tt>.</t>
<t>The <tt>type</tt> attribute should reference a name or identifier of the
project or authority associated with the contents of the
<tt><extension></tt> element.</t>
<t>The <tt>type</tt> attribute is <bcp14>REQUIRED</bcp14>.</t>
</section>
</section>
<section anchor="xml-schema-for-ebml-schema"><name>XML Schema for EBML Schema</name>
<t>The following provides an XML Schema <xref target="XML-SCHEMA"></xref> for
facilitating verification of an EBML Schema described in
<xref target="ebml-schema"></xref>.</t>
<sourcecode type="xml"><?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="urn:ietf:rfc:8794"
targetNamespace="urn:ietf:rfc:8794"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xhtml="http://www.w3.org/1999/xhtml"
elementFormDefault="qualified" version="01">
<!-- for HTML in comments -->
<xs:import namespace="http://www.w3.org/1999/xhtml"
schemaLocation="http://www.w3.org/MarkUp/SCHEMA/xhtml11.xsd"/>
<xs:element name="EBMLSchema" type="EBMLSchemaType"/>
<xs:complexType name="EBMLSchemaType">
<xs:sequence>
<xs:element name="element" type="elementType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="docType" use="required"/>
<xs:attribute name="version" use="required" type="xs:integer"/>
<xs:attribute name="ebml" type="xs:positiveInteger"
default="1"/>
</xs:complexType>
<xs:complexType name="elementType">
<xs:sequence>
<xs:element name="documentation" type="documentationType"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="implementation_note" type="noteType"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="restriction" type="restrictionType"
minOccurs="0" maxOccurs="1"/>
<xs:element name="extension" type="extensionType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="name" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:pattern value="[0-9A-Za-z.-]([0-9A-Za-z.-])*"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="path" use="required">
<!-- <xs:simpleType>
<xs:restriction base="xs:integer">
<xs:pattern value="[0-9]*\*[0-9]*()"/>
</xs:restriction>
</xs:simpleType> -->
</xs:attribute>
<xs:attribute name="id" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:pattern value="0x([0-9A-F][0-9A-F])+"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="minOccurs" default="0">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="maxOccurs" default="1">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="range"/>
<xs:attribute name="length"/>
<xs:attribute name="default"/>
<xs:attribute name="type" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="integer"/>
<xs:enumeration value="uinteger"/>
<xs:enumeration value="float"/>
<xs:enumeration value="string"/>
<xs:enumeration value="date"/>
<xs:enumeration value="utf-8"/>
<xs:enumeration value="master"/>
<xs:enumeration value="binary"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="unknownsizeallowed" type="xs:boolean"
default="false"/>
<xs:attribute name="recursive" type="xs:boolean"
default="false"/>
<xs:attribute name="recurring" type="xs:boolean"
default="false"/>
<xs:attribute name="minver" default="1">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="maxver">
<xs:simpleType>
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="restrictionType">
<xs:sequence>
<xs:element name="enum" type="enumType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="extensionType">
<xs:sequence>
<xs:any processContents="skip"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="type" use="required"/>
<xs:anyAttribute processContents="skip"/>
</xs:complexType>
<xs:complexType name="enumType">
<xs:sequence>
<xs:element name="documentation" type="documentationType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="label"/>
<xs:attribute name="value" use="required"/>
</xs:complexType>
<xs:complexType name="documentationType" mixed="true">
<xs:sequence>
<xs:element name="a" type="xhtml:xhtml.a.type"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="br" type="xhtml:xhtml.br.type"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="strong" type="xhtml:xhtml.strong.type"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="lang"/>
<xs:attribute name="purpose" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="definition"/>
<xs:enumeration value="rationale"/>
<xs:enumeration value="references"/>
<xs:enumeration value="usage notes"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="noteType">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="note_attribute" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="minOccurs"/>
<xs:enumeration value="maxOccurs"/>
<xs:enumeration value="range"/>
<xs:enumeration value="length"/>
<xs:enumeration value="default"/>
<xs:enumeration value="minver"/>
<xs:enumeration value="maxver"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
</sourcecode>
</section>
<section anchor="identically-recurring-elements"><name>Identically Recurring Elements</name>
<t>An Identically Recurring Element is an EBML Element that <bcp14>MAY</bcp14>
occur within its Parent Element more than once, but each recurrence of it
within that Parent Element <bcp14>MUST</bcp14> be identical both in storage
and semantics. Identically Recurring Elements are permitted to be stored
multiple times within the same Parent Element in order to increase data
resilience and optimize the use of EBML in transmission. For instance, a
pertinent Top-Level Element could be periodically resent within a datastream
so that an EBML Reader that starts reading the stream from the middle could
better interpret the contents. Identically Recurring Elements
