



Internet Engineering Task Force (IETF)                         S. Lhomme
Request for Comments: 8794                                              
Category: Standards Track                                        D. Rice
ISSN: 2070-1721                                                         
                                                               M. Bunkus
                                                               July 2020


                    Extensible Binary Meta Language

Abstract

   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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8794.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Notation and Conventions
   3.  Structure
   4.  Variable-Size Integer
     4.1.  VINT_WIDTH
     4.2.  VINT_MARKER
     4.3.  VINT_DATA
     4.4.  VINT Examples
   5.  Element ID
   6.  Element Data Size
     6.1.  Data Size Format
     6.2.  Unknown Data Size
     6.3.  Data Size Values
   7.  EBML Element Types
     7.1.  Signed Integer Element
     7.2.  Unsigned Integer Element
     7.3.  Float Element
     7.4.  String Element
     7.5.  UTF-8 Element
     7.6.  Date Element
     7.7.  Master Element
     7.8.  Binary Element
   8.  EBML Document
     8.1.  EBML Header
     8.2.  EBML Body
   9.  EBML Stream
   10. EBML Versioning
     10.1.  EBML Header Version
     10.2.  EBML Document Version
   11. Elements semantics
     11.1.  EBML Schema
       11.1.1.  EBML Schema Example
       11.1.2.  "<EBMLSchema>" Element
       11.1.3.  "<EBMLSchema>" Namespace
       11.1.4.  "<EBMLSchema>" Attributes
       11.1.5.  "<element>" Element
       11.1.6.  "<element>" Attributes
       11.1.7.  "<documentation>" Element
       11.1.8.  "<documentation>" Attributes
       11.1.9.  "<implementation_note>" Element
       11.1.10. "<implementation_note>" Attributes
       11.1.11. "<restriction>" Element
       11.1.12. "<enum>" Element
       11.1.13. "<enum>" Attributes
       11.1.14. "<extension>" Element
       11.1.15. "<extension>" Attributes
       11.1.16. XML Schema for EBML Schema
       11.1.17. Identically Recurring Elements
       11.1.18. Textual expression of floats
       11.1.19. Note on the use of default attributes to define
               Mandatory EBML Elements
     11.2.  EBML Header Elements
       11.2.1.  EBML Element
       11.2.2.  EBMLVersion Element
       11.2.3.  EBMLReadVersion Element
       11.2.4.  EBMLMaxIDLength Element
       11.2.5.  EBMLMaxSizeLength Element
       11.2.6.  DocType Element
       11.2.7.  DocTypeVersion Element
       11.2.8.  DocTypeReadVersion Element
       11.2.9.  DocTypeExtension Element
       11.2.10. DocTypeExtensionName Element
       11.2.11. DocTypeExtensionVersion Element
     11.3.  Global Elements
       11.3.1.  CRC-32 Element
       11.3.2.  Void Element
   12. Considerations for Reading EBML Data
   13. Terminating Elements
   14. Guidelines for Updating Elements
     14.1.  Reducing Element Data in Size
       14.1.1.  Adding a Void Element
       14.1.2.  Extending the Element Data Size
       14.1.3.  Terminating Element Data
     14.2.  Considerations when Updating Elements with Cyclic
            Redundancy Check (CRC)
   15. Backward and Forward Compatibility
     15.1.  Backward Compatibility
     15.2.  Forward Compatibility
   16. Security Considerations
   17. IANA Considerations
     17.1.  EBML Element IDs Registry
     17.2.  EBML DocTypes Registry
   18. Normative References
   19. Informative References
   Authors' Addresses

1.  Introduction

   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).

   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 [Matroska].  The applicability of EBML for other use cases
   is beyond the scope of this document.

   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 [WebM].

   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.

   A typical EBML file has the following structure:

   EBML Header (master)
     + DocType (string)
     + DocTypeVersion (unsigned integer)
   EBML Body Root (master)
     + ElementA (utf-8)
     + Parent (master)
       + ElementB (integer)
     + Parent (master)
       + ElementB (integer)

2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document defines specific terms in order to define the format
   and application of "EBML".  Specific terms are defined below:

   "EBML":  Extensible Binary Meta Language

   "EBML Document Type":  A name provided by an "EBML Schema" to
      designate a particular implementation of "EBML" for a data format
      (e.g., Matroska and WebM).

   "EBML Schema":  A standardized definition for the structure of an
      "EBML Document Type".

   "EBML Document":  A datastream comprised of only two components, an
      "EBML Header" and an "EBML Body".

   "EBML Reader":  A data parser that interprets the semantics of an
      "EBML Document" and creates a way for programs to use "EBML".

   "EBML Stream":  A file that consists of one or more "EBML Documents"
      that are concatenated together.

   "EBML Header":  A declaration that provides processing instructions
      and identification of the "EBML Body".  The "EBML Header" is
      analogous to an XML Declaration [XML] (see Section 2.8 on "Prolog
      and Document Type Declaration").

   "EBML Body":  All data of an "EBML Document" following the "EBML
      Header".

   "Variable-Size Integer":  A compact variable-length binary value that
      defines its own length.

   "VINT":  Also known as "Variable-Size Integer".

   "EBML Element":  A foundation block of data that contains three
      parts: an "Element ID", an "Element Data Size", and "Element
      Data".

   "Element ID":  A binary value, encoded as a "Variable-Size Integer",
      used to uniquely identify a defined "EBML Element" within a
      specific "EBML Schema".

   "Element Data Size":  An expression, encoded as a "Variable-Size
      Integer", of the length in octets of "Element Data".

   "VINTMAX":  The maximum possible value that can be stored as "Element
      Data Size".

   "Unknown-Sized Element":  An "Element" with an unknown "Element Data
      Size".

   "Element Data":  The value(s) of the "EBML Element", which is
      identified by its "Element ID" and "Element Data Size".  The form
      of the "Element Data" is defined by this document and the
      corresponding "EBML Schema" of the Element's "EBML Document Type".

   "Root Level":  The starting level in the hierarchy of an "EBML
      Document".

   "Root Element":  A mandatory, nonrepeating "EBML Element" that occurs
      at the top level of the path hierarchy within an "EBML Body" and
      contains all other "EBML Elements" of the "EBML Body", excepting
      optional "Void Elements".

   "Top-Level Element":  An "EBML Element" defined to only occur as a
      "Child Element" of the "Root Element".

   "Master Element":  The "Master Element" contains zero, one, or many
      other "EBML Elements".

   "Child Element":  A "Child Element" is a relative term to describe
      the "EBML Elements" immediately contained within a "Master
      Element".

   "Parent Element":  A relative term to describe the "Master Element"
      that contains a specified element.  For any specified "EBML
      Element" that is not at "Root Level", the "Parent Element" refers
      to the "Master Element" in which that "EBML Element" is directly
      contained.

   "Descendant Element":  A relative term to describe any "EBML
      Elements" contained within a "Master Element", including any of
      the "Child Elements" of its "Child Elements", and so on.

   "Void Element":  An "Element" used to overwrite data or reserve space
      within a "Master Element" for later use.

   "Element Name":  The human-readable name of the "EBML Element".

   "Element Path":  The hierarchy of "Parent Element" where the "EBML
      Element" is expected to be found in the "EBML Body".

   "Empty Element":  An "EBML Element" that has an "Element Data Size"
      with all "VINT_DATA" bits set to zero, which indicates that the
      "Element Data" of the "Element" is zero octets in length.

