Extensible Binary Meta Language

4.1. VINT_WIDTH

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

Within the EBML Header the VINT_WIDTH of a VINT MUST NOT exceed three bits in length (meaning that the Variable Size Integer MUST NOT exceed four octets in length) except if said VINT is used to express the Element Data Size of an EBML Element with Element Name EBML and Element ID 0x1A45DFA3 (see Section 11.2.1) in which case the VINT_WIDTH MUST NOT exceed seven bits in length. Within the EBML Body, when a VINT is used to express an Element ID, the maximum length allowed for the VINT_WIDTH is one less than the value set in the EBMLMaxIDLength Element. Within the EBML Body, when a VINT is used to express an Element Data Size, the maximum length allowed for the VINT_WIDTH is one less than the value set in the EBMLMaxSizeLength Element.¶

4.2. VINT_MARKER

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

4.3. VINT_DATA

The VINT_DATA portion of the Variable Size Integer includes all data that follows (but not including) the VINT_MARKER until end of the Variable Size Integer whose length is derived from the VINT_WIDTH. The bits required for the VINT_WIDTH and the VINT_MARKER 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 are less than the bit size of VINT_DATA, then VINT_DATA MUST be zero-padded to the left to a size that fits. The VINT_DATA value MUST be expressed as a big-endian unsigned integer.¶

4.4. VINT Examples

Table 1 shows examples of Variable Size Integers with lengths from 1 to 5 octets. The Usable Bits column refers to the number of bits that can be used in the VINT_DATA. The Representation column depicts a binary expression of Variable Size Integers where VINT_WIDTH is depicted by 0, the VINT_MARKER as 1, and the VINT_DATA as x.¶

Data encoded as a Variable Size Integer may be rendered at octet lengths larger than needed to store the data in order to facilitate overwriting it at a later date, e.g. when its final size isn't known in advance. In Table 2 a 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.¶

6.1. Data Size Format

The Element Data Size expresses the length in octets of Element Data. The Element Data Size itself is encoded as a Variable Size Integer. By default, Element Data Sizes can be encoded in lengths from one octet to eight octets, although Element Data Sizes of greater lengths MAY be used if the octet length of the longest Element Data Size of the EBML Document is declared in the EBMLMaxSizeLength Element of the EBML Header (see Section 11.2.5). Unlike the VINT_DATA of the Element ID, the VINT_DATA component of the Element Data Size is not mandated to be encoded at the shortest valid length. For example, an Element Data Size with binary encoding of 1011 1111 or a binary encoding of 0100 0000 0011 1111 are both valid Element Data Sizes and both store a semantically equal value (both 0b00000000111111 and 0b0111111, the VINT_DATA sections of the examples, represent the integer 63).¶

Although an Element ID with all VINT_DATA bits set to zero is invalid, an Element Data Size with all VINT_DATA bits set to zero is allowed for EBML Element Types which do not mandate a non-zero length (see Section 7). An Element Data Size with all VINT_DATA bits set to zero indicates that the Element Data is zero octets in length. Such an EBML Element is referred to as an Empty Element. If an Empty Element has a default value declared then the EBML Reader MUST interpret the value of the Empty Element as the default value. If an Empty Element has no default value declared then the EBML Reader MUST use the value of the Empty Element for the corresponding EBML Element Type of the Element ID, 0 for numbers and an empty string for strings.¶

6.2. Unknown Data Size

An Element Data Size with all VINT_DATA bits set to one is reserved as an indicator that the size of the EBML Element is unknown. The only reserved value for the VINT_DATA of Element Data Size is all bits set to one. An EBML Element with an unknown Element Data Size is referred to as an Unknown-Sized Element. A Master Element MAY be an Unknown-Sized Element; however an EBML Element that is not a Master Element MUST NOT be an Unknown-Sized Element. Master Elements MUST NOT use an unknown size unless the unknownsizeallowed attribute of their EBML Schema is set to true (see Section 11.1.5.10).¶

The use of Unknown-Sized Elements allows for an EBML Element to be written and read before the size of the EBML Element is known. Unknown-Sized Elements MUST only be used if the Element Data Size is not known before the Element Data is written, such as in some cases of data streaming. The end of an Unknown-Sized Element is determined by whichever comes first:¶