<bcp14>SHOULD</bcp14> include a CRC-32 Element as a Child Element; this is
especially recommended when EBML is used for long-term storage or
transmission. If a Parent Element contains more than one copy of an
Identically Recurring Element that includes a CRC-32 Element as a Child
Element, then the first instance of the Identically Recurring Element with a
valid CRC-32 value should be used for interpretation. If a Parent Element
contains more than one copy of an Identically Recurring Element that does not
contain a CRC-32 Element, or if CRC-32 Elements are present but none are valid,
then the first instance of the Identically Recurring Element should be used
for interpretation.</t>
</section>
<section anchor="textual-expression-of-floats"><name>Textual expression of floats</name>
<t>When a float value is represented textually in an EBML Schema, such as
within a default or range value, the float values <bcp14>MUST</bcp14> be
expressed as Hexadecimal Floating-Point Constants as defined in the C11
standard <xref target="ISO9899"></xref> (see Section 6.4.4.2 on Floating
Constants). <xref target="tableFloatExamples"></xref> provides examples of
expressions of float ranges.</t>
<table anchor="tableFloatExamples"><name>Example of Floating-Point values and
ranges as decimal and Hexadecimal Floating-Point Constants</name>
<thead>
<tr>
<th align="left">as decimal</th>
<th align="left">as Hexadecimal Floating-Point Constants</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">0.0</td>
<td align="left"><tt>0x0p+1</tt></td>
</tr>
<tr>
<td align="left">0.0-1.0</td>
<td align="left"><tt>0x0p+1-0x1p+0</tt></td>
</tr>
<tr>
<td align="left">1.0-256.0</td>
<td align="left"><tt>0x1p+0-0x1p+8</tt></td>
</tr>
<tr>
<td align="left">0.857421875</td>
<td align="left"><tt>0x1.b7p-1</tt></td>
</tr>
<tr>
<td align="left">-1.0--0.857421875</td>
<td align="left"><tt>-0x1p+0--0x1.b7p-1</tt></td>
</tr>
</tbody>
</table><t>Within an expression of a float range, as in an integer range, the
- (hyphen) character is the separator between the minimum and maximum values
permitted by the range. Hexadecimal Floating-Point Constants also use a -
(hyphen) when indicating a negative binary power. Within a float range, when a
- (hyphen) is immediately preceded by a letter p, then the - (hyphen) is a
part of the Hexadecimal Floating-Point Constant that notes negative binary
power. Within a float range, when a - (hyphen) is not immediately preceded by
a letter p, then the - (hyphen) represents the separator between the minimum
and maximum values permitted by the range.</t>
</section>
<section anchor="note-on-the-use-of-default-attributes-to-define-mandatory-ebml-elements"><name>Note on the use of default attributes to define Mandatory EBML Elements</name>
<t>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 <bcp14>MUST</bcp14> be read by the EBML Reader,
although the EBML Element is not present within its Parent Element.</t>
<t>If a Mandatory EBML Element has no default value declared by an EBML Schema
and its Parent Element is present, then the EBML Element <bcp14>MUST</bcp14>
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
<bcp14>MUST</bcp14> be present.</t>
<t><xref target="tableVintRequirements"></xref> clarifies whether a Mandatory
EBML Element <bcp14>MUST</bcp14> be written, according to whether the default value
is declared, the value of the EBML Element is equal to the declared default
value, and/or the Parent Element is used.</t>
<table anchor="tableVintRequirements"><name>Demonstration of the conditional
requirements of VINT Storage</name>
<thead>
<tr>
<th align="center">Is the default value declared?</th>
<th align="center">Is the value equal to default?</th>
<th align="center">Is the Parent Element present?</th>
<th align="center">Then is storing the EBML Element <bcp14>REQUIRED</bcp14>?</th>
</tr>
</thead>
<tbody>
<tr>
<td align="center">Yes</td>
<td align="center">Yes</td>
<td align="center">Yes</td>
<td align="center">No</td>
</tr>
<tr>
<td align="center">Yes</td>
<td align="center">Yes</td>
<td align="center">No</td>
<td align="center">No</td>
</tr>
<tr>
<td align="center">Yes</td>
<td align="center">No</td>
<td align="center">Yes</td>
<td align="center">Yes</td>
</tr>
<tr>
<td align="center">Yes</td>
<td align="center">No</td>
<td align="center">No</td>
<td align="center">No</td>
</tr>
<tr>
<td align="center">No</td>
<td align="center">n/a</td>
<td align="center">Yes</td>
<td align="center">Yes</td>
</tr>
<tr>
<td align="center">No</td>
<td align="center">n/a</td>
<td align="center">No</td>
<td align="center">No</td>
</tr>
</tbody>
</table></section>
</section>
<section anchor="ebml-header-elements"><name>EBML Header Elements</name>
<t>This document contains definitions of all EBML Elements of the EBML Header.</t>
<section anchor="ebml-element"><name>EBML Element</name>
<dl>
<dt>name:</dt>
<dd>EBML</dd>
<dt>path:</dt>
<dd><tt>\EBML</tt></dd>
<dt>id:</dt>
<dd>0x1A45DFA3</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>type:</dt>
<dd>Master Element</dd>
<dt>description:</dt>
<dd>Set the EBML characteristics of the data to follow. Each EBML Document has
to start with this.</dd>
</dl>
</section>
<section anchor="ebmlversion-element"><name>EBMLVersion Element</name>
<dl>
<dt>name:</dt>
<dd>EBMLVersion</dd>
<dt>path:</dt>
<dd><tt>\EBML\EBMLVersion</tt></dd>
<dt>id:</dt>
<dd>0x4286</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>not 0</dd>
<dt>default:</dt>
