4.1. VINT_WIDTH

Each Variable-Size Integer starts with a VINT_WIDTH followed by a VINT_MARKER. VINT_WIDTH is a sequence of zero or more bits of value 0 and is terminated by the VINT_MARKER, which is a single bit of value 1. The total length in bits of both VINT_WIDTH and VINT_MARKER is the total length in octets in of the Variable-Size Integer.¶

The single bit 1 starts a Variable-Size Integer with a length of one octet. The sequence of bits 01 starts a Variable-Size Integer with a length of two octets. 001 starts a Variable-Size Integer with a length of three octets, and so on, with each additional 0 bit adding one octet to the length of the Variable-Size Integer.¶

4.2. VINT_MARKER

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

4.3. VINT_DATA

The VINT_DATA portion of the Variable-Size Integer includes all data following (but not including) the VINT_MARKER until end of the Variable-Size Integer whose length is derived from the VINT_WIDTH. The bits required for the VINT_WIDTH and the VINT_MARKER use one out of every eight bits of the total length of the Variable-Size Integer. Thus, a Variable-Size Integer of 1-octet length supplies 7 bits for VINT_DATA, a 2-octet length supplies 14 bits for VINT_DATA, and a 3-octet length supplies 21 bits for VINT_DATA. If the number of bits required for VINT_DATA is less than the bit size of VINT_DATA, then VINT_DATA MUST be zero-padded to the left to a size that fits. The VINT_DATA value MUST be expressed as a big-endian unsigned integer.¶

4.4. VINT Examples

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

A Variable-Size Integer may be rendered at octet lengths larger than needed to store the data in order to facilitate overwriting it at a later date -- e.g., when its final size isn't known in advance. In Table 2, an integer 2 (with a corresponding binary value of 0b10) is shown encoded as different Variable-Size Integers with lengths from one octet to four octets. All four encoded examples have identical semantic meaning, though the VINT_WIDTH and the padding of the VINT_DATA vary.¶

6.1. Data Size Format

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

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

6.2. Unknown Data Size

An Element Data Size with all VINT_DATA bits set to one is reserved as an indicator that the size of the EBML Element is unknown. The only reserved value for the VINT_DATA of Element Data Size is all bits set to one. An EBML Element with an unknown Element Data Size is referred to as an Unknown-Sized Element. Only a Master Element is allowed to be of unknown size, and it can only be so if the unknownsizeallowed attribute of its EBML Schema is set to true (see Section 11.1.6.10).¶

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

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

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

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

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

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

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

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 2⁵⁶-2 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) that could be negative, positive, or zero. Signed Integers are stored with two's complement notation with the leftmost bit being the sign bit. Because EBML limits Signed Integers to 8 octets in length, a Signed Integer Element stores a number from -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807.¶

7.2. Unsigned Integer Element

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

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

7.3. Float Element

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

A Float Element stores a floating-point number in the 32-bit and 64-bit binary interchange format, as defined in [IEEE.754].¶

7.4. String Element

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

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

7.5. UTF-8 Element

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

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

7.6. Date Element

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

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

7.7. Master Element

A Master Element MUST declare a length in octets from zero to VINTMAX or be of unknown length. See Section 6 for rules that apply to elements of unknown length.¶

The Master Element contains zero or more other elements. EBML Elements contained within a Master Element MUST have the EBMLParentPath of their Element Path equal to the EBMLFullPath of the Master Element Element Path (see Section 11.1.6.2). Element Data stored within Master Elements SHOULD only consist of EBML Elements and SHOULD NOT contain any data that is not part of an EBML Element. The EBML Schema identifies what Element IDs are valid within the Master Elements for that version of the EBML Document Type. Any data contained within a Master Element that is not part of a Child Element MUST be ignored.¶

7.8. Binary Element

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

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

8.1. EBML Header

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

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

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

Elements within an EBML Header can be at most 4 octets long, except for the EBML Element with Element Name EBML and Element ID 0x1A45DFA3 (see Section 11.2.1); this Element can be up to 8 octets long.¶

8.2. EBML Body

All data of an EBML Document following the EBML Header is the EBML Body. The end of the EBML Body, as well as the end of the EBML Document that contains the EBML Body, is reached at whichever comes first: the beginning of a new EBML Header at the Root Level or the end of the file. This document defines precisely which EBML Elements are to be used within the EBML Header but does not name or define which EBML Elements are to be used within the EBML Body. The definition of which EBML Elements are to be used within the EBML Body is defined by an EBML Schema.¶

Within the EBML Body, the maximum octet length allowed for any Element ID is set by the EBMLMaxIDLength Element of the EBML Header, and the maximum octet length allowed for any Element Data Size is set by the EBMLMaxSizeLength Element of the EBML Header.¶

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 [XML] that defines the properties, arrangement, and usage of EBML Elements that compose a specific EBML Document Type. The relationship of an EBML Schema to an EBML Document is analogous to the relationship of an XML Schema [XML-SCHEMA] to an XML Document [XML]. An EBML Schema MUST be clearly associated with one or more EBML Document Types. An EBML Document Type is identified by a string stored within the EBML Header in the DocType Element -- for example, Matroska or WebM (see Section 11.2.6). The DocType value for an EBML Document Type MUST be unique, persistent, and described in the IANA registry (see Section 17.2).¶

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

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

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

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

<?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>

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

<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>

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="urn:ietf:rfc:8794"
  targetNamespace="urn:ietf:rfc:8794"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:xhtml="http://www.w3.org/1999/xhtml"
  elementFormDefault="qualified" version="01">