• Any EBML Element that is a valid Parent Element of the Unknown-Sized Element according to the EBML Schema, Global Elements excluded.¶

• Any valid EBML Element according to the EBML Schema, Global Elements excluded, that is not a Descendant Element of the Unknown-Sized Element but share a common direct parent, such as a Top-Level Element.¶

• Any EBML Element that is a valid Root Element according to the EBML Schema, Global Elements excluded.¶

• The end of the Parent Element with a known size has been reached.¶

• The end of the EBML Document, either when reaching the end of the file or because a new EBML Header started.¶

Consider an Unknown-Sized Element which EBML path is \root\level1\level2\elt. When reading a new Element ID, assuming the EBML Path of that new Element is valid, here are some possible and impossible ways that this new Element is ending elt:¶

6.3. Data Size Values

For Element Data Sizes encoded at octet lengths from one to eight, Table 6 depicts the range of possible values that can be encoded as an Element Data Size. An Element Data Size with an octet length of 8 is able to express a size of 2^56-2 or 72,057,594,037,927,934 octets (or about 72 petabytes). The maximum possible value that can be stored as Element Data Size is referred to as VINTMAX.¶

If the length of Element Data equals 2^(n*7)-1 then the octet length of the Element Data Size MUST be at least n+1. This rule prevents an Element Data Size from being expressed as the unknown size value. Table 7 clarifies this rule by showing a valid and invalid expression of an Element Data Size with a VINT_DATA of 127 (which is equal to 2^(1*7)-1) and 16,383 (which is equal to 2^(2*7)-1).¶

7.1. Signed Integer Element

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

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

7.2. Unsigned Integer Element

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

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

7.3. Float Element

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

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

7.4. String Element

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

A String Element MUST either be empty (zero-length) or contain printable ASCII characters [RFC0020] in the range of 0x20 to 0x7E, with an exception made for termination (see Section 13).¶

7.5. UTF-8 Element

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

A UTF-8 Element contains only a valid Unicode string as defined in [RFC3629], with an exception made for termination (see Section 13).¶

7.6. Date Element

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

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

7.7. Master Element

A Master Element MUST declare a length in octets from zero to VINTMAX. The Master Element MAY also use an unknown length. See Section 6 for rules that apply to elements of unknown length.¶

The Master Element contains zero, one, or many other elements. EBML Elements contained within a Master Element MUST have the EBMLParentPath of their Element Path equal to the EBMLFullPath of the Master Element Element Path (see Section 11.1.5.2). Element Data stored within Master Elements SHOULD only consist of EBML Elements and SHOULD NOT contain any data that is not part of an EBML Element. The EBML Schema identifies what Element IDs are valid within the Master Elements for that version of the EBML Document Type. Any data contained within a Master Element that is not part of a Child Element MUST be ignored.¶

7.8. Binary Element

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

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

8.1. EBML Header

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

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

The EBML Header MUST contain a single Master Element with an Element Name of EBML and Element ID of 0x1A45DFA3 (see Section 11.2.1) and any number of additional EBML Elements within it. The EBML Header of an EBML Document that uses an EBMLVersion of 1 MUST only contain EBML Elements that are defined as part of this document.¶

8.2. EBML Body

All data of an EBML Document following the EBML Header is the EBML Body. The end of the EBML Body, as well as the end of the EBML Document that contains the EBML Body, is reached at whichever comes first: the beginning of a new EBML Header at the Root Level or the end of the file. The EBML Body MUST NOT contain any data that is not part of an EBML Element. 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.¶

10.1. EBML Header Version

The version of the EBML Header is found in EBMLVersion. An EBML parser can read an EBML Header if it can read either the EBMLVersion version or a version equal or higher than the one found in EBMLReadVersion.¶

10.2. EBML Document Version

The version of the EBML Body is found in EBMLDocTypeVersion. A parser for the particular DocType format can read the EBML Document if it can read either the EBMLDocTypeVersion version of that format or a version equal or higher than the one found in EBMLDocTypeReadVersion.¶