<dd>1</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The version of EBML specifications used to create the EBML Document. The
version of EBML defined in this document is 1, so EBMLVersion
<bcp14>SHOULD</bcp14> be 1.</dd>
</dl>
</section>
<section anchor="ebmlreadversion-element"><name>EBMLReadVersion Element</name>
<dl>
<dt>name:</dt>
<dd>EBMLReadVersion</dd>
<dt>path:</dt>
<dd><tt>\EBML\EBMLReadVersion</tt></dd>
<dt>id:</dt>
<dd>0x42F7</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>1</dd>
<dt>default:</dt>
<dd>1</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The minimum EBML version an EBML Reader has to support to read this EBML
Document. The EBMLReadVersion Element <bcp14>MUST</bcp14> be less than or equal to EBMLVersion.</dd>
</dl>
</section>
<section anchor="ebmlmaxidlength-element"><name>EBMLMaxIDLength Element</name>
<dl>
<dt>name:</dt>
<dd>EBMLMaxIDLength</dd>
<dt>path:</dt>
<dd><tt>\EBML\EBMLMaxIDLength</tt></dd>
<dt>id:</dt>
<dd>0x42F2</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>>=4</dd>
<dt>default:</dt>
<dd>4</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The EBMLMaxIDLength Element stores the maximum permitted length in octets
of the Element IDs to be found within the EBML Body. An EBMLMaxIDLength Element value of four
is <bcp14>RECOMMENDED</bcp14>, though larger values are allowed.</dd>
</dl>
</section>
<section anchor="ebmlmaxsizelength-element"><name>EBMLMaxSizeLength Element</name>
<dl>
<dt>name:</dt>
<dd>EBMLMaxSizeLength</dd>
<dt>path:</dt>
<dd><tt>\EBML\EBMLMaxSizeLength</tt></dd>
<dt>id:</dt>
<dd>0x42F3</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>not 0</dd>
<dt>default:</dt>
<dd>8</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The EBMLMaxSizeLength Element stores the maximum permitted length in
octets of the expressions of all Element Data Sizes to be found within the EBML Body. The
EBMLMaxSizeLength Element documents an upper bound for the <tt>length</tt> of
all Element Data Size expressions within the EBML Body and not an upper bound
for the <tt>value</tt> of all Element Data Size expressions
within the EBML Body. EBML Elements that have an Element Data Size expression that is larger in octets
than what is expressed by EBMLMaxSizeLength Element are invalid.</dd>
</dl>
</section>
<section anchor="doctype-element"><name>DocType Element</name>
<dl>
<dt>name:</dt>
<dd>DocType</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocType</tt></dd>
<dt>id:</dt>
<dd>0x4282</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>length:</dt>
<dd>>0</dd>
<dt>type:</dt>
<dd>String</dd>
<dt>description:</dt>
<dd>A string that describes and identifies the content of the EBML Body that
follows this EBML Header.</dd>
</dl>
</section>
<section anchor="doctypeversion-element"><name>DocTypeVersion Element</name>
<dl>
<dt>name:</dt>
<dd>DocTypeVersion</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocTypeVersion</tt></dd>
<dt>id:</dt>
<dd>0x4287</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>not 0</dd>
<dt>default:</dt>
<dd>1</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The version of DocType interpreter used to create the EBML Document.</dd>
</dl>
</section>
<section anchor="doctypereadversion-element"><name>DocTypeReadVersion Element</name>
<dl>
<dt>name:</dt>
<dd>DocTypeReadVersion</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocTypeReadVersion</tt></dd>
<dt>id:</dt>
<dd>0x4285</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>not 0</dd>
<dt>default:</dt>
<dd>1</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The minimum DocType version an EBML Reader has to support to read this
EBML Document. The value of the DocTypeReadVersion Element <bcp14>MUST</bcp14>
be less than or equal to the value of the DocTypeVersion Element.</dd>
</dl>
</section>
<section anchor="doctypeextension-element"><name>DocTypeExtension Element</name>
<dl>
<dt>name:</dt>
<dd>DocTypeExtension</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocTypeExtension</tt></dd>
<dt>id:</dt>
<dd>0x4281</dd>
<dt>minOccurs:</dt>
<dd>0</dd>
<dt>type:</dt>
<dd>Master Element</dd>
<dt>description:</dt>
<dd>A DocTypeExtension adds extra Elements to the main DocType+DocTypeVersion
tuple it's attached to. An EBML Reader <bcp14>MAY</bcp14> know these extra Elements and how
to use them. A DocTypeExtension <bcp14>MAY</bcp14> be used to iterate between
experimental Elements before they are integrated into a regular
DocTypeVersion. Reading one DocTypeExtension version of a
DocType+DocTypeVersion tuple doesn't imply one should be able to read upper
versions of this DocTypeExtension.</dd>
</dl>
</section>
<section anchor="doctypeextensionname-element"><name>DocTypeExtensionName Element</name>
<dl>
<dt>name:</dt>
<dd>DocTypeExtensionName</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocTypeExtension\DocTypeExtensionName</tt></dd>
<dt>id:</dt>
<dd>0x4283</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>length:</dt>
<dd>>0</dd>
<dt>type:</dt>
<dd>String</dd>
<dt>description:</dt>
<dd>The name of the DocTypeExtension to differentiate it from other
DocTypeExtensions of the same DocType+DocTypeVersion tuple. A DocTypeExtensionName value
<bcp14>MUST</bcp14> be unique within the EBML Header.</dd>
</dl>
</section>
<section anchor="doctypeextensionversion-element"><name>DocTypeExtensionVersion Element</name>
<dl>
<dt>name:</dt>
<dd>DocTypeExtensionVersion</dd>
<dt>path:</dt>
<dd><tt>\EBML\DocTypeExtension\DocTypeExtensionVersion</tt></dd>
<dt>id:</dt>
<dd>0x4284</dd>
<dt>minOccurs:</dt>
<dd>1</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>range:</dt>
<dd>not 0</dd>
<dt>type:</dt>
<dd>Unsigned Integer</dd>
<dt>description:</dt>
<dd>The version of the DocTypeExtension. Different DocTypeExtensionVersion
values of the same DocType + DocTypeVersion + DocTypeExtensionName tuple
<bcp14>MAY</bcp14> contain completely different sets of extra Elements. An
EBML Reader <bcp14>MAY</bcp14> support multiple versions
of the same tuple, only one version of the tuple, or not support the tuple at all.</dd>
</dl>
</section>
</section>
<section anchor="global-elements"><name>Global Elements</name>
<t>EBML allows some special Elements to be found within more than one parent
in an EBML Document or optionally at the Root Level of an EBML Body. These
Elements are called Global Elements. There are two Global Elements that can be
found in any EBML Document: the CRC-32 Element and the Void Element. An EBML
Schema <bcp14>MAY</bcp14> add other Global Elements to the format it
defines. These extra elements apply only to the EBML Body, not the EBML
Header.</t>
<t>Global Elements are EBML Elements whose EBMLLastParent part of the path has
a GlobalPlaceholder. Because it is the last Parent part of the path, a Global
Element might also have EBMLParentPath parts in its path. In this case, the
Global Element can only be found within this EBMLParentPath path -- i.e., it's
not fully "global".</t>
<t>A Global Element can be found in many Parent Elements, allowing the same number of occurrences in each Parent where this Element is found.</t>
<section anchor="crc-32-element"><name>CRC-32 Element</name>
<dl>
<dt>name:</dt>
<dd>CRC-32</dd>
<dt>path:</dt>
<dd><tt>\(1-\)CRC-32</tt></dd>
<dt>id:</dt>
<dd>0xBF</dd>
<dt>minOccurs:</dt>
<dd>0</dd>
<dt>maxOccurs:</dt>
<dd>1</dd>
<dt>length:</dt>
<dd>4</dd>
<dt>type:</dt>
<dd>Binary</dd>
<dt>description:</dt>
<dd>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
<bcp14>MUST</bcp14> 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 <bcp14>SHOULD</bcp14> include a CRC-32 Element as a Child
Element. The CRC in use is the IEEE-CRC-32 algorithm as used in the
<xref target="ISO3309"></xref> standard and in Section 8.1.1.6.2 of
<xref target="ITU.V42"></xref>, with initial value of 0xFFFFFFFF. The CRC value
<bcp14>MUST</bcp14> be computed on a little-endian bytestream and
<bcp14>MUST</bcp14> use little-endian storage.</dd>
</dl>
</section>
<section anchor="void-element"><name>Void Element</name>
<dl>
<dt>name:</dt>
<dd>Void</dd>
<dt>path:</dt>
<dd><tt>\(-\)Void</tt></dd>
<dt>id:</dt>
<dd>0xEC</dd>
<dt>minOccurs:</dt>
<dd>0</dd>
<dt>type:</dt>
<dd>Binary</dd>
<dt>description:</dt>
<dd>Used to void data or to avoid unexpected behaviors when using damaged
data. The content is discarded. Also used to reserve space in a subelement for
later use.</dd>
</dl>
</section>
</section>
</section>
<section anchor="considerations-for-reading-ebml-data"><name>Considerations for Reading EBML Data</name>
<t>The following scenarios describe events to consider when reading EBML
Documents, as well as the recommended design of an EBML Reader.</t>
<t>If a Master Element contains a CRC-32 Element that doesn't validate, then
the EBML Reader <bcp14>MAY</bcp14> ignore all contained data except for
Descendant Elements that contain their own valid CRC-32 Element.</t>
<t>In the following XML representation of a simple, hypothetical EBML
fragment, a Master Element called CONTACT contains two Child Elements, NAME
and ADDRESS. In this example, some data within the NAME Element had been
altered so that the CRC-32 of the NAME Element does not validate, and thus any
Ancestor Element with a CRC-32 would therefore also no longer
validate. However, even though the CONTACT Element has a CRC-32 that does not
validate (because of the changed data within the NAME Element), the CRC-32 of
the ADDRESS Element does validate, and thus the contents and semantics of the
ADDRESS Element <bcp14>MAY</bcp14> be used.</t>
<sourcecode type="xml"><CONTACT>
<CRC-32>c119a69b</CRC-32><!-- does not validate -->
<NAME>
<CRC-32>1f59ee2b</CRC-32><!-- does not validate -->
<FIRST-NAME>invalid data</FIRST-NAME>
<LAST-NAME>invalid data</LAST-NAME>
</NAME>
<ADDRESS>
<CRC-32>df941cc9</CRC-32><!-- validates -->
<STREET>valid data</STREET>
<CITY>valid data</CITY>
</ADDRESS>
</CONTACT>
</sourcecode>
<t>If a Master Element contains more occurrences of a Child Master Element
than permitted according to the <tt>maxOccurs</tt> and <tt>recurring</tt>
attributes of the
definition of that Element, then the occurrences in addition to <tt>maxOccurs</tt>
<bcp14>MAY</bcp14> be ignored.</t>
<t>If a Master Element contains more occurrences of a Child Element than
permitted according to the <tt>maxOccurs</tt> attribute of the definition of that
Element, then all instances of that Element after the first <tt>maxOccurs</tt>
occurrences from the beginning of its Parent Element <bcp14>SHOULD</bcp14> be
ignored.</t>
</section>
<section anchor="terminating-elements"><name>Terminating Elements</name>
<t>Null Octets, which are octets with all bits set to zero, <bcp14>MAY</bcp14> follow the value of a String Element or UTF-8 Element to serve as a terminator.
An EBML Writer <bcp14>MAY</bcp14> terminate a String Element or UTF-8 Element
with Null Octets in order to overwrite a stored value with a new value of
lesser length while maintaining the same Element Data Size; this can prevent
the need to rewrite large portions of an EBML Document. Otherwise, the use of
Null Octets within a String Element or UTF-8 Element is <bcp14>NOT RECOMMENDED</bcp14>.
