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Versions: (draft-lhomme-cellar-ebml) 00 01 02 03

cellar                                                         S. Lhomme
Internet-Draft
Intended status: Standards Track                                 D. Rice
Expires: January 3, 2018
                                                               M. Bunkus
                                                            July 2, 2017


                    Extensible Binary Meta Language
                       draft-ietf-cellar-ebml-03

Abstract

   This document defines the Extensible Binary Meta Language (EBML)
   format as a generalized file format for any type of data in a
   hierarchical form.  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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 January 3, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notation and Conventions  . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Structure . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Variable Size Integer . . . . . . . . . . . . . . . . . . . .   7
     6.1.  VINT_WIDTH  . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  VINT_MARKER . . . . . . . . . . . . . . . . . . . . . . .   8
     6.3.  VINT_DATA . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.4.  VINT Examples . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Element ID  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Element Data Size . . . . . . . . . . . . . . . . . . . . . .  11
   9.  EBML Element Types  . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Signed Integer Element  . . . . . . . . . . . . . . . . .  13
     9.2.  Unsigned Integer Element  . . . . . . . . . . . . . . . .  13
     9.3.  Float Element . . . . . . . . . . . . . . . . . . . . . .  13
     9.4.  String Element  . . . . . . . . . . . . . . . . . . . . .  14
     9.5.  UTF-8 Element . . . . . . . . . . . . . . . . . . . . . .  14
     9.6.  Date Element  . . . . . . . . . . . . . . . . . . . . . .  14
     9.7.  Master Element  . . . . . . . . . . . . . . . . . . . . .  14
     9.8.  Binary Element  . . . . . . . . . . . . . . . . . . . . .  15
   10. Terminating Elements  . . . . . . . . . . . . . . . . . . . .  15
   11. Guidelines for Updating Elements  . . . . