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cellar                                                         S. Lhomme
Internet-Draft
Intended status: Standards Track                                 D. Rice
Expires: 30 July 2020
                                                               M. Bunkus
                                                         27 January 2020


                    Extensible Binary Meta Language
                       draft-ietf-cellar-ebml-17

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 30 July 2020.

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



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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notation and Conventions  . . . . . . . . . . . . . . . . . .   4
   3.  Structure . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Variable Size Integer . . . . . . . . . . . . . . . . . . . .   7
     4.1.  VINT_WIDTH  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  VINT_MARKER . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  VINT_DATA . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  VINT Examples . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Element ID  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Element Data Size . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Data Size Format  . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Unknown Data Size . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Data Size Values  . . . . . . . . . . . . . . . . . . . .  14
   7.  EBML Element Types  . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Signed Integer Element  . . . . . . . . . . . . . . . . .  15
     7.2.  Unsigned Integer Element  . . . . . . . . . . . . . . . .  16
     7.3.  Float Element . . . . . . . . . . . . . . . . . . . . . .  16
     7.4.  String Element  . . . . . . . . . . . . . . . . . . . . .  16
     7.5.  UTF-8 Element . . . . . . . . . . . . . . . . . . . . . .  16
     7.6.  Date Element  . . . . . . . . . . . . . . . . . . . . . .  17
     7.7.  Master Element  . . . . . . . . . . . . . . . . . . . . .  17
     7.8.  Binary Element  . . . . . . . . . . . . . . . . . . . . .  17
   8.  EBML Document . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  EBML Header . . . . . . . . . . . . . . . . . . . . . . .  18
     8.2.  EBML Body . . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  EBML Stream . . . . . . . . . . . . . . . . . . . . . . . . .  19
   10. EBML Versioning . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  EBML Header Version  . . . . . . . . . . . . . . . . . .  19
     10.2.  EBML Document Version  . . . . . . . . . . . . . . . . .  19
   11. Elements semantic . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  EBML Schema  . . . . . . . . . . . . . . . . . . . . . .  19
       11.1.1.  EBML Schema Example  . . . . . . . . . . . . . . . .  20
       11.1.2.  <EBMLSchema> Element . . . . . . . . . . . . . . . .  21
       11.1.3.  <EBMLSchema> Attributes  . . . . . . . . . . . . . .  21
       11.1.4.  <element> Element  . . . . . . . . . . . . . . . . .  22
       11.1.5.  <element> Attributes . . . . . . . . . . . . . . . .  22
       11.1.6.  <documentation> Element  . . . . . . . . . . . . . .  30
       11.1.7.  <documentation> Attributes . . . . . . . . . . . . .  30
       11.1.8.  <implementation_note> Element  . . . . . . . . . . .  31
       11.1.9.  <implementation_note> Attributes . . . . . . . . . .  32
       11.1.10. <restriction> Element  . . . . . . . . . . . . . . .  33
       11.1.11. <enum> Element . . . . . . . . . . . . . . . . . . .  33