3.  Structure

   EBML uses a system of Elements to compose an EBML Document.  EBML
   Elements incorporate three parts: an Element ID, an Element Data
   Size, and Element Data.  The Element Data, which is described by the
   Element ID, includes either binary data, one or more other EBML
   Elements, or both.

4.  Variable-Size Integer

   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
   MUST 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.

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".

      +==============+=============+===============================+
      | Octet Length | Usable Bits | Representation                |
      +==============+=============+===============================+
      | 1            | 7           | 1xxx xxxx                     |
      +--------------+-------------+-------------------------------+
      | 2            | 14          | 01xx xxxx xxxx xxxx           |
      +--------------+-------------+-------------------------------+
      | 3            | 21          | 001x xxxx xxxx xxxx xxxx xxxx |
      +--------------+-------------+-------------------------------+
      | 4            | 28          | 0001 xxxx xxxx xxxx xxxx xxxx |
      |              |             | xxxx xxxx                     |
      +--------------+-------------+-------------------------------+
      | 5            | 35          | 0000 1xxx xxxx xxxx xxxx xxxx |
      |              |             | xxxx xxxx xxxx xxxx           |
      +--------------+-------------+-------------------------------+

               Table 1: VINT examples depicting usable bits

   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.

   +=========+==============+=====================+====================+
   | Integer | Octet        |   As Represented in |  As Represented in |
   |         | Length       |       VINT (binary) | VINT (hexadecimal) |
   +=========+==============+=====================+====================+
   | 2       | 1            |           1000 0010 |               0x82 |
   +---------+--------------+---------------------+--------------------+
   | 2       | 2            | 0100 0000 0000 0010 |             0x4002 |
   +---------+--------------+---------------------+--------------------+
   | 2       | 3            | 0010 0000 0000 0000 |           0x200002 |
   |         |              |           0000 0010 |                    |
   +---------+--------------+---------------------+--------------------+
   | 2       | 4            | 0001 0000 0000 0000 |         0x10000002 |
   |         |              | 0000 0000 0000 0010 |                    |
   +---------+--------------+---------------------+--------------------+

      Table 2: VINT examples depicting the same integer value rendered
                         at different VINT lengths

5.  Element ID

   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 MAY be used if the EBMLMaxIDLength Element of the
   EBML Header is set to a value greater than four (see Section 11.2.4).
   The bits of the VINT_DATA component of the Element ID MUST NOT be all
   "0" values or all "1" values.  The VINT_DATA component of the Element
   ID MUST be encoded at the shortest valid length.  For example, an
   Element ID with binary encoding of "1011 1111" is valid, whereas an
   Element ID with binary encoding of "0100 0000 0011 1111" stores a
   semantically equal VINT_DATA but is invalid, because a shorter VINT
   encoding is possible.  Additionally, an Element ID with binary
   encoding of "1111 1111" is invalid, since the VINT_DATA section is
   set to all one values, whereas an Element ID with binary encoding of
   "0100 0000 0111 1111" stores a semantically equal VINT_DATA and is
   the shortest-possible VINT encoding.

   Table 3 details these specific examples further:

    +============+=============+================+====================+
    | VINT_WIDTH | VINT_MARKER |      VINT_DATA | Element ID Status  |
    +============+=============+================+====================+
    |            |           1 |        0000000 | Invalid: VINT_DATA |
    |            |             |                | MUST NOT be set to |
    |            |             |                | all 0              |
    +------------+-------------+----------------+--------------------+
    |          0 |           1 | 00000000000000 | Invalid: VINT_DATA |
    |            |             |                | MUST NOT be set to |
    |            |             |                | all 0              |
    +------------+-------------+----------------+--------------------+
    |            |           1 |        0000001 | Valid              |
    +------------+-------------+----------------+--------------------+
    |          0 |           1 | 00000000000001 | Invalid: A shorter |
    |            |             |                | VINT_DATA encoding |
    |            |             |                | is available.      |
    +------------+-------------+----------------+--------------------+
    |            |           1 |        0111111 | Valid              |
    +------------+-------------+----------------+--------------------+
    |          0 |           1 | 00000000111111 | Invalid: A shorter |
    |            |             |                | VINT_DATA encoding |
    |            |             |                | is available.      |
    +------------+-------------+----------------+--------------------+
    |            |           1 |        1111111 | Invalid: VINT_DATA |
    |            |             |                | MUST NOT be set to |
    |            |             |                | all 1              |
    +------------+-------------+----------------+--------------------+
    |          0 |           1 | 00000001111111 | Valid              |
    +------------+-------------+----------------+--------------------+

            Table 3: Examples of valid and invalid Element IDs

   The range and count of possible Element IDs are determined by their
   octet length.  Examples of this are provided in Table 4.

      +=========================+================+=================+
      | Element ID Octet Length | Range of Valid | Number of Valid |
      |                         |  Element IDs   |     Element IDs |
      +=========================+================+=================+
      |            1            |  0x81 - 0xFE   |             126 |
      +-------------------------+----------------+-----------------+
      |            2            |    0x407F -    |          16,256 |
      |                         |     0x7FFE     |                 |
      +-------------------------+----------------+-----------------+
      |            3            |   0x203FFF -   |       2,080,768 |
      |                         |    0x3FFFFE    |                 |
      +-------------------------+----------------+-----------------+
      |            4            |  0x101FFFFF -  |     268,338,304 |
      |                         |   0x1FFFFFFE   |                 |
      +-------------------------+----------------+-----------------+

         Table 4: Examples of count and range for Element IDs at
                          various octet lengths

6.  Element Data Size

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":

     +==================================+============================+
     | EBML Path of new element         | Status                     |
     +==================================+============================+
     | \root\level1\level2              | Ends the Unknown-Sized     |
     |                                  | Element, as it is a new    |
     |                                  | Parent Element             |
     +----------------------------------+----------------------------+
     | \root\level1                     | Ends the Unknown-Sized     |
     |                                  | Element, as it is a new    |
     |                                  | Parent Element             |
     +----------------------------------+----------------------------+
     | \root                            | Ends the Unknown-Sized     |
     |                                  | Element, as it is a new    |
     |                                  | Root Element               |
     +----------------------------------+----------------------------+
     | \root2                           | Ends the Unknown-Sized     |
     |                                  | Element, as it is a new    |
     |                                  | Root Element               |
     +----------------------------------+----------------------------+
     | \root\level1\level2\other        | Ends the Unknown-Sized     |
     |                                  | Element, as they share the |
     |                                  | same parent                |
     +----------------------------------+----------------------------+
     | \root\level1\level2\elt          | Ends the Unknown-Sized     |
     |                                  | Element, as they share the |
     |                                  | same parent                |
     +----------------------------------+----------------------------+
     | \root\level1\level2\elt\inside   | Doesn't end the Unknown-   |
     |                                  | Sized Element; it's a      |
     |                                  | child of "elt"             |
     +----------------------------------+----------------------------+
     | \root\level1\level2\elt\<global> | Global Element is valid;   |
     |                                  | it's a child of "elt"      |
     +----------------------------------+----------------------------+
     | \root\level1\level2\<global>     | Global Element cannot be   |
     |                                  | interpreted with this      |
     |                                  | path; while parsing "elt", |
     |                                  | a Global Element can only  |
     |                                  | be a child of "elt"        |
     +----------------------------------+----------------------------+

        Table 5: Examples of determining the end of an Unknown-Sized
                                  Element

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.