11.1. EBML Schema

An EBML Schema is a well-formed XML Document [W3C.REC-xml-20081126] 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 [W3C.REC-xmlschema-0-20041028] to an XML Document [W3C.REC-xml-20081126]. An EBML Schema MUST be clearly associated with one or more EBML Document Types. An EBML Document Type is identified by a string stored within the EBML Header in the DocType Element; for example matroska or webm (see Section 11.2.6). The DocType value for an EBML Document Type MUST be unique and persistent.¶

An EBML Schema MUST declare exactly one EBML Element at Root Level (referred to as the Root Element) that occurs exactly once within an EBML Document. The Void Element MAY also occur at Root Level but is not a Root Element (see Section 11.3.2).¶

The EBML Schema MUST document all Elements of the EBML Body. The EBML Schema does not document Global Elements that are defined by this document (namely the Void Element and the CRC-32 Element).¶

The EBML Schema MUST NOT use the Element ID 0x1A45DFA3 which is reserved for the EBML Header for resynchronization purpose.¶

An EBML Schema MAY constrain the use of EBML Header Elements (see Section 11.2) by adding or constraining that Element's range attribute. For example, an EBML Schema MAY constrain the EBMLMaxSizeLength to a maximum value of 8 or MAY constrain the EBMLVersion to only support a value of 1. If an EBML Schema adopts the EBML Header Element as-is, then it is not required to document that Element within the EBML Schema. If an EBML Schema constrains the range of an EBML Header Element, then that Element MUST be documented within an <element> node of the EBML Schema. This document provides an example of an EBML Schema, see Section 11.1.1.¶

<?xml version="1.0" encoding="utf-8"?>
<EBMLSchema xmlns="https://ietf.org/cellar/ebml"
  docType="files-in-ebml-demo" version="1">
 <!-- constraints to the range of two EBML Header Elements -->
 <element name="EBMLReadVersion" path="1*1(\EBML\EBMLReadVersion)"
   id="0x42F7" minOccurs="1" maxOccurs="1" range="1" default="1"
   type="uinteger"/>
 <element name="EBMLMaxSizeLength"
   path="1*1(\EBML\EBMLMaxSizeLength)" id="0x42F3" minOccurs="1"
   maxOccurs="1" range="8" default="8" type="uinteger"/>
 <!-- Root Element-->
 <element name="Files" path="*1(\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="1*(\Files\File)" id="0x6146"
   type="master" minOccurs="1">
  <documentation lang="en" purpose="definition">
    An attached file.
  </documentation>
 </element>
 <element name="FileName" path="1*1(\Files\File\FileName)"
   id="0x614E" type="utf-8"
   minOccurs="1">
  <documentation lang="en" purpose="definition">
    Filename of the attached file.
  </documentation>
 </element>
 <element name="MimeType" path="1*1(\Files\File\MimeType)"
   id="0x464D" type="string"
     minOccurs="1">
  <documentation lang="en" purpose="definition">
    MIME type of the file.
  </documentation>
 </element>
 <element name="ModificationTimestamp"
   path="1*1(\Files\File\ModificationTimestamp)" id="0x4654"
   type="date" minOccurs="1">
  <documentation lang="en" purpose="definition">
    Modification timestamp of the file.
  </documentation>
 </element>
 <element name="Data" path="1*1(\Files\File\Data)" id="0x4664" 
   type="binary" minOccurs="1">
  <documentation lang="en" purpose="definition">
    The data of the file.
  </documentation>
 </element>
</EBMLSchema>

EBMLFullPath          = [EBMLParentPath] EBMLElementPath
EBMLParentPath        = EBMLFixedParent EBMLLastParent
EBMLFixedParent       = *(EBMLPathAtom)
EBMLElementPath       = EBMLPathAtom / EBMLPathAtomRecursive
EBMLPathAtom          = PathDelimiter EBMLAtomName
EBMLPathAtomRecursive = "(1*(" EBMLPathAtom "))"
EBMLLastParent        = EBMLPathAtom / EBMLGlobalParent
EBMLGlobalParent      = "(" GlobalParentOccurence "\)"
EBMLAtomName          = 1*(EBMLNameChar)
EBMLNameChar          = ALPHA / DIGIT / "-" / "."
PathDelimiter         = "\"
GlobalParentOccurence = [PathMinOccurrence] "*" [PathMaxOccurrence]
PathMinOccurrence     = 1*DIGIT ; no upper limit
PathMaxOccurrence     = 1*DIGIT ; no upper limit