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       11.1.12. <enum> Attributes  . . . . . . . . . . . . . . . . .  34
       11.1.13. <extension> Element  . . . . . . . . . . . . . . . .  34
       11.1.14. <extension> Attributes . . . . . . . . . . . . . . .  34
       11.1.15. XML Schema for EBML Schema . . . . . . . . . . . . .  35
       11.1.16. Identically Recurring Elements . . . . . . . . . . .  38
       11.1.17. Textual expression of floats . . . . . . . . . . . .  39
       11.1.18. Note on the use of default attributes to define
               Mandatory EBML Elements . . . . . . . . . . . . . . .  40
     11.2.  EBML Header Elements . . . . . . . . . . . . . . . . . .  41
       11.2.1.  EBML Element . . . . . . . . . . . . . . . . . . . .  41
       11.2.2.  EBMLVersion Element  . . . . . . . . . . . . . . . .  41
       11.2.3.  EBMLReadVersion Element  . . . . . . . . . . . . . .  41
       11.2.4.  EBMLMaxIDLength Element  . . . . . . . . . . . . . .  42
       11.2.5.  EBMLMaxSizeLength Element  . . . . . . . . . . . . .  42
       11.2.6.  DocType Element  . . . . . . . . . . . . . . . . . .  43
       11.2.7.  DocTypeVersion Element . . . . . . . . . . . . . . .  43
       11.2.8.  DocTypeReadVersion Element . . . . . . . . . . . . .  44
       11.2.9.  DocTypeExtension Element . . . . . . . . . . . . . .  44
       11.2.10. DocTypeExtensionName Element . . . . . . . . . . . .  45
       11.2.11. DocTypeExtensionVersion Element  . . . . . . . . . .  45
     11.3.  Global Elements  . . . . . . . . . . . . . . . . . . . .  46
       11.3.1.  CRC-32 Element . . . . . . . . . . . . . . . . . . .  46
       11.3.2.  Void Element . . . . . . . . . . . . . . . . . . . .  47
   12. Considerations for Reading EBML Data  . . . . . . . . . . . .  47
   13. Terminating Elements  . . . . . . . . . . . . . . . . . . . .  48
   14. Guidelines for Updating Elements  . . . . . . . . . . . . . .  49
     14.1.  Reducing a Element Data in Size  . . . . . . . . . . . .  49
       14.1.1.  Adding a Void Element  . . . . . . . . . . . . . . .  49
       14.1.2.  Extending the Element Data Size  . . . . . . . . . .  49
       14.1.3.  Terminating Element Data . . . . . . . . . . . . . .  50
     14.2.  Considerations when Updating Elements with Cyclic
            Redundancy Check (CRC) . . . . . . . . . . . . . . . . .  51
   15. Backward and Forward Compatibility  . . . . . . . . . . . . .  51
     15.1.  Backward Compatibility . . . . . . . . . . . . . . . . .  51
     15.2.  Forward Compatibility  . . . . . . . . . . . . . . . . .  52
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  52
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  53
     17.1.  EBML Element ID Registry . . . . . . . . . . . . . . . .  54
     17.2.  EBML DocType Registry  . . . . . . . . . . . . . . . . .  57
   18. Normative References  . . . . . . . . . . . . . . . . . . . .  57
   19. Informative References  . . . . . . . . . . . . . . . . . . .  59
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  59

1.  Introduction

   EBML, short for Extensible Binary Meta Language, specifies a binary
   and octet (byte) aligned format inspired by the principle of XML (a
   framework for structuring data).



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   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




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   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 [W3C.REC-xml-20081126] (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 which
   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": The "Element ID" is a binary value, encoded as a
   "Variable Size Integer", used to uniquely identify a defined "EBML
   Element" within a specific "EBML Schema".

   "Element 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".



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

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

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






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






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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.

















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   +---------+--------------+---------------------+--------------------+
   | 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:














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    +------------+-------------+----------------+--------------------+
    | 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.

















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      +-------------------------+----------------+-----------------+
      | 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 which do not mandate a non-zero length
   (see Section 7).  An Element Data Size with all VINT_DATA bits set to
   zero indicates that the Element Data is zero octets in length.  Such
   an EBML Element is referred to as an Empty Element.  If an Empty
   Element has a default value declared then the EBML Reader MUST
   interpret the value of the Empty Element as the default value.  If an
   Empty Element has no default value declared then the EBML Reader MUST
   use the value of the Empty Element for the corresponding EBML Element



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   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 11.1.5.10).

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

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

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

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

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

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

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









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    +------------------------------------+---------------------------+
    | 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








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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).)












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    +------------+-------------+-----------------------+--------------+
    | 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 which 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.





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

7.2.  Unsigned Integer Element

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

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

7.3.  Float Element

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

   A Float Element stores a floating-point number in the 32-bit and
   64-bit binary interchange format as defined in [IEEE.754.1985].

7.4.  String Element

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

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

7.5.  UTF-8 Element

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




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   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 11.1.5.2).  Element Data
   stored within Master Elements SHOULD only consist of EBML Elements
   and SHOULD NOT contain any data that is not part of an EBML Element.
   The EBML Schema identifies what Element IDs are valid within the
   Master Elements for that version of the EBML Document Type.  Any data
   contained within a Master Element that is not part of a Child Element
   MUST be ignored.