                  +==============+======================+
                  | Octet Length | Possible Value Range |
                  +==============+======================+
                  | 1            | 0 to 2^(7) - 2       |
                  +--------------+----------------------+
                  | 2            | 0 to 2^(14) - 2      |
                  +--------------+----------------------+
                  | 3            | 0 to 2^(21) - 2      |
                  +--------------+----------------------+
                  | 4            | 0 to 2^(28) - 2      |
                  +--------------+----------------------+
                  | 5            | 0 to 2^(35) - 2      |
                  +--------------+----------------------+
                  | 6            | 0 to 2^(42) - 2      |
                  +--------------+----------------------+
                  | 7            | 0 to 2^(49) - 2      |
                  +--------------+----------------------+
                  | 8            | 0 to 2^(56) - 2      |
                  +--------------+----------------------+

                     Table 6: Possible range of values
                      that can be stored in VINTs, by
                                octet length

   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).

    +============+=============+=======================+==============+
    | VINT_WIDTH | VINT_MARKER |             VINT_DATA | Element Data |
    |            |             |                       | Size Status  |
    +============+=============+=======================+==============+
    |            |           1 |               1111111 | Reserved     |
    |            |             |                       | (meaning     |
    |            |             |                       | Unknown)     |
    +------------+-------------+-----------------------+--------------+
    |          0 |           1 |        00000001111111 | Valid        |
    |            |             |                       | (meaning 127 |
    |            |             |                       | octets)      |
    +------------+-------------+-----------------------+--------------+
    |         00 |           1 | 000000000000001111111 | Valid        |
    |            |             |                       | (meaning 127 |
    |            |             |                       | octets)      |
    +------------+-------------+-----------------------+--------------+
    |          0 |           1 |        11111111111111 | Reserved     |
    |            |             |                       | (meaning     |
    |            |             |                       | Unknown)     |
    +------------+-------------+-----------------------+--------------+
    |         00 |           1 | 000000011111111111111 | Valid        |
    |            |             |                       | (16,383      |
    |            |             |                       | octets)      |
    +------------+-------------+-----------------------+--------------+

        Table 7: Demonstration of VINT_DATA reservation for VINTs of
                                unknown size

7.  EBML Element Types

   EBML Elements are defined by an EBML Schema (see Section 11.1), which
   MUST declare one of the following EBML Element Types for each EBML
   Element.  An EBML Element Type defines a concept of storing data
   within an EBML Element that describes such characteristics as length,
   endianness, and definition.

   EBML Elements that are defined as a Signed Integer Element, Unsigned
   Integer Element, Float Element, or Date Element use big-endian
   storage.

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.  EBML Document

   An EBML Document is composed of only two components, an EBML Header
   and an EBML Body.  An EBML Document MUST start with an EBML Header
   that declares significant characteristics of the entire EBML Body.
   An EBML Document consists of EBML Elements and MUST NOT contain any
   data that is not part of an EBML Element.

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.

9.  EBML Stream

   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.

10.  EBML Versioning

   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.

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.  Elements semantics

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.

11.1.1.  EBML Schema Example

   <?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>

11.1.2.  "<EBMLSchema>" Element

   Within an EBML Schema, the XPath [XPath] of the "<EBMLSchema>"
   element is "/EBMLSchema".

   When used as an XML Document, the EBML Schema MUST use "<EBMLSchema>"
   as the top-level element.  The "<EBMLSchema>" element can contain
   "<element>" subelements.

11.1.3.  "<EBMLSchema>" Namespace

   The namespace URI for elements of the EBML Schema is a URN as defined
   by [RFC8141] that uses the namespace identifier 'ietf' defined by
   [RFC2648] and extended by [RFC3688].  This URN is
   "urn:ietf:rfc:8794".

11.1.4.  "<EBMLSchema>" Attributes

   Within an EBML Schema, the "<EBMLSchema>" element uses the following
   attributes to define an EBML Element:

11.1.4.1.  docType

   Within an EBML Schema, the XPath of the "@docType" attribute is
   "/EBMLSchema/@docType".

   The docType lists the official name of the EBML Document Type that is
   defined by the EBML Schema; for example, "<EBMLSchema
   docType="matroska">".

   The "docType" attribute is REQUIRED within the "<EBMLSchema>"
   Element.

11.1.4.2.  version

   Within an EBML Schema, the XPath of the "@version" attribute is
   "/EBMLSchema/@version".

   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 "minver" and
   "maxver" attributes of "<element>".

   The "version" attribute is REQUIRED within the "<EBMLSchema>"
   Element.

11.1.4.3.  ebml

   Within an EBML Schema, the XPath of the "@ebml" attribute is
   "/EBMLSchema/@ebml".

   The "ebml" attribute is a positive integer that specifies the version
   of the EBML Header (see Section 11.2.2) used by the EBML Schema.  If
   the attribute is omitted, the EBML Header version is 1.

11.1.5.  "<element>" Element

   Within an EBML Schema, the XPath of the "<element>" element is
   "/EBMLSchema/element".

   Each "<element>" defines one EBML Element through the use of several
   attributes that are defined in Section 11.1.6.  EBML Schemas MAY
   contain additional attributes to extend the semantics but MUST NOT
   conflict with the definitions of the "<element>" attributes defined
   within this document.

   The "<element>" nodes contain a description of the meaning and use of
   the EBML Element stored within one or more "<documentation>"
   subelements, followed by optional "<implementation_note>"
   subelements, followed by zero or one "<restriction>" subelement,
   followed by optional "<extension>" subelements.  All "<element>"
   nodes MUST be subelements of the "<EBMLSchema>".

11.1.6.  "<element>" Attributes

   Within an EBML Schema, the "<element>" uses the following attributes
   to define an EBML Element:

11.1.6.1.  name

   Within an EBML Schema, the XPath of the "@name" attribute is
   "/EBMLSchema/element/@name".

   The name provides the human-readable name of the EBML Element.  The
   value of the name MUST be in the form of characters "A" to "Z", "a"
   to "z", "0" to "9", "-", and ".".  The first character of the name
   MUST be in the form of an "A" to "Z", "a" to "z", or "0" to "9"
   character.

   The "name" attribute is REQUIRED.

11.1.6.2.  path

   Within an EBML Schema, the XPath of the "@path" attribute is
   "/EBMLSchema/element/@path".

   The path defines the allowed storage locations of the EBML Element
   within an EBML Document.  This path MUST be defined with the full
   hierarchy of EBML Elements separated with a "\".  The top EBML
   Element in the path hierarchy is the first in the value.  The syntax
   of the "path" attribute is defined using this Augmented Backus-Naur
   Form (ABNF) [RFC5234] with the case-sensitive update [RFC7405]
   notation:

   The "path" attribute is REQUIRED.

   EBMLFullPath           = 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

   The "*", "(", and ")" symbols are interpreted as defined in
   [RFC5234].

   The EBMLAtomName of the EBMLElement part MUST be equal to the "@name"
   attribute of the EBML Schema.  If the EBMLElement part contains an
   IsRecursive part, the EBML Element can occur within itself
   recursively (see Section 11.1.6.11).

   The starting PathDelimiter of EBMLParentPath corresponds to the root
   of the EBML Document.

   The "@path" value MUST be unique within the EBML Schema.  The "@id"
   value corresponding to this "@path" MUST NOT be defined for use
   within another EBML Element with the same EBMLParentPath as this
   "@path".

   A path with a GlobalPlaceholder as the EBMLLastParent defines a
   Global Element; see Section 11.3.  If the element has no
   EBMLLastParent part, or the EBMLLastParent part is not a
   GlobalPlaceholder, then the Element is not a Global Element.

   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.

   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 MUST NOT 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 MUST be bigger than, or equal to,
   PathMinOccurrence.