The Element Data of a UTF-8 Element <bcp14>MUST</bcp14>
be a valid UTF-8 string up to whichever comes first: the end of the Element or
the first occurring Null octet. Within the Element Data of a String or UTF-8 Element,
any Null octet itself and any following data within that Element
<bcp14>SHOULD</bcp14> be ignored. A string value and a copy of that string
value terminated by one or more Null Octets are semantically equal.</t>
<t><xref target="tableNullOctetSemantics"></xref> shows examples of semantics
and validation for the use of Null Octets. Values to represent Stored Values
and the Semantic Meaning as represented as hexadecimal values.</t>
<table anchor="tableNullOctetSemantics"><name>Examples of semantics for Null
Octets in VINT_DATA</name>
<thead>
<tr>
<th align="left">Stored Value</th>
<th align="left">Semantic Meaning</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">0x65 0x62 0x6D 0x6C</td>
<td align="left">0x65 0x62 0x6D 0x6C</td>
</tr>
<tr>
<td align="left">0x65 0x62 0x00 0x6C</td>
<td align="left">0x65 0x62</td>
</tr>
<tr>
<td align="left">0x65 0x62 0x00 0x00</td>
<td align="left">0x65 0x62</td>
</tr>
<tr>
<td align="left">0x65 0x62</td>
<td align="left">0x65 0x62</td>
</tr>
</tbody>
</table></section>
<section anchor="guidelines-for-updating-elements"><name>Guidelines for Updating Elements</name>
<t>An EBML Document can be updated without requiring that the entire EBML
Document be rewritten. These recommendations describe strategies for changing
the Element Data of a written EBML Element with minimal disruption to the rest
of the EBML Document.</t>
<section anchor="reducing-element-data-in-size"><name>Reducing Element Data in Size</name>
<t>There are three methods to reduce the size of Element Data of a written EBML Element.</t>
<section anchor="adding-a-void-element"><name>Adding a Void Element</name>
<t>When an EBML Element is changed to reduce its total length by more than one octet, an EBML Writer <bcp14>SHOULD</bcp14> fill the freed space with a Void Element.</t>
</section>
<section anchor="extending-the-element-data-size"><name>Extending the Element Data Size</name>
<t>The same value for Element Data Size <bcp14>MAY</bcp14> be written in various lengths, so for minor reductions of the Element Data, the Element Size <bcp14>MAY</bcp14> be written to a longer octet length to fill the freed space.</t>
<t>For example, the first row of
<xref target="tableShortenVintOneOctet"></xref> depicts a String Element that stores
an Element ID (3 octets), Element Data Size (1 octet), and Element Data (4
octets). If the Element Data is changed to reduce the length by one octet, and
if the current length of the Element Data Size is less than its maximum
permitted length, then the Element Data Size of that Element
<bcp14>MAY</bcp14> be rewritten to increase its length by one octet. Thus,
before and after the change, the EBML Element maintains the same length of 8
octets, and data around the Element does not need to be moved.</t>
<table anchor="tableShortenVintOneOctet"><name>Example of editing a VINT to
reduce VINT_DATA length by one octet</name>
<thead>
<tr>
<th>Status</th>
<th>Element ID</th>
<th>Element Data Size</th>
<th>Element Data</th>
</tr>
</thead>
<tbody>
<tr>
<td>Before edit</td>
<td>0x3B4040</td>
<td>0x84</td>
<td>0x65626D6C</td>
</tr>
<tr>
<td>After edit</td>
<td>0x3B4040</td>
<td>0x4003</td>
<td>0x6D6B76</td>
</tr>
</tbody>
</table><t>This method is <bcp14>RECOMMENDED</bcp14> when the Element Data is
reduced by a single octet; for reductions by two or more octets, it is
<bcp14>RECOMMENDED</bcp14> to fill the freed space with a Void Element.</t>
<t>Note that if the Element Data length needs to be rewritten as shortened by
one octet and the Element Data Size could be rewritten as a shorter VINT, then
it is <bcp14>RECOMMENDED</bcp14> to rewrite the Element Data Size as one octet
shorter, shorten the Element Data by one octet, and follow that Element with a
Void Element. For example,
<xref target="tableShortenVintMoreThanOneOctet"></xref> depicts a String Element
that stores an Element ID (3 octets), Element Data Size (2 octets, but could
be rewritten in one octet), and Element Data (3 octets). If the Element Data
is to be rewritten to a two-octet length, then another octet can be taken from
Element Data Size so that there is enough space to add a two-octet Void
Element.</t>
<table anchor="tableShortenVintMoreThanOneOctet"><name>Example of editing a
VINT to reduce VINT_DATA length by more than one octet</name>
<thead>
<tr>
<th>Status</th>
<th>Element ID</th>
<th>Element Data Size</th>
<th>Element Data</th>
<th>Void Element</th>
</tr>
</thead>
<tbody>
<tr>
<td>Before</td>
<td>0x3B4040</td>
<td>0x4003</td>
<td>0x6D6B76</td>
<td></td>
</tr>
<tr>
<td>After</td>
<td>0x3B4040</td>
<td>0x82</td>
<td>0x6869</td>
<td>0xEC80</td>
</tr>
</tbody>
</table></section>
<section anchor="terminating-element-data"><name>Terminating Element Data</name>
<t>For String Elements and UTF-8 Elements, the length of Element Data could be
reduced by adding Null Octets to terminate the Element Data (see
<xref target="terminating-elements"></xref>).</t>
<t>In <xref target="tableExampleNullPadding"></xref>, Element Data four octets
long is changed to a value three octets long, followed by a Null Octet; the
Element Data Size includes any Null Octets used to terminate Element Data and therefore
remains unchanged.</t>
<table anchor="tableExampleNullPadding"><name>Example of terminating VINT_DATA
with a Null Octet when reducing VINT length during an edit</name>
<thead>
<tr>
<th>Status</th>
<th>Element ID</th>
<th>Element Data Size</th>
<th>Element Data</th>
</tr>
</thead>
<tbody>
<tr>
<td>Before edit</td>
<td>0x3B4040</td>
<td>0x84</td>
<td>0x65626D6C</td>
</tr>
<tr>
<td>After edit</td>
<td>0x3B4040</td>
<td>0x84</td>
<td>0x6D6B7600</td>
</tr>
</tbody>
</table><t>Note that this method is <bcp14>NOT RECOMMENDED</bcp14>. For
reductions of one octet, the method for Extending the Element Data Size
<bcp14>SHOULD</bcp14> be used. For reduction by more than one octet, the
method for Adding a Void Element <bcp14>SHOULD</bcp14> be used.</t>
</section>
</section>
<section anchor="considerations-when-updating-elements-with-cyclic-redundancy-check-crc"><name>Considerations when Updating Elements with Cyclic Redundancy Check (CRC)</name>
<t>If the Element to be changed is a Descendant Element of any Master Element
that contains a CRC-32 Element (see <xref target="crc-32-element"></xref>),
then the CRC-32 Element <bcp14>MUST</bcp14> be verified before permitting the
change. Additionally, the CRC-32 Element value <bcp14>MUST</bcp14> be
subsequently updated to reflect the changed data.</t>
</section>
</section>
<section anchor="backward-and-forward-compatibility"><name>Backward and Forward Compatibility</name>
<t>Elements of an EBML format <bcp14>SHOULD</bcp14> be designed with backward and forward compatibility in mind.</t>
<section anchor="backward-compatibility"><name>Backward Compatibility</name>
<t>Backward compatibility of new EBML Elements can be achieved by using default values for mandatory elements. The default value <bcp14>MUST</bcp14> represent the state that was assumed for previous versions of the EBML Schema, without this new EBML Element. If such a state doesn't make sense for previous versions, then the new EBML Element <bcp14>SHOULD NOT</bcp14> be mandatory.</t>
<t>Non-mandatory EBML Elements can be added in a new EBMLDocTypeVersion. Since
they are not mandatory, they won't be found in older versions of the
EBMLDocTypeVersion, just as they might not be found in newer versions. This
causes no compatibility issue.</t>
</section>
<section anchor="forward-compatibility"><name>Forward Compatibility</name>
<t>EBML Elements <bcp14>MAY</bcp14> be marked as deprecated in a new
EBMLDocTypeVersion using the <tt>maxver</tt> attribute of the EBML Schema. If such an
Element is found in an EBML Document with a newer version of the
EBMLDocTypeVersion, it <bcp14>SHOULD</bcp14> be discarded.</t>
</section>
</section>
<section anchor="security-considerations"><name>Security Considerations</name>
<t>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.</t>
<t>Even if the semantic layer offers any kind of encryption, EBML itself could
leak information at both the semantic layer (as declared via the DocType
Element) and within the EBML structure (the presence of EBML Elements can be
derived even with an unknown semantic layer using a heuristic approach -- not
without errors, of course, but with a certain degree of confidence).</t>
<t>An EBML Document that has the following issues may still be handled by the
EBML Reader and the data accepted as such, depending on how strict the EBML
Reader wants to be:</t>
<ul>
<li><t>Invalid Element IDs that are longer than the limit stated in the EBMLMaxIDLength Element of the EBML Header.</t>
</li>
<li><t>Invalid Element IDs that are not encoded in the shortest-possible way.</t>
</li>
<li><t>Invalid Element Data Size values that are longer than the limit stated in the EBMLMaxSizeLength Element of the EBML Header.</t>
</li>
</ul>
<t>Element IDs that are unknown to the EBML Reader <bcp14>MAY</bcp14> be
accepted as valid EBML IDs in order to skip such elements.</t>
<t>EBML Elements with a string type may contain extra data after the first
0x00. These data <bcp14>MUST</bcp14> be discarded according to the
<xref target="terminating-elements"></xref> rules.</t>
<t>An EBML Reader may discard some or all data if the following errors are found in the EBML Document:</t>
<ul>
<li><t>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).</t>
</li>
<li><t>Very high lengths in order to force out-of-memory situations resulting in a denial of service, access-out-of-bounds issues, etc.</t>
</li>
<li><t>Missing EBML Elements that are mandatory in a Master Element and have no declared default value, making the semantic invalid at that Master Element level.</t>
</li>
<li><t>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).</t>
</li>
<li><t>Usage of invalid data in EBML Elements with a date type, triggering bogus date accesses.</t>
</li>
<li><t>The CRC-32 Element (see <xref target="crc-32-element"></xref>) of a Master Element doesn't match the rest of the content of that Master Element.</t>
</li>
</ul>
<t>Side-channel attacks could exploit:</t>
<ul>
<li><t>The semantic equivalence of the same string stored in a String Element or UTF-8 Element with and without zero-bit padding, making comparison at the semantic level invalid.</t>
</li>
<li><t>The semantic equivalence of VINT_DATA within Element Data Size with two different lengths due to left-padding zero bits, making comparison at the semantic level invalid.</t>
</li>
<li><t>Data contained within a Master Element that is not itself part of a Child Element, which can trigger incorrect parsing behavior in EBML Readers.</t>
</li>
<li><t>Extraneous copies of Identically Recurring Element, making parsing unnecessarily slow to the point of not being usable.</t>
</li>
<li><t>Copies of Identically Recurring Element within a Parent Element that contain invalid CRC-32 Elements. EBML Readers not checking the CRC-32 might use the version of the element with mismatching CRC-32s.</t>
</li>
<li><t>Use of Void Elements that could be used to hide content or create bogus resynchronization points seen by some EBML Readers and not others.</t>
</li>
</ul>
</section>
<section anchor="iana-considerations"><name>IANA Considerations</name>
<section anchor="ebml-element-ids-registry"><name>EBML Element IDs Registry</name>
<t>This document creates a new IANA registry called the
"EBML Element IDs" registry.</t>
<t>Element IDs are described in <xref target="element-id"></xref>. Element
IDs are encoded using the VINT mechanism described in
<xref target="variable-size-integer"></xref> and can be between one and five
octets long. Five-octet-long Element IDs are possible only if declared
in the header.</t>
<t>This IANA registry only applies to Elements that can be contained
in the EBML Header, thus including Global Elements. Elements only
found in the EBML Body have their own set of independent Element IDs
and are not part of this IANA registry.</t>