7.8.  Binary Element

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

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









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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) and
   any number of additional EBML Elements within it.  The EBML Header of
   an EBML Document that uses an EBMLVersion of 1 MUST only contain EBML
   Elements that are defined as part of this document.

   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), which 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.



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9.  EBML Stream

   An EBML Stream is a file that consists of one or more EBML Documents
   that are concatenated together.  An occurrence of a 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 semantic

11.1.  EBML Schema

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




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

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

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

11.1.1.  EBML Schema Example

   <?xml version="1.0" encoding="utf-8"?>
   <EBMLSchema xmlns="https://ietf.org/cellar/ebml"
     docType="files-in-ebml-demo" version="1">
    <!-- constraints to the range of two EBML Header Elements -->
    <element name="EBMLReadVersion" path="\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>



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    </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 [W3C.REC-xpath-19991116] of
   "<EBMLSchema>" element is "/EBMLSchema".

   As an XML Document, the EBML Schema MUST use "<EBMLSchema>" as the
   top level element.  The "<EBMLSchema>" element can contain
   "<element>" sub-elements.

11.1.3.  <EBMLSchema> Attributes

   Within an EBML Schema the "<EBMLSchema>" element uses the following
   attributes:

11.1.3.1.  docType

   Within an EBML Schema, the XPath of "@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.




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11.1.3.2.  version

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

   The version lists a non-negative integer that specifies the version
   of the docType documented by the EBML Schema.  Unlike XML Schemas, an
   EBML Schema documents all versions of a docType's definition rather
   than using separate EBML Schemas for each version of a docType.  EBML
   Elements may be introduced and deprecated by using the minver and
   maxver attributes of "<element>".

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

11.1.3.3.  ebml

   Within an EBML Schema, the XPath of "@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.

11.1.4.  <element> Element

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

   Each "<element>" defines one EBML Element through the use of several
   attributes that are defined in Section 11.1.5.  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>" sub-
   elements, followed by optional "<implementation_note>" sub-elements,
   followed by zero or one "<restriction>" sub-element, followed by
   optional "<extension>" sub-elements.  All "<element>" nodes MUST be
   sub-elements of the "<EBMLSchema>".

11.1.5.  <element> Attributes

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






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11.1.5.1.  name

   Within an EBML Schema, the XPath of "@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", "0" to "9"
   character.

   The name attribute is REQUIRED.

11.1.5.2.  path

   Within an EBML Schema, the XPath of "@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 being the first in the value.  The
   syntax of the path attribute is defined using this Augmented Backus-
   Naur Form (ABNF) [RFC5234] with the case sensitive update [RFC7405]
   notation:

   The path attribute is REQUIRED.

   EBMLFullPath          = 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     = "(" GlobalParentOccurence "\)"
   GlobalParentOccurence = [PathMinOccurrence] "-" [PathMaxOccurrence]
   PathMinOccurrence     = 1*DIGIT ; no upper limit
   PathMaxOccurrence     = 1*DIGIT ; no upper limit



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   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 11.1.5.11).

   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 GlobalParentOccurence part is interpreted as the amount of valid
   EBMLPathAtom parts that can replace the GlobalPlaceholder in the
   path.  PathMinOccurrence represents the minimum amount of
   EBMLPathAtom required to replace the GlobalPlaceholder.
   PathMaxOccurrence represents the maximum amount of EBMLPathAtom
   possible to replace the GlobalPlaceholder.

   If PathMinOccurrence is not present then that GlobalParentOccurence
   has a PathMinOccurrence value of 0.  If PathMaxOccurrence is not
   present then there is no upper bound for the permitted amount of
   EBMLPathAtom 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 or equal to PathMinOccurrence.