   For example, in "\a\(0-1\)global", the Element path "\a\x\global"
   corresponds to an EBMLPathAtom occurrence of 1.  The Element
   "\a\x\y\global" corresponds to an EBMLPathAtom occurrence of 2, etc.
   In those cases, "\a\x" or "\a\x\y" MUST be valid paths to be able to
   contain the element "global".

   Consider another EBML Path, "\a\(1-\)global".  There has to be at
   least one EBMLPathAtom between the "\a\" part and "global".  So the
   "global" EBML Element cannot be found inside the "\a" EBML Element,
   as it means the resulting path "\a\global" has no EBMLPathAtom
   between the "\a\" and "global".  However, the "global" EBML Element
   can be found inside the "\a\b" EBML Element, because the resulting
   path, "\a\b\global", has one EBMLPathAtom between the "\a\" and
   "global".  Alternatively, it can be found inside the "\a\b\c" EBML
   Element (two EBMLPathAtom), or inside the "\a\b\c\d" EBML Element
   (three EBMLPathAtom), etc.

   Consider another EBML Path, "\a\(0-1\)global".  There has to be at
   most one EBMLPathAtom between the "\a\" part and "global".  So the
   "global" EBML Element can be found inside either the "\a" EBML
   Element (0 EBMLPathAtom replacing GlobalPlaceholder) or the "\a\b"
   EBML Element (one replacement EBMLPathAtom).  But it cannot be found
   inside the "\a\b\c" EBML Element, because the resulting path,
   "\a\b\c\global", has two EBMLPathAtom between "\a\" and "global".

11.1.6.3.  id

   Within an EBML Schema, the XPath of the "@id" attribute is
   "/EBMLSchema/element/@id".

   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 SHOULD be at least 4 octets in length.
   Element IDs defined for use at Root Level or directly under the Root
   Level MAY use shorter octet lengths to facilitate padding and
   optimize edits to EBML Documents; for instance, the Void Element uses
   an Element ID with a length of one octet to allow its usage in more
   writing and editing scenarios.

   The Element ID of any Element found within an EBML Document MUST only
   match a single "@path" value of its corresponding EBML Schema, but a
   separate instance of that Element ID value defined by the EBML Schema
   MAY occur within a different "@path".  If more than one Element is
   defined to use the same "@id" value, then the "@path" values of those
   Elements MUST NOT share the same EBMLParentPath.  Elements MUST NOT
   be defined to use the same "@id" value if one of their common Parent
   Elements could be an Unknown-Sized Element.

   The "id" attribute is REQUIRED.

11.1.6.4.  minOccurs

   Within an EBML Schema, the XPath of the "@minOccurs" attribute is
   "/EBMLSchema/element/@minOccurs".

   "minOccurs" is a nonnegative integer expressing the minimum permitted
   number of occurrences of this EBML Element within its Parent Element.

   Each instance of the Parent Element MUST contain at least this many
   instances of this EBML Element.  If the EBML Element has an empty
   EBMLParentPath, then "minOccurs" refers to constraints on the
   occurrence of the EBML Element within the EBML Document.  EBML
   Elements with "minOccurs" set to "1" that also have a default value
   (see Section 11.1.6.8) declared are not REQUIRED to be stored but are
   REQUIRED to be interpreted; see Section 11.1.19.

   An EBML Element defined with a "minOccurs" value greater than zero is
   called a Mandatory EBML Element.

   The "minOccurs" attribute is OPTIONAL.  If the "minOccurs" attribute
   is not present, then that EBML Element has a "minOccurs" value of 0.

   The semantic meaning of "minOccurs" within an EBML Schema is
   analogous to the meaning of "minOccurs" within an XML Schema.

11.1.6.5.  maxOccurs

   Within an EBML Schema, the XPath of the "@maxOccurs" attribute is
   "/EBMLSchema/element/@maxOccurs".

   "maxOccurs" is a nonnegative integer expressing the maximum permitted
   number of occurrences of this EBML Element within its Parent Element.

   Each instance of the Parent Element MUST contain at most this many
   instances of this EBML Element, including the unwritten mandatory
   element with a default value; see Section 11.1.19.  If the EBML
   Element has an empty EBMLParentPath, then "maxOccurs" refers to
   constraints on the occurrence of the EBML Element within the EBML
   Document.

   The "maxOccurs" attribute is OPTIONAL.  If the "maxOccurs" 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.

   The semantic meaning of "maxOccurs" within an EBML Schema is
   analogous to the meaning of "maxOccurs" within an XML Schema; when it
   is not present, it's similar to xml:maxOccurs="unbounded" in an XML
   Schema.

11.1.6.6.  range

   Within an EBML Schema, the XPath of the "@range" attribute is
   "/EBMLSchema/element/@range".

   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 MUST be within the defined range.  See
   Section 11.1.6.6.1 for rules applied to expression of range values.

   The "range" attribute is OPTIONAL.  If the "range" attribute is not
   present, then any value legal for the "type" attribute is valid.

11.1.6.6.1.  Expression of range

   The "range" attribute MUST 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 MUST NOT convey meaning.

   To set a fixed value for the range, the value is used as the
   attribute value.  For example, "1234" means the EBML element always
   has the value 1234.  The value can be prefixed with "not" to indicate
   that the fixed value MUST NOT be used for that Element.  For example,
   "not 1234" means the Element can use all values of its type except
   1234.

   The ">" sign is used for an exclusive lower boundary, and the ">="
   sign is used for an inclusive lower boundary.  For example, ">3"
   means the Element value MUST be greater than 3, and ">=0x1p+0" means
   the Element value MUST be greater than or equal to the floating value
   1.0; see Section 11.1.18.

   The "<" sign is used for an exclusive upper boundary, and the "<="
   sign is used for an inclusive upper boundary.  For example, "<-2"
   means the Element value MUST be less than -2, and "<=10" means the
   Element value MUST be less than or equal to 10.

   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, ">3,<= 20" means
   the Element value MUST be greater than 3 and less than or equal to
   20.

   A special form of lower and upper bounds using the "-" separator is
   possible, meaning the Element value MUST be greater than, or equal
   to, the first value and MUST be less than or equal to the second
   value.  For example, "1-10" is equivalent to ">=1,<=10".  If the
   upper boundary is negative, the "range" attribute MUST only use the
   latter form.

11.1.6.7.  length

   Within an EBML Schema, the XPath of the "@length" attribute is
   "/EBMLSchema/element/@length".

   The "length" 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 MUST be expressed as
   either a nonnegative integer or a range (see Section 11.1.6.6.1) that
   consists of only nonnegative integers and valid operators.

   The "length" attribute is OPTIONAL.  If the "length" 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.

11.1.6.8.  default

   Within an EBML Schema, the XPath of the "@default" attribute is
   "/EBMLSchema/element/@default".

   If an Element is mandatory (has a "minOccurs" value greater than
   zero) but not written within its Parent Element or stored as an Empty
   Element, then the EBML Reader of the EBML Document MUST semantically
   interpret the EBML Element as present with this specified default
   value for the EBML Element.  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 MUST NOT declare a default value.
   EBML Elements with a "minOccurs" value greater than 1 MUST NOT
   declare a default value.

   The default attribute is OPTIONAL.

11.1.6.9.  type

   Within an EBML Schema, the XPath of the "@type" attribute is
   "/EBMLSchema/element/@type".

   The type MUST be set to one of the following values: "integer"
   (signed integer), "uinteger" (unsigned integer), "float", "string",
   "date", "utf-8", "master", or "binary".  The content of each type is
   defined in Section 7.