<t>One-octet Element IDs <bcp14>MUST</bcp14> be between 0x81 and
0xFE. These items are valuable because they are short, and they need
to be used for commonly repeated elements. Element IDs are to be
allocated within this range according to the "RFC Required"
policy <xref target="RFC8126"></xref>.</t>
<t>The following one-octet Element IDs are RESERVED: 0xFF and
0x80.</t>
<t>Values in the one-octet range of 0x00 to 0x7F are not valid for use
as an Element ID.</t>
<t>Two-octet Element IDs <bcp14>MUST</bcp14> be between 0x407F and
0x7FFE. Element IDs are to be allocated within this range according to
the "Specification Required" policy
<xref target="RFC8126"></xref>.</t>
<t>The following two-octet Element IDs are RESERVED: 0x7FFF and
0x4000.</t>
<t>Values in the two-octet ranges of 0x0000 to 0x3FFF and 0x8000 to 0xFFFF are
not valid for use as an Element ID.</t>
<t>Three-octet Element IDs <bcp14>MUST</bcp14> be between 0x203FFF and 0x3FFFFE. Element IDs are to be allocated within this range according to the "First Come First Served" policy <xref target="RFC8126"></xref>.</t>
<t>The following three-octet Element IDs are RESERVED: 0x3FFFFF and
0x200000.</t>
<t>Values in the three-octet ranges of 0x000000 to 0x1FFFFF and
0x400000 to 0xFFFFFF are not valid for use as an Element ID.</t>
<t>Four-octet Element IDs <bcp14>MUST</bcp14> be between 0x101FFFFF
and 0x1FFFFFFE. Four-octet Element IDs are somewhat special in that
they are useful for resynchronizing to major structures in the event
of data corruption or loss. As such, four-octet Element IDs are split
into two categories. Four-octet Element IDs whose lower three octets
(as encoded) would make printable 7-bit ASCII values (0x20 to 0x7E,
inclusive) <bcp14>MUST</bcp14> be allocated by the "Specification
Required" policy. Sequential allocation of values is not
required: specifications <bcp14>SHOULD</bcp14> include a specific
request and are encouraged to do early allocations.</t>
<t>To be clear about the above category: four-octet Element IDs always start
with hex 0x10 to 0x1F, and that octet may be chosen so that the entire VINT
has some desirable property, such as a specific CRC. The other three octets,
when ALL having values between 0x20 (32, ASCII Space) and 0x7E (126, ASCII
"~"), fall into this category.</t>
<t>Other four-octet Element IDs may be allocated by the "First Come First Served" policy.</t>
<t>The following four-octet Element IDs are RESERVED: 0x1FFFFFFF and 0x10000000.</t>
<t>Values in the four-octet ranges of 0x00000000 to 0x0FFFFFFF and 0x20000000
to 0xFFFFFFFF are not valid for use as an Element ID.</t>
<t>Five-octet Element IDs (values from 0x080FFFFFFF to 0x0FFFFFFFFE) are RESERVED according to the "Experimental Use" policy <xref target="RFC8126"></xref>: they may be used by anyone at any time, but there is no coordination.</t>
<t>ID Values found in this document are assigned as initial values as follows:</t>
<table><name>IDs and Names for EBML Elements assigned by this document
</name>
<thead>
<tr>
<th align="right">Element ID</th>
<th align="left">Element Name</th>
<th align="left">Reference</th>
</tr>
</thead>
<tbody>
<tr>
<td align="right">0x1A45DFA3</td>
<td align="left">EBML</td>
<td align="left">Described in <xref target="ebml-element"></xref></td>
</tr>
<tr>
<td align="right">0x4286</td>
<td align="left">EBMLVersion</td>
<td align="left">Described in <xref target="ebmlversion-element"></xref></td>
</tr>
<tr>
<td align="right">0x42F7</td>
<td align="left">EBMLReadVersion</td>
<td align="left">Described in <xref target="ebmlreadversion-element"></xref></td>
</tr>
<tr>
<td align="right">0x42F2</td>
<td align="left">EBMLMaxIDLength</td>
<td align="left">Described in <xref target="ebmlmaxidlength-element"></xref></td>
</tr>
<tr>
<td align="right">0x42F3</td>
<td align="left">EBMLMaxSizeLength</td>
<td align="left">Described in <xref target="ebmlmaxsizelength-element"></xref></td>
</tr>
<tr>
<td align="right">0x4282</td>
<td align="left">DocType</td>
<td align="left">Described in <xref target="doctype-element"></xref></td>
</tr>
<tr>
<td align="right">0x4287</td>
<td align="left">DocTypeVersion</td>
<td align="left">Described in <xref target="doctypeversion-element"></xref></td>
</tr>
<tr>
<td align="right">0x4285</td>
<td align="left">DocTypeReadVersion</td>
<td align="left">Described in <xref target="doctypereadversion-element"></xref></td>
</tr>
<tr>
<td align="right">0x4281</td>
<td align="left">DocTypeExtension</td>
<td align="left">Described in <xref target="doctypeextension-element"></xref></td>
</tr>
<tr>
<td align="right">0x4283</td>
<td align="left">DocTypeExtensionName</td>
<td align="left">Described in <xref target="doctypeextensionname-element"></xref></td>
</tr>
<tr>
<td align="right">0x4284</td>
<td align="left">DocTypeExtensionVersion</td>
<td align="left">Described in <xref target="doctypeextensionversion-element"></xref></td>
</tr>
<tr>
<td align="right">0xBF</td>
<td align="left">CRC-32</td>
<td align="left">Described in <xref target="crc-32-element"></xref></td>
</tr>
<tr>
<td align="right">0xEC</td>
<td align="left">Void</td>
<td align="left">Described in <xref target="void-element"></xref></td>
</tr>
</tbody>
</table></section>
<section anchor="ebml-doctypes-registry"><name>EBML DocTypes Registry</name>
<t>This document creates a new IANA registry called the "EBML DocTypes" registry.</t>
<t>To register a new DocType in this registry, one needs a DocType name, a Description of the DocType, a Change Controller (IESG or email of registrant), and an optional Reference to a document describing the DocType.</t>
<t>DocType values are described in <xref target="doctype"></xref>. DocTypes
are ASCII strings, defined in <xref target="string-element"></xref>, which
label the official name of the EBML Document Type. The strings may be
allocated according to the "First Come First Served" policy.</t>
<t>The use of ASCII corresponds to the types and code already in use; the
value is not meant to be visible to the user.</t>
<t>DocType string values of "matroska" and "webm" are RESERVED to the IETF for future use.