   For example in "\a\(0-1\)global", the Element path "\a\x\global"
   corresponds to an EBMLPathAtom occurence of 1.  The Element
   "\a\x\y\global" corresponds to an EBMLPathAtom occurence of 2, etc.
   In those case "\a\x" or "\a\x\y" MUST be valid pathes 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".  But the "global" EBML Element can be found
   inside the "\a\b" EBML Element as the resulting path "\a\b\global"



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   has one EBMLPathAtom between the "\a\" and "global".  Or 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 the "\a" EBML Element (0
   EBMLPathAtom replacing GlobalPlaceholder) or inside the "\a\b" EBML
   Element (one replacement EBMLPathAtom).  But it cannot be found
   inside the "\a\b\c" EBML Element as the resulting path
   "\a\b\c\global" has two EBMLPathAtom between "\a\" and "global".

11.1.5.3.  id

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

   The Element ID encoded as a Variable Size Integer expressed in
   hexadecimal notation prefixed by a 0x that is read and stored in big-
   endian order.  To reduce the risk of false positives while parsing
   EBML Streams, the Element IDs of the Root Element and Top-Level
   Elements SHOULD be at least 4 octets in length.  Element IDs defined
   for use at Root Level or directly under the Root Level MAY use
   shorter octet lengths to facilitate padding and optimize edits to
   EBML Documents; for instance, the Void Element uses an Element ID
   with a one octet length to allow its usage in more writing and
   editing scenarios.

   The 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-Size Element.

   The id attribute is REQUIRED.

11.1.5.4.  minOccurs

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

   The minOccurs is a non-negative integer expressing the minimum
   permitted number of occurrences of this EBML Element within its
   Parent Element.




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   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 11.1.5.8) declared are not REQUIRED to be stored but are
   REQUIRED to be interpreted, see Section 11.1.18.

   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.

11.1.5.5.  maxOccurs

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

   The maxOccurs is a non-negative 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 11.1.18.  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 the EBMLParentPath of the EBML
   Element is empty or not.

   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.

11.1.5.6.  range

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




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   A numerical range for EBML Elements which are of numerical types
   (Unsigned Integer, Signed Integer, Float, and Date).  If specified
   the value of the EBML Element MUST be within the defined range.  See
   Section 11.1.5.6.1 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.

11.1.5.6.1.  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.

   For an exclusive lower boundary the ">" sign is used and the ">="
   sign is used for an inclusive lower boundary.  For example ">3"
   meaning the Element value MUST be greater than 3 or ">=0x1p+0"
   meaning the Element value MUST be greater than or equal to the
   floating value 1.0, see Section 11.1.17.

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

   The lower and upper bounds can be combined into an expression to form
   a closed boundary.  The lower boundary coming 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 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, only the latter form MUST be used.




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11.1.5.7.  length

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

   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 non-negative integer
   or a range (see Section 11.1.5.6.1) that consists of only non-
   negative 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.

11.1.5.8.  default

   Within an EBML Schema, the XPath of "@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.

11.1.5.9.  type

   Within an EBML Schema, the XPath of "@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 within Section 7.

   The type attribute is REQUIRED.






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11.1.5.10.  unknownsizeallowed

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

   A boolean to express if an EBML Element is permitted to be 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.

11.1.5.11.  recursive

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

   A boolean to express if an EBML Element is permitted to be stored
   recursively.  In this case the EBML Element MAY be stored within
   another EBML Element that has the same Element ID.  Which itself can
   be stored in an EBML Element that has the same Element ID, and so on.
   EBML Elements that are not Master Elements MUST NOT set recursive to
   true.

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

   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.

11.1.5.12.  recurring

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

   A boolean to express if an EBML Element is defined as an Identically
   Recurring Element or not; see Section 11.1.16.




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

11.1.5.13.  minver

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

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

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

11.1.5.14.  maxver

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

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

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

11.1.6.  <documentation> Element

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

   The "<documentation>" element provides additional information about
   the EBML Element.  Within the "<documentation>" element the following
   XHTML [W3C.SPSD-xhtml-basic-20180327] elements MAY be used: "<a>",
   "<br>", "<strong>".

11.1.7.  <documentation> Attributes

11.1.7.1.  lang

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

   A lang attribute which is set to the [RFC5646] value of the language
   of the element's documentation.