   The "type" attribute is REQUIRED.

11.1.6.10.  unknownsizeallowed

   Within an EBML Schema, the XPath of the "@unknownsizeallowed"
   attribute is "/EBMLSchema/element/@unknownsizeallowed".

   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 MUST NOT set "unknownsizeallowed" to true.  An EBML Element
   that is defined with an "unknownsizeallowed" attribute set to 1 MUST
   also have the "unknownsizeallowed" attribute of its Parent Element
   set to 1.

   An EBML Element with the "unknownsizeallowed" attribute set to 1 MUST
   NOT have its "recursive" attribute set to 1.

   The "unknownsizeallowed" attribute is OPTIONAL.  If the
   "unknownsizeallowed" attribute is not used, then that EBML Element is
   not allowed to use an unknown Element Data Size.

11.1.6.11.  recursive

   Within an EBML Schema, the XPath of the "@recursive" attribute is
   "/EBMLSchema/element/@recursive".

   This attribute is a boolean to express whether an EBML Element is
   permitted to be stored recursively.  If it is allowed, the EBML
   Element MAY be stored within another EBML Element that has the same
   Element ID, which itself can be stored in an EBML Element that has
   the same Element ID, and so on.  EBML Elements that are not Master
   Elements MUST NOT set recursive to true.

   If the EBMLElement part of the "@path" contains an IsRecursive part,
   then the "recursive" value MUST be true; otherwise, it MUST be false.

   An EBML Element with the "recursive" attribute set to 1 MUST NOT have
   its "unknownsizeallowed" attribute set to 1.

   The "recursive" attribute is OPTIONAL.  If the "recursive" attribute
   is not present, then the EBML Element MUST NOT be used recursively.

11.1.6.12.  recurring

   Within an EBML Schema, the XPath of the "@recurring" attribute is
   "/EBMLSchema/element/@recurring".

   This attribute is a boolean to express whether or not an EBML Element
   is defined as an Identically Recurring Element; see Section 11.1.17.

   The "recurring" attribute is OPTIONAL.  If the "recurring" attribute
   is not present, then the EBML Element is not an Identically Recurring
   Element.

11.1.6.13.  minver

   Within an EBML Schema, the XPath of the "@minver" attribute is
   "/EBMLSchema/element/@minver".

   The "minver" (minimum version) attribute stores a nonnegative integer
   that represents the first version of the docType to support the EBML
   Element.

   The "minver" attribute is OPTIONAL.  If the "minver" attribute is not
   present, then the EBML Element has a minimum version of "1".

11.1.6.14.  maxver

   Within an EBML Schema, the XPath of the "@maxver" attribute is
   "/EBMLSchema/element/@maxver".

   The "maxver" (maximum version) attribute stores a nonnegative integer
   that represents the last or most recent version of the docType to
   support the element. "maxver" MUST be greater than or equal to
   "minver".

   The "maxver" attribute is OPTIONAL.  If the "maxver" attribute is not
   present, then the EBML Element has a maximum version equal to the
   value stored in the "version" attribute of "<EBMLSchema>".

11.1.7.  "<documentation>" Element

   Within an EBML Schema, the XPaths of the "<documentation>" elements
   are "/EBMLSchema/element/documentation" and
   "/EBMLSchema/element/restriction/enum/documentation".

   The "<documentation>" element provides additional information about
   EBML Elements or enumeration values.  Within the "<documentation>"
   element, the following XHTML [XHTML] elements MAY be used: "<a>",
   "<br>", and "<strong>".

11.1.8.  "<documentation>" Attributes

11.1.8.1.  lang

   Within an EBML Schema, the XPath of the "@lang" attribute is
   "/EBMLSchema/element/documentation/@lang".

   The "lang" attribute is set to the value from [RFC5646] of the
   language of the element's documentation.

   The "lang" attribute is OPTIONAL.

11.1.8.2.  purpose

   Within an EBML Schema, the XPath of the "@purpose" attribute is
   "/EBMLSchema/element/documentation/@purpose".

   A "purpose" attribute distinguishes the meaning of the documentation.
   Values for the "<documentation>" subelement's "purpose" attribute
   MUST include one of the values listed in Table 8.

     +============+=================================================+
     | value of   | definition                                      |
     | "purpose"  |                                                 |
     | attribute  |                                                 |
     +============+=================================================+
     | definition | A "definition" is recommended for every defined |
     |            | EBML Element.  This documentation explains the  |
     |            | semantic meaning of the EBML Element.           |
     +------------+-------------------------------------------------+
     | rationale  | An explanation about the reason or catalyst for |
     |            | the definition of the Element.                  |
     +------------+-------------------------------------------------+
     | usage      | Recommended practices or guidelines for both    |
     | notes      | reading, writing, or interpreting the Element.  |
     +------------+-------------------------------------------------+
     | references | Informational references to support the         |
     |            | contextualization and understanding of the      |
     |            | value of the Element.                           |
     +------------+-------------------------------------------------+

           Table 8: Definitions of the permitted values for the
             "purpose" attribute of the documentation Element

   The "purpose" attribute is REQUIRED.

11.1.9.  "<implementation_note>" Element

   Within an EBML Schema, the XPath of the "<implementation_note>"
   element is "/EBMLSchema/element/implementation_note".

   In some cases within an EBML Document Type, the attributes of the
   "<element>" 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 "<implementation_note>"
   elements can be used to store that information.  The
   "<implementation_note>" refers to a specific attribute of the parent
   "<element>" as expressed by the "note_attribute" attribute (see
   Section 11.1.10.1).

11.1.10.  "<implementation_note>" Attributes

11.1.10.1.  note_attribute

   Within an EBML Schema, the XPath of the "@note_attribute" attribute
   is "/EBMLSchema/element/implementation_note/@note_attribute".

   The "note_attribute" attribute references which of the attributes of
   the "<element>" the "<implementation_note>" relates to.  The
   "note_attribute" attribute MUST be set to one of the following values
   (corresponding to that attribute of the parent "<element>"):
   "minOccurs", "maxOccurs", "range", "length", "default", "minver", or
   "maxver".  The "<implementation_note>" SHALL supersede the parent
   "<element>"'s attribute that is named in the "note_attribute"
   attribute.  An "<element>" SHALL NOT have more than one
   "<implementation_note>" of the same "note_attribute".

   The "note_attribute" attribute is REQUIRED.

11.1.10.2.  "<implementation_note>" Example

   The following fragment of an EBML Schema demonstrates how an
   "<implementation_note>" 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 "<implementation_note>" to say that the
   Currency Element is REQUIRED if the Cost Element is set, otherwise
   not.

   <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>

11.1.11.  "<restriction>" Element

   Within an EBML Schema, the XPath of the "<restriction>" element is
   "/EBMLSchema/element/restriction".

   The "<restriction>" element provides information about restrictions
   to the allowable values for the EBML Element, which are listed in
   "<enum>" elements.

11.1.12.  "<enum>" Element

   Within an EBML Schema, the XPath of the "<enum>" element is
   "/EBMLSchema/element/restriction/enum".

   The "<enum>" element stores a list of values allowed for storage in
   the EBML Element.  The values MUST match the type of the EBML Element
   (for example, "<enum value="Yes">" cannot be a valid value for an
   EBML Element that is defined as an unsigned integer).  An "<enum>"
   element MAY also store "<documentation>" elements to further describe
   the "<enum>".

11.1.13.  "<enum>" Attributes

11.1.13.1.  label

   Within an EBML Schema, the XPath of the "@label" attribute is
   "/EBMLSchema/element/restriction/enum/@label".