These can be assigned via the "IESG Approval" or "RFC Required" policies <xref target="RFC8126"></xref>.</t>
</section>
</section>
</middle>
<back>
<displayreference target="I-D.ietf-cellar-matroska" to="Matroska"/>
<references><name>Normative References</name>
<reference anchor="IEEE.754" target="https://standards.ieee.org/standard/754-2019.html">
<front>
<title>IEEE Standard for Binary Floating-Point Arithmetic</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2019" month="June" day="13"></date>
</front>
</reference>
<reference anchor="ISO3309" target="https://www.iso.org/standard/8558.html">
<front>
<title>Data communication -- High-level data link control procedures -- Frame structure</title>
<author>
<organization>International Organization for Standardization</organization>
</author>
<date year="1984" month="October"></date>
</front>
<seriesInfo name="ISO" value="3309, 3rd Edition"></seriesInfo>
</reference>
<reference anchor="ISO9899" target="https://www.iso.org/standard/57853.html">
<front>
<title>Information technology -- Programming languages -- C</title>
<author>
<organization>International Organization for Standardization</organization>
</author>
<date year="2011"></date>
</front>
<seriesInfo name="ISO/IEC" value="9899:2011"></seriesInfo>
</reference>
<reference anchor="ITU.V42" target="https://www.itu.int/rec/T-REC-V.42">
<front>
<title>Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion</title>
<author>
<organization>International Telecommunications Union</organization>
</author>
<date year="2002" month="March"></date>
</front>
<seriesInfo name="ITU-T Recommendation" value="V.42"></seriesInfo>
</reference>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0020.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2141.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2648.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3339.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3629.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3688.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5646.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7405.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
<reference anchor="XHTML" target="https://www.w3.org/TR/2018/SPSD-xhtml-basic-20180327/">
<front>
<title>XHTML(tm) Basic 1.1 -- Second Edition</title>
<author fullname="Shane McCarron" initials="S." surname="McCarron">
</author>
<date year="2018" month="March" day="27"></date>
</front>
<seriesInfo name="Latest version available at" value="https://www.w3.org/TR/xhtml-basic"></seriesInfo>
</reference>
<reference anchor="XML" target="https://www.w3.org/TR/2008/REC-xml-20081126/">
<front>
<title>Extensible Markup Language (XML) 1.0 (Fifth Edition)</title>
<author initials="T." surname="Bray" role="editor">
<organization>Textuality and Netscape</organization>
</author>
<author initials="J" surname="Paoli" role="editor">
</author>
<author initials="C.M" surname="Sperberg-McQueen" role="editor">
</author>
<author initials="E." surname="Maler" role="editor">
</author>
<author initials="F." surname="Yergeau" role="editor">
</author>
<date year="2008" month="November" day="26"></date>
</front>
<seriesInfo name="Latest version available at" value="https://www.w3.org/TR/xml/"></seriesInfo>
</reference>
<reference anchor="XML-SCHEMA" target="https://www.w3.org/TR/2004/REC-xmlschema-0-20041028/">
<front>
<title>XML Schema Part 0: Primer Second Edition</title>
<author fullname="David C. Fallside" initials="D.C." surname="Fallside">
</author>
<author fullname="Priscilla Walmsley" initials="P" surname="Walmsley">
</author>
<date year="2004" month="October" day="28"></date>
</front>
<seriesInfo name="Latest version available at" value="http://www.w3.org/TR/xmlschema-0/"></seriesInfo>
</reference>
</references>
<references><name>Informative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml-ids/reference.I-D.ietf-cellar-matroska.xml"/>
<reference anchor="WebM" target="https://www.webmproject.org/docs/container/">
<front>
<title>WebM Container Guidelines</title>
<author>
<organization>The WebM Project</organization>
</author>
<date year="2017" month="November" day="28"></date>
</front>
</reference>
<reference anchor="XPath" target="https://www.w3.org/TR/1999/REC-xpath-19991116">
<front>
<title>XML Path Language (XPath) Version 1.0</title>
<author initials="J." surname="Clark" role="editor">
</author>
<author initials="S" surname="DeRose">
</author>
<date year="1999" month="November" day="16"></date>
</front>
<seriesInfo name="Latest version available at" value="https://www.w3.org/TR/xpath"></seriesInfo>
</reference>
</references>
</back>
</rfc>