<element name="Item" path="1*1(\Items)" id="0x4025" type="master"
  minOccurs="1" maxOccurs="1">
  <documentation lang="en" purpose="definition">
    A set of items.
  </documentation>
</element>
<element name="Item" path="0*(\Items\Item)" id="0x4026"
  type="master">
  <documentation lang="en" purpose="definition">
    An item.
  </documentation>
</element>
<element name="Cost" path="0*1(\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="0*1(\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>

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="https://ietf.org/cellar/ebml"
  targetNamespace="https://ietf.org/cellar/ebml"
  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: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"/>
    <xs:attribute name="recurring" type="xs:boolean"/>
    <xs:attribute name="minver" default="1">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="maxver">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
  </xs:complexType>

  <xs:complexType name="restrictionType">
    <xs:sequence>
      <xs:element name="enum" type="enumType"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="extensionType">
    <xs:sequence>
      <xs:any processContents="skip"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="type" use="required"/>
    <xs:anyAttribute processContents="skip"/>
  </xs:complexType>

  <xs:complexType name="enumType">
    <xs:sequence>
      <xs:element name="documentation" type="documentationType"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="label"/>
    <xs:attribute name="value" use="required"/>
  </xs:complexType>

  <xs:complexType name="documentationType" mixed="true">
    <xs:sequence>
      <xs:element name="a"      type="xhtml:xhtml.a.type" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xs:element name="br"     type="xhtml:xhtml.br.type"
        minOccurs="0" maxOccurs="unbounded"/>
      <xs:element name="strong" type="xhtml:xhtml.strong.type"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="lang"/>
    <xs:attribute name="purpose" use="required">
      <xs:simpleType>
        <xs:restriction base="xs:string">
          <xs:enumeration value="definition"/>
          <xs:enumeration value="rationale"/>
          <xs:enumeration value="references"/>
          <xs:enumeration value="usage notes"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
  </xs:complexType>

  <xs:complexType name="noteType">
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute name="note_attribute" use="required">
          <xs:simpleType>
            <xs:restriction base="xs:string">
              <xs:enumeration value="minOccurs"/>
              <xs:enumeration value="maxOccurs"/>
              <xs:enumeration value="range"/>
              <xs:enumeration value="length"/>
              <xs:enumeration value="default"/>
              <xs:enumeration value="minver"/>
              <xs:enumeration value="maxver"/>
            </xs:restriction>
          </xs:simpleType>
        </xs:attribute>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
</xs:schema>

11.2. EBML Header Elements

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

11.3. Global Elements

EBML allows some special Elements to be found within more than one parent in an EBML Document or optionally at the Root Level of an EBML Body. These Elements are called Global Elements. There are 2 Global Elements that can be found in any EBML Document: the CRC-32 Element and the Void Element. An EBML Schema MAY add other Global Elements to the format it defines. These extra elements apply only to the EBML Body, not the EBML Header.¶

Global Elements are EBML Elements whose path have a EBMLGlobalParent as their EBMLLastParent. Because it is the last Parent part of the path, a Global Element might also have non-EBMLGlobalParent parts in its path. In this case the Global Element can only be found within this non-EBMLGlobalParent path, i.e. it's not fully "global".¶

A Global Element can be found in many Parent Elements, allowing the same number of occurrences in each Parent where this Element is found.¶

14.1. Reducing a Element Data in Size

There are three methods to reduce the size of Element Data of a written EBML Element.¶

14.2. Considerations when Updating Elements with Cyclic Redundancy Check (CRC)

If the Element to be changed is a Descendant Element of any Master Element that contains a CRC-32 Element (see Section 11.3.1) then the CRC-32 Element MUST be verified before permitting the change. Additionally the CRC-32 Element value MUST be subsequently updated to reflect the changed data.¶