   The label provides a concise expression for human consumption that
   describes what the value of "<enum>" represents.

   The "label" attribute is OPTIONAL.

11.1.13.2.  value

   Within an EBML Schema, the XPath of the "@value" attribute is
   "/EBMLSchema/element/restriction/enum/@value".

   The value represents data that MAY be stored within the EBML Element.

   The "value" attribute is REQUIRED.

11.1.14.  "<extension>" Element

   Within an EBML Schema, the XPath of the "<extension>" element is
   "/EBMLSchema/element/extension".

   The "<extension>" element provides an unconstrained element to
   contain information about the associated EBML "<element>", which is
   undefined by this document but MAY be defined by the associated EBML
   Document Type.  The "<extension>" element MUST contain a "type"
   attribute and also MAY contain any other attribute or subelement as
   long as the EBML Schema remains as a well-formed XML Document.  All
   "<extension>" elements MUST be subelements of the "<element>".

11.1.15.  "<extension>" Attributes

11.1.15.1.  type

   Within an EBML Schema, the XPath of the "@type" attribute is
   "/EBMLSchema/element/extension/@type".

   The "type" attribute should reference a name or identifier of the
   project or authority associated with the contents of the
   "<extension>" element.

   The "type" attribute is REQUIRED.

11.1.16.  XML Schema for EBML Schema

   The following provides an XML Schema [XML-SCHEMA] for facilitating
   verification of an EBML Schema described in Section 11.1.

   <?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>

11.1.17.  Identically Recurring Elements

   An Identically Recurring Element is an EBML Element that MAY occur
   within its Parent Element more than once, but each recurrence of it
   within that Parent Element MUST be identical both in storage and
   semantics.  Identically Recurring Elements are permitted to be stored
   multiple times within the same Parent Element in order to increase
   data resilience and optimize the use of EBML in transmission.  For
   instance, a pertinent Top-Level Element could be periodically resent
   within a datastream so that an EBML Reader that starts reading the
   stream from the middle could better interpret the contents.
   Identically Recurring Elements SHOULD include a CRC-32 Element as a
   Child Element; this is especially recommended when EBML is used for
   long-term storage or transmission.  If a Parent Element contains more
   than one copy of an Identically Recurring Element 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.

11.1.18.  Textual expression of floats

   When a float value is represented textually in an EBML Schema, such
   as within a default or range value, the float values MUST be
   expressed as Hexadecimal Floating-Point Constants as defined in the
   C11 standard [ISO9899] (see Section 6.4.4.2 on Floating Constants).
   Table 9 provides examples of expressions of float ranges.

      +===================+=========================================+
      | as decimal        | as Hexadecimal Floating-Point Constants |
      +===================+=========================================+
      | 0.0               | 0x0p+1                                  |
      +-------------------+-----------------------------------------+
      | 0.0-1.0           | 0x0p+1-0x1p+0                           |
      +-------------------+-----------------------------------------+
      | 1.0-256.0         | 0x1p+0-0x1p+8                           |
      +-------------------+-----------------------------------------+
      | 0.857421875       | 0x1.b7p-1                               |
      +-------------------+-----------------------------------------+
      | -1.0--0.857421875 | -0x1p+0--0x1.b7p-1                      |
      +-------------------+-----------------------------------------+

          Table 9: Example of Floating-Point values and ranges as
              decimal and Hexadecimal Floating-Point Constants

   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.

11.1.19.  Note on the use of default attributes to define Mandatory EBML
          Elements

   If a Mandatory EBML Element has a default value declared by an EBML
   Schema and the value of the EBML Element is equal to the declared
   default value, then that EBML Element is not required to be present
   within the EBML Document if its Parent Element is present.  In this
   case, the default value of the Mandatory EBML Element MUST be read by
   the EBML Reader, although the EBML Element is not present within its
   Parent Element.

   If a Mandatory EBML Element has no default value declared by an EBML
   Schema and its Parent Element is present, then the EBML Element MUST
   be present, as well.  If a Mandatory EBML Element has a default value
   declared by an EBML Schema, and its Parent Element is present, and
   the value of the EBML Element is NOT equal to the declared default
   value, then the EBML Element MUST be present.

   Table 10 clarifies whether a Mandatory EBML Element MUST 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.

   +=================+=============+===============+==================+
   |  Is the default |    Is the   | Is the Parent | Then is storing  |
   | value declared? | value equal |    Element    | the EBML Element |
   |                 | to default? |    present?   |    REQUIRED?     |
   +=================+=============+===============+==================+
   |       Yes       |     Yes     |      Yes      |        No        |
   +-----------------+-------------+---------------+------------------+
   |       Yes       |     Yes     |       No      |        No        |
   +-----------------+-------------+---------------+------------------+
   |       Yes       |      No     |      Yes      |       Yes        |
   +-----------------+-------------+---------------+------------------+
   |       Yes       |      No     |       No      |        No        |
   +-----------------+-------------+---------------+------------------+
   |        No       |     n/a     |      Yes      |       Yes        |
   +-----------------+-------------+---------------+------------------+
   |        No       |     n/a     |       No      |        No        |
   +-----------------+-------------+---------------+------------------+

     Table 10: Demonstration of the conditional requirements of VINT
                                 Storage

11.2.  EBML Header Elements

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

11.2.1.  EBML Element

   name:  EBML

   path:  "\EBML"

   id:  0x1A45DFA3

   minOccurs:  1

   maxOccurs:  1

   type:  Master Element

   description:  Set the EBML characteristics of the data to follow.
      Each EBML Document has to start with this.

11.2.2.  EBMLVersion Element

   name:  EBMLVersion

   path:  "\EBML\EBMLVersion"

   id:  0x4286

   minOccurs:  1

   maxOccurs:  1

   range:  not 0

   default:  1

   type:  Unsigned Integer

   description:  The version of EBML specifications used to create the
      EBML Document.  The version of EBML defined in this document is 1,
      so EBMLVersion SHOULD be 1.

11.2.3.  EBMLReadVersion Element

   name:  EBMLReadVersion

   path:  "\EBML\EBMLReadVersion"

   id:  0x42F7

   minOccurs:  1

   maxOccurs:  1

   range:  1

   default:  1

   type:  Unsigned Integer

   description:  The minimum EBML version an EBML Reader has to support
      to read this EBML Document.  The EBMLReadVersion Element MUST be
      less than or equal to EBMLVersion.

11.2.4.  EBMLMaxIDLength Element

   name:  EBMLMaxIDLength

   path:  "\EBML\EBMLMaxIDLength"

   id:  0x42F2

   minOccurs:  1

   maxOccurs:  1

   range:  >=4

   default:  4

   type:  Unsigned Integer

   description:  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
      RECOMMENDED, though larger values are allowed.

11.2.5.  EBMLMaxSizeLength Element

   name:  EBMLMaxSizeLength

   path:  "\EBML\EBMLMaxSizeLength"

   id:  0x42F3

   minOccurs:  1

   maxOccurs:  1

   range:  not 0

   default:  8

   type:  Unsigned Integer

   description:  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 "length" of all Element
      Data Size expressions within the EBML Body and not an upper bound
      for the "value" 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.

11.2.6.  DocType Element

   name:  DocType

   path:  "\EBML\DocType"

   id:  0x4282

   minOccurs:  1

   maxOccurs:  1

   length:  >0

   type:  String

   description:  A string that describes and identifies the content of
      the EBML Body that follows this EBML Header.