  <!-- for HTML in comments -->
  <xs:import namespace="http://www.w3.org/1999/xhtml"
    schemaLocation="http://www.w3.org/MarkUp/SCHEMA/xhtml11.xsd"/>

  <xs:element name="EBMLSchema" type="EBMLSchemaType"/>

  <xs:complexType name="EBMLSchemaType">
    <xs:sequence>
      <xs:element name="element" type="elementType"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="docType" use="required"/>
    <xs:attribute name="version" use="required" type="xs:integer"/>
    <xs:attribute name="ebml" type="xs:positiveInteger"
      default="1"/>
  </xs:complexType>

  <xs:complexType name="elementType">
    <xs:sequence>
      <xs:element name="documentation" type="documentationType"
        minOccurs="0" maxOccurs="unbounded"/>
      <xs:element name="implementation_note" type="noteType"
        minOccurs="0" maxOccurs="unbounded"/>
      <xs:element name="restriction" type="restrictionType"
        minOccurs="0" maxOccurs="1"/>
      <xs:element name="extension" type="extensionType"
        minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="name" use="required">
      <xs:simpleType>
        <xs:restriction base="xs:string">
          <xs:pattern value="[0-9A-Za-z.-]([0-9A-Za-z.-])*"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="path" use="required">
      <!-- <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:pattern value="[0-9]*\*[0-9]*()"/>
        </xs:restriction>
      </xs:simpleType> -->
    </xs:attribute>
    <xs:attribute name="id" use="required">
      <xs:simpleType>
        <xs:restriction base="xs:string">
          <xs:pattern value="0x([0-9A-F][0-9A-F])+"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="minOccurs" default="0">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="maxOccurs" default="1">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="range"/>
    <xs:attribute name="length"/>
    <xs:attribute name="default"/>
    <xs:attribute name="type" use="required">
      <xs:simpleType>
        <xs:restriction base="xs:string">
          <xs:enumeration value="integer"/>
          <xs:enumeration value="uinteger"/>
          <xs:enumeration value="float"/>
          <xs:enumeration value="string"/>
          <xs:enumeration value="date"/>
          <xs:enumeration value="utf-8"/>
          <xs:enumeration value="master"/>
          <xs:enumeration value="binary"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="unknownsizeallowed" type="xs:boolean"
      default="false"/>
    <xs:attribute name="recursive" type="xs:boolean"
      default="false"/>
    <xs:attribute name="recurring" type="xs:boolean"
      default="false"/>
    <xs:attribute name="minver" default="1">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="maxver">
      <xs:simpleType>
        <xs:restriction base="xs:integer">
          <xs:minInclusive value="0"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:attribute>
  </xs:complexType>

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

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

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

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

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

11.2. 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 two Global Elements that can be found in any EBML Document: the CRC-32 Element and the Void Element. An EBML Schema MAY add other Global Elements to the format it defines. These extra elements apply only to the EBML Body, not the EBML Header.¶

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

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

14.1. Reducing 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 a newer version of the EBMLDocTypeVersion, it SHOULD be discarded.¶

17.1. EBML Element IDs Registry

This document creates a new IANA registry called the "EBML Element IDs" registry.¶

Element IDs are described in Section 5. Element IDs are encoded using the VINT mechanism described in Section 4 and can be between one and five octets long. Five-octet-long Element IDs are possible only if declared in the header.¶

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

One-octet Element IDs MUST be between 0x81 and 0xFE. These items are valuable because they are short, and they need to be used for commonly repeated elements. Element IDs are to be allocated within this range according to the "RFC Required" policy [RFC8126].¶

The following one-octet Element IDs are RESERVED: 0xFF and 0x80.¶

Values in the one-octet range of 0x00 to 0x7F are not valid for use as an Element ID.¶

Two-octet Element IDs MUST be between 0x407F and 0x7FFE. Element IDs are to be allocated within this range according to the "Specification Required" policy [RFC8126].¶

The following two-octet Element IDs are RESERVED: 0x7FFF and 0x4000.¶

Values in the two-octet ranges of 0x0000 to 0x3FFF and 0x8000 to 0xFFFF are not valid for use as an Element ID.¶

Three-octet Element IDs MUST be between 0x203FFF and 0x3FFFFE. Element IDs are to be allocated within this range according to the "First Come First Served" policy [RFC8126].¶

The following three-octet Element IDs are RESERVED: 0x3FFFFF and 0x200000.¶

Values in the three-octet ranges of 0x000000 to 0x1FFFFF and 0x400000 to 0xFFFFFF are not valid for use as an Element ID.¶

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

To be clear about the above category: four-octet Element IDs always start with hex 0x10 to 0x1F, and that octet may be chosen so that the entire VINT has some desirable property, such as a specific CRC. The other three octets, when ALL having values between 0x20 (32, ASCII Space) and 0x7E (126, ASCII "~"), fall into this category.¶

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

The following four-octet Element IDs are RESERVED: 0x1FFFFFFF and 0x10000000.¶

Values in the four-octet ranges of 0x00000000 to 0x0FFFFFFF and 0x20000000 to 0xFFFFFFFF are not valid for use as an Element ID.¶

Five-octet Element IDs (values from 0x080FFFFFFF to 0x0FFFFFFFFE) are RESERVED according to the "Experimental Use" policy [RFC8126]: they may be used by anyone at any time, but there is no coordination.¶

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

17.2. EBML DocTypes Registry

This document creates a new IANA registry called the "EBML DocTypes" registry.¶

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

DocType values are described in Section 11.1.4.1. DocTypes are ASCII strings, defined in Section 7.4, which label the official name of the EBML Document Type. The strings may be allocated according to the "First Come First Served" policy.¶

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

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