15.1. Backward Compatibility

Backward compatibility of new EBML Elements can be achieved by using default values for mandatory elements. The default value MUST represent the state that was assumed for previous versions of the EBML Schema, without this new EBML Element. If such a state doesn't make sense for previous versions, then the new EBML Element SHOULD NOT be mandatory.¶

Non mandatory EBML Elements can be added in a new EBMLDocTypeVersion. Since they are not mandatory they won't be found in older versions of the EBMLDocTypeVersion, just as they might not be found in newer versions. This causes no compatibility issue.¶

15.2. Forward Compatibility

EBML Elements MAY be marked as deprecated in a new EBMLDocTypeVersion using the maxver attribute of the EBML Schema. If such an Element is found in an EBML Document with newer version of the EBMLDocTypeVersion it SHOULD be discarded.¶

17.1. CELLAR EBML Element ID Registry

This document creates a new IANA Registry called "CELLAR EBML Element ID Registry".¶

Element IDs are described in section Element ID. Element IDs are encoded using the VINT mechanism described in section Section 4 can be between one and five octets long. Five octet long Element IDs are possible only if declared in the header.¶

This IANA Registry only applies to Elements that can be contained in the EBML Header, thus including Global Elements. Elements only found in the EBML Body have their own set of independent Element IDs and are not part of this IANA Registry.¶

The VINT Data value of one-octet Element IDs MUST be between 0x01 and 0x7E. These items are valuable because they are short, and need to be used for commonly repeated elements. Values from 1 to 126 are to be allocated according to the "RFC Required" policy [RFC8126].¶

The VINT Data value of two-octet Element IDs MUST be between 0x007F and 0x3FFE. Numbers are to be allocated within this range according to the "Specification Required" policy [RFC8126].¶

The numbers 0x3FFF and 0x4000 are RESERVED.¶

The VINT Data value of three-octet Element IDs MUST be between 0x4001 and 0x1FFFFE. Numbers may be allocated within this range according to the "First Come First Served" policy [RFC8126].¶

The numbers 0x1FFFFF and 0x200000 are RESERVED.¶

Four-octet Element IDs are numbers between 0x101FFFFF and 0x1FFFFFFE. Four-octet Element IDs are somewhat special in that they are useful for resynchronizing to major structures in the event of data corruption or loss. As such four-octet Element IDs are split into two categories. Four-octet Element IDs whose lower three octets (as encoded) would make printable 7-bit ASCII values (0x20 to 0x7E, inclusive) MUST be allocated by the "Specification Required" policy. Sequential allocation of values is not required: specifications SHOULD include a specific request, and are encouraged to do early allocations.¶

To be clear about the above category: four-octet Element IDs always start with hex 0x10 to 0x1F, and that octet may be chosen so that the entire number has some desirable property, such as a specific CRC. The other three octets, when ALL having values between 0x21 (33, ASCII !) and 0x7E (126, ASCII ~), fall into this category.¶

Other four-octet Element IDs may be allocated by the "First Come First Served" policy.¶

The numbers 0xFFFFFFF and 0x1000000 are RESERVED.¶

Five octet Element IDs (values from 0x10000001 upwards) are RESERVED according to the "Experimental Use" policy [RFC8126]: they may be used by anyone at any time, but there is no coordination.¶

ID Values found in this document are assigned as initial values as follows:¶

17.2. CELLAR EBML DocType Registry

This document creates a new IANA Registry called "CELLAR EBML DocType Registry".¶

To register a new DocType in this registry one needs a DocType name, a Description of the DocType, a Change Controller (IESG or email of registrant) and an optional Reference to a document describing the DocType.¶

DocType values are described in Section 11.1.3.1. DocTypes are ASCII strings, defined in Section 7.4, which label the official name of the EBML Document Type. The strings may be allocated according to the "First Come First Served" policy.¶

The use of ASCII corresponds to the types and code already in use, the value is not meant to be visible to the user.¶

DocType string values of "matroska" and "webm" are RESERVED to the IETF for future use. These can be assigned via the "IESG Approval" or "RFC Required" policies [RFC8126].¶