11.2.7.  DocTypeVersion Element

   name:  DocTypeVersion

   path:  "\EBML\DocTypeVersion"

   id:  0x4287

   minOccurs:  1

   maxOccurs:  1

   range:  not 0

   default:  1

   type:  Unsigned Integer

   description:  The version of DocType interpreter used to create the
      EBML Document.

11.2.8.  DocTypeReadVersion Element

   name:  DocTypeReadVersion

   path:  "\EBML\DocTypeReadVersion"

   id:  0x4285

   minOccurs:  1

   maxOccurs:  1

   range:  not 0

   default:  1

   type:  Unsigned Integer

   description:  The minimum DocType version an EBML Reader has to
      support to read this EBML Document.  The value of the
      DocTypeReadVersion Element MUST be less than or equal to the value
      of the DocTypeVersion Element.

11.2.9.  DocTypeExtension Element

   name:  DocTypeExtension

   path:  "\EBML\DocTypeExtension"

   id:  0x4281

   minOccurs:  0

   type:  Master Element

   description:  A DocTypeExtension adds extra Elements to the main
      DocType+DocTypeVersion tuple it's attached to.  An EBML Reader MAY
      know these extra Elements and how to use them.  A DocTypeExtension
      MAY 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.

11.2.10.  DocTypeExtensionName Element

   name:  DocTypeExtensionName

   path:  "\EBML\DocTypeExtension\DocTypeExtensionName"

   id:  0x4283

   minOccurs:  1

   maxOccurs:  1

   length:  >0

   type:  String

   description:  The name of the DocTypeExtension to differentiate it
      from other DocTypeExtensions of the same DocType+DocTypeVersion
      tuple.  A DocTypeExtensionName value MUST be unique within the
      EBML Header.

11.2.11.  DocTypeExtensionVersion Element

   name:  DocTypeExtensionVersion

   path:  "\EBML\DocTypeExtension\DocTypeExtensionVersion"

   id:  0x4284

   minOccurs:  1

   maxOccurs:  1

   range:  not 0

   type:  Unsigned Integer

   description:  The version of the DocTypeExtension.  Different
      DocTypeExtensionVersion values of the same DocType +
      DocTypeVersion + DocTypeExtensionName tuple MAY contain completely
      different sets of extra Elements.  An EBML Reader MAY support
      multiple versions of the same tuple, only one version of the
      tuple, or not support the tuple at all.

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.

11.3.1.  CRC-32 Element

   name:  CRC-32

   path:  "\(1-\)CRC-32"

   id:  0xBF

   minOccurs:  0

   maxOccurs:  1

   length:  4

   type:  Binary

   description:  The CRC-32 Element contains a 32-bit Cyclic Redundancy
      Check value of all the Element Data of the Parent Element as
      stored except for the CRC-32 Element itself.  When the CRC-32
      Element is present, the CRC-32 Element MUST be the first ordered
      EBML Element within its Parent Element for easier reading.  All
      Top-Level Elements of an EBML Document that are Master Elements
      SHOULD include a CRC-32 Element as a Child Element.  The CRC in
      use is the IEEE-CRC-32 algorithm as used in the [ISO3309] standard
      and in Section 8.1.1.6.2 of [ITU.V42], with initial value of
      0xFFFFFFFF.  The CRC value MUST be computed on a little-endian
      bytestream and MUST use little-endian storage.

11.3.2.  Void Element

   name:  Void

   path:  "\(-\)Void"

   id:  0xEC

   minOccurs:  0

   type:  Binary

   description:  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.

12.  Considerations for Reading EBML Data

   The following scenarios describe events to consider when reading EBML
   Documents, as well as the recommended design of an EBML Reader.

   If a Master Element contains a CRC-32 Element that doesn't validate,
   then the EBML Reader MAY ignore all contained data except for
   Descendant Elements that contain their own valid CRC-32 Element.

   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
   MAY be used.

   <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>

   If a Master Element contains more occurrences of a Child Master
   Element than permitted according to the "maxOccurs" and "recurring"
   attributes of the definition of that Element, then the occurrences in
   addition to "maxOccurs" MAY be ignored.

   If a Master Element contains more occurrences of a Child Element than
   permitted according to the "maxOccurs" attribute of the definition of
   that Element, then all instances of that Element after the first
   "maxOccurs" occurrences from the beginning of its Parent Element
   SHOULD be ignored.

13.  Terminating Elements

   Null Octets, which are octets with all bits set to zero, MAY follow
   the value of a String Element or UTF-8 Element to serve as a
   terminator.  An EBML Writer MAY 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 NOT RECOMMENDED.  The Element Data of a UTF-8
   Element MUST 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 SHOULD be ignored.  A string
   value and a copy of that string value terminated by one or more Null
   Octets are semantically equal.

   Table 11 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.

               +=====================+=====================+
               | Stored Value        | Semantic Meaning    |
               +=====================+=====================+
               | 0x65 0x62 0x6D 0x6C | 0x65 0x62 0x6D 0x6C |
               +---------------------+---------------------+
               | 0x65 0x62 0x00 0x6C | 0x65 0x62           |
               +---------------------+---------------------+
               | 0x65 0x62 0x00 0x00 | 0x65 0x62           |
               +---------------------+---------------------+
               | 0x65 0x62           | 0x65 0x62           |
               +---------------------+---------------------+

                  Table 11: Examples of semantics for Null
                            Octets in VINT_DATA

14.  Guidelines for Updating Elements

   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.

14.1.  Reducing Element Data in Size

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

14.1.1.  Adding a Void Element

   When an EBML Element is changed to reduce its total length by more
   than one octet, an EBML Writer SHOULD fill the freed space with a
   Void Element.

14.1.2.  Extending the Element Data Size

   The same value for Element Data Size MAY be written in various
   lengths, so for minor reductions of the Element Data, the Element
   Size MAY be written to a longer octet length to fill the freed space.

   For example, the first row of Table 12 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 MAY 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.

      +=============+============+===================+==============+
      | Status      | Element ID | Element Data Size | Element Data |
      +=============+============+===================+==============+
      | Before edit | 0x3B4040   | 0x84              | 0x65626D6C   |
      +-------------+------------+-------------------+--------------+
      | After edit  | 0x3B4040   | 0x4003            | 0x6D6B76     |
      +-------------+------------+-------------------+--------------+

          Table 12: Example of editing a VINT to reduce VINT_DATA
                            length by one octet

   This method is RECOMMENDED when the Element Data is reduced by a
   single octet; for reductions by two or more octets, it is RECOMMENDED
   to fill the freed space with a Void Element.

   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 RECOMMENDED 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, Table 13
   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.

   +========+============+===================+==============+=========+
   | Status | Element ID | Element Data Size | Element Data | Void    |
   |        |            |                   |              | Element |
   +========+============+===================+==============+=========+
   | Before | 0x3B4040   | 0x4003            | 0x6D6B76     |         |
   +--------+------------+-------------------+--------------+---------+
   | After  | 0x3B4040   | 0x82              | 0x6869       | 0xEC80  |
   +--------+------------+-------------------+--------------+---------+

      Table 13: Example of editing a VINT to reduce VINT_DATA length
                          by more than one octet

14.1.3.  Terminating Element Data

   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 Section 13).

   In Table 14, 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.