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   The lang attribute is OPTIONAL.

11.1.7.2.  purpose

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

   A purpose attribute distinguishes the meaning of the documentation.
   Values for the "<documentation>" sub-element'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 guideline 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.

11.1.8.  <implementation_note> Element

   Within an EBML Schema, the XPath of "<implementation_note>" attribute
   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, or as another example, the default value of an EBML Element
   might derive from a related Element's content.  In these cases where
   the Element's definition is conditional or advanced implementation



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   notes are needed, one or many "<implementation_note>" elements can be
   used to store that information.  The "<implementation_note>" refer to
   a specific attribute of the parent "<element>" as expressed by the
   "note_attribute" attribute Section 11.1.9.1.

11.1.9.  <implementation_note> Attributes

11.1.9.1.  note_attribute

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

   The note_attribute attribute references which of the "<element>"'s
   attributes that the implementation_note is in regards 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.

11.1.9.2.  <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.



















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   <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>

11.1.10.  <restriction> Element

   Within an EBML Schema, the XPath of "<restriction>" attribute 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.

11.1.11.  <enum> Element

   Within an EBML Schema, the XPath of "<enum>" attribute 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 a EBML
   Element that is defined as an unsigned integer).  An "<enum>" element




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   MAY also store "<documentation>" elements to further describe the
   "<enum>".

11.1.12.  <enum> Attributes

11.1.12.1.  label

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

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

   The label attribute is OPTIONAL.

11.1.12.2.  value

   Within an EBML Schema, the XPath of "@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.

11.1.13.  <extension> Element

   Within an EBML Schema, the XPath of "<extension>" attribute 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 sub-element as
   long as the EBML Schema remains as a well-formed XML Document.  All
   "<extension>" elements MUST be sub-elements of the "<element>".

11.1.14.  <extension> Attributes

11.1.14.1.  type

   Within an EBML Schema, the XPath of "@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.




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   The type attribute is REQUIRED.

11.1.15.  XML Schema for EBML Schema

   This following provides an XML Schema [W3C.REC-xmlschema-0-20041028]
   for facilitating verification of an EBML Schema to the definition
   described in Section 8.1.

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

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

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

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



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         </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"



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         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>



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         <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>

11.1.16.  Identically Recurring Elements

   An Identically Recurring Element is an EBML Element that MAY occur
   within its Parent Element more than once but that each recurrence
   within that Parent Element MUST be identical both in storage and
   semantics.  Identically Recurring Elements are permitted to be stored
   multiple times within the same Parent Element in order to increase



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   data resilience and optimize the use of EBML in transmission.  For
   instance a pertinent Top-Level Element could be periodically resent
   within a data stream so that an EBML Reader which starts reading the
   stream from the middle could better interpret the contents.
   Identically Recurring Elements SHOULD include a CRC-32 Element as a
   Child Element; this is especially recommended when EBML is used for
   long-term storage or transmission.  If a Parent Element contains more
   than one copy of an Identically Recurring Element which includes a
   CRC-32 Element as a Child Element then the first instance of the
   Identically Recurring Element with a valid CRC-32 value should be
   used for interpretation.  If a Parent Element contains more than one
   copy of an Identically Recurring Element which does not contain a
   CRC-32 Element or if CRC-32 Elements are present but none are valid
   then the first instance of the Identically Recurring Element should
   be used for interpretation.

11.1.17.  Textual expression of floats

   When a float value is represented textually in an EBML Schema, such
   as within a default or range value, the float values MUST be
   expressed as Hexadecimal Floating-Point Constants as defined in the
   C11 standard [ISO.9899.2011] (see section 6.4.4.2 on Floating
   Constants).  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 as Hexadecimal Floating-Point Constants.

   Within an expression of a float range, as in an integer range, the -
   (hyphen) character is the separator between the minimal and maximum
   value permitted by the range.  Hexadecimal Floating-Point Constants
   also use a - (hyphen) when indicating a negative binary power.
   Within a float range, when a - (hyphen) is immediately preceded by a
   letter p, then the - (hyphen) is a part of the Hexadecimal Floating-



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   Point Constant which notes negative binary power.  Within a float
   range, when a - (hyphen) is not immediately preceded by a letter p,
   then the - (hyphen) represents the separator between the minimal and
   maximum value permitted by the range.