      +=============+============+===================+==============+
      | Status      | Element ID | Element Data Size | Element Data |
      +=============+============+===================+==============+
      | Before edit | 0x3B4040   | 0x84              | 0x65626D6C   |
      +-------------+------------+-------------------+--------------+
      | After edit  | 0x3B4040   | 0x84              | 0x6D6B7600   |
      +-------------+------------+-------------------+--------------+

           Table 14: Example of terminating VINT_DATA with a Null
               Octet when reducing VINT length during an edit

   Note that this method is NOT RECOMMENDED.  For reductions of one
   octet, the method for Extending the Element Data Size SHOULD be used.
   For reduction by more than one octet, the method for Adding a Void
   Element SHOULD be used.

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.  Backward and Forward Compatibility

   Elements of an EBML format SHOULD be designed with backward and
   forward compatibility in mind.

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.

16.  Security Considerations

   EBML itself does not offer any kind of security and does not provide
   confidentiality.  EBML does not provide any kind of authorization.
   EBML only offers marginally useful and effective data integrity
   options, such as CRC elements.

   Even if the semantic layer offers any kind of encryption, EBML itself
   could leak information at both the semantic layer (as declared via
   the DocType Element) and within the EBML structure (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).

   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:

   *  Invalid Element IDs that are longer than the limit stated in the
      EBMLMaxIDLength Element of the EBML Header.

   *  Invalid Element IDs that are not encoded in the shortest-possible
      way.

   *  Invalid Element Data Size values that are longer than the limit
      stated in the EBMLMaxSizeLength Element of the EBML Header.

   Element IDs that are unknown to the EBML Reader MAY be accepted as
   valid EBML IDs in order to skip such elements.

   EBML Elements with a string type may contain extra data after the
   first 0x00.  These data MUST be discarded according to the Section 13
   rules.

   An EBML Reader may discard some or all data if the following errors
   are found in the EBML Document:

   *  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).

   *  Very high lengths in order to force out-of-memory situations
      resulting in a denial of service, access-out-of-bounds issues,
      etc.

   *  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.

   *  Usage of invalid UTF-8 encoding in EBML Elements of UTF-8 type
      (e.g., in order to trigger access-out-of-bounds or buffer-overflow
      issues).

   *  Usage of invalid data in EBML Elements with a date type,
      triggering bogus date accesses.

   *  The CRC-32 Element (see Section 11.3.1) of a Master Element
      doesn't match the rest of the content of that Master Element.

   Side-channel attacks could exploit:

   *  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.

   *  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.

   *  Data contained within a Master Element that is not itself part of
      a Child Element, which can trigger incorrect parsing behavior in
      EBML Readers.

   *  Extraneous copies of Identically Recurring Element, making parsing
      unnecessarily slow to the point of not being usable.

   *  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.

   *  Use of Void Elements that could be used to hide content or create
      bogus resynchronization points seen by some EBML Readers and not
      others.

17.  IANA Considerations

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:

        +============+=========================+=================+
        | Element ID | Element Name            | Reference       |
        +============+=========================+=================+
        | 0x1A45DFA3 | EBML                    | Described in    |
        |            |                         | Section 11.2.1  |
        +------------+-------------------------+-----------------+
        |     0x4286 | EBMLVersion             | Described in    |
        |            |                         | Section 11.2.2  |
        +------------+-------------------------+-----------------+
        |     0x42F7 | EBMLReadVersion         | Described in    |
        |            |                         | Section 11.2.3  |
        +------------+-------------------------+-----------------+
        |     0x42F2 | EBMLMaxIDLength         | Described in    |
        |            |                         | Section 11.2.4  |
        +------------+-------------------------+-----------------+
        |     0x42F3 | EBMLMaxSizeLength       | Described in    |
        |            |                         | Section 11.2.5  |
        +------------+-------------------------+-----------------+
        |     0x4282 | DocType                 | Described in    |
        |            |                         | Section 11.2.6  |
        +------------+-------------------------+-----------------+
        |     0x4287 | DocTypeVersion          | Described in    |
        |            |                         | Section 11.2.7  |
        +------------+-------------------------+-----------------+
        |     0x4285 | DocTypeReadVersion      | Described in    |
        |            |                         | Section 11.2.8  |
        +------------+-------------------------+-----------------+
        |     0x4281 | DocTypeExtension        | Described in    |
        |            |                         | Section 11.2.9  |
        +------------+-------------------------+-----------------+
        |     0x4283 | DocTypeExtensionName    | Described in    |
        |            |                         | Section 11.2.10 |
        +------------+-------------------------+-----------------+
        |     0x4284 | DocTypeExtensionVersion | Described in    |
        |            |                         | Section 11.2.11 |
        +------------+-------------------------+-----------------+
        |       0xBF | CRC-32                  | Described in    |
        |            |                         | Section 11.3.1  |
        +------------+-------------------------+-----------------+
        |       0xEC | Void                    | Described in    |
        |            |                         | Section 11.3.2  |
        +------------+-------------------------+-----------------+

          Table 15: IDs and Names for EBML Elements assigned by
                              this document

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].

18.  Normative References

   [IEEE.754] IEEE, "IEEE Standard for Binary Floating-Point
              Arithmetic", 13 June 2019,
              <https://standards.ieee.org/standard/754-2019.html>.

   [ISO3309]  International Organization for Standardization, "Data
              communication -- High-level data link control procedures
              -- Frame structure", ISO 3309, 3rd Edition, October 1984,
              <https://www.iso.org/standard/8558.html>.

   [ISO9899]  International Organization for Standardization,
              "Information technology -- Programming languages -- C",
              ISO/IEC 9899:2011, 2011,
              <https://www.iso.org/standard/57853.html>.

   [ITU.V42]  International Telecommunications Union, "Error-correcting
              procedures for DCEs using asynchronous-to-synchronous
              conversion", ITU-T Recommendation V.42, March 2002,
              <https://www.itu.int/rec/T-REC-V.42>.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
              DOI 10.17487/RFC2648, August 1999,
              <https://www.rfc-editor.org/info/rfc2648>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
              <https://www.rfc-editor.org/info/rfc8141>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [XHTML]    McCarron, S., "XHTML(tm) Basic 1.1 -- Second Edition",
              Latest version available
              at https://www.w3.org/TR/xhtml-basic, 27 March 2018,
              <https://www.w3.org/TR/2018/SPSD-xhtml-basic-20180327/>.

   [XML]      Bray, T., Ed., Paoli, J., Ed., Sperberg-McQueen, C.M.,
              Ed., Maler, E., Ed., and F. Yergeau, Ed., "Extensible
              Markup Language (XML) 1.0 (Fifth Edition)", Latest version
              available at https://www.w3.org/TR/xml/, 26 November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126/>.

   [XML-SCHEMA]
              Fallside, D.C. and P. Walmsley, "XML Schema Part 0: Primer
              Second Edition", Latest version available at 
              http://www.w3.org/TR/xmlschema-0/, 28 October 2004,
              <https://www.w3.org/TR/2004/REC-xmlschema-0-20041028/>.

19.  Informative References

   [Matroska] Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
              Container Format Specifications", Work in Progress,
              Internet-Draft, draft-ietf-cellar-matroska-05, 17 April
              2020, <https://tools.ietf.org/html/draft-ietf-cellar-
              matroska-05>.

   [WebM]     The WebM Project, "WebM Container Guidelines", 28 November
              2017, <https://www.webmproject.org/docs/container/>.

   [XPath]    Clark, J., Ed. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", Latest version available
              at https://www.w3.org/TR/xpath, 16 November 1999,
              <https://www.w3.org/TR/1999/REC-xpath-19991116>.

Authors' Addresses

   Steve Lhomme

   Email: slhomme@matroska.org


   Dave Rice

   Email: dave@dericed.com


   Moritz Bunkus

   Email: moritz@bunkus.org