11.1.18.  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 if a Mandatory EBML Element MUST be written,
   according to if the default value is declared, if the value of the
   EBML Element is equal to the declared default value, and if the
   Parent Element is used.

   +-----------------+-------------+---------------+------------------+
   |  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.



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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




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   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



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   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 which 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



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   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



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   know these extra Elements and how to use them.  A DocTypeExtension
   MAY be used to iterate between experimental Elements before they are
   integrated in 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\Name"

   id 0x4283

   minOccurs: 1

   maxOccurs: 1

   length: >0

   type: String

   description: The name of the DocTypeExtension to differentiate it
   from other DocTypeExtension 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\Version"

   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 DocTypeExtension, only one or
   none.



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11.3.  Global Elements

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

   Global Elements are EBML Elements whose EBMLLastParent part of the
   path has a GlobalPlaceholder.  Because it is the last Parent part of
   the path, a Global Element might also have an 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 [ISO.3309.1979] standard and in section
   8.1.1.6.2 of [ITU.V42.1994], with initial value of 0xFFFFFFFF.  The
   CRC value MUST be computed on a little endian bytestream and MUST use
   little endian storage.




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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 sub-element for later use.

12.  Considerations for Reading EBML Data

   The following scenarios describe events to consider when reading EBML
   Documents and 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.
















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   <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
   maxOccur 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.






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               +---------------------+---------------------+
               | 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: Exmaples 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 to change the Element Data of a written EBML Element with
   minimal disruption to the rest of the EBML Document.

14.1.  Reducing a Element Data in Size

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

14.1.1.  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.



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      +-------------+------------+-------------------+--------------+
      | 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, a four octets long Element Data is changed to a three
   octet long value followed by a Null Octet; the Element Data Size




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   includes any Null Octets used to terminate Element Data so 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.





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15.2.  Forward Compatibility

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

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).




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   *  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 which is not itself part of
      a Child Element 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-32.

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

17.  IANA Considerations







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17.1.  EBML Element ID Registry

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

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

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

   One-octet Element IDs MUST be between 0x81 and 0xFE.  These items are
   valuable because they are short, and 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.

   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.

   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.

   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



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   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.

   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:























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        +------------+-------------------------+-----------------+
        | 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.






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17.2.  EBML DocType Registry

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

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

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

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

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

18.  Normative References

   [W3C.REC-xmlschema-0-20041028]
              Fallside, D. and P. Walmsley, "XML Schema Part 0: Primer
              Second Edition", World Wide Web Consortium Recommendation
              REC-xmlschema-0-20041028, 28 October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-0-20041028>.

   [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>.

   [ITU.V42.1994]
              International Telecommunications Union, "Error-correcting
              Procedures for DCEs Using Asynchronous-to-Synchronous
              Conversion", 1994.

   [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>.

   [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>.




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   [ISO.9899.2011]
              International Organization for Standardization,
              "Programming languages - C", 2011.

   [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>.

   [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>.

   [W3C.SPSD-xhtml-basic-20180327]
              McCarron, S., "XHTML(tm) Basic 1.1 - Second Edition", 27
              March 2018.

   [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>.

   [IEEE.754.1985]
              Institute of Electrical and Electronics Engineers,
              "Standard for Binary Floating-Point Arithmetic", August
              1985.

   [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>.

   [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>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, 26 November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

   [ISO.3309.1979]
              International Organization for Standardization, "Data



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              communication - High-level data link control procedures -
              Frame structure", 1979.

19.  Informative References

   [Matroska] IETF, "Matroska Specifications", 2019,
              <https://datatracker.ietf.org/doc/draft-ietf-cellar-
              matroska/>.

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

   [W3C.REC-xpath-19991116]
              Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", World Wide Web Consortium Recommendation
              REC-xpath-19991116, 16 November 1999,
              <http://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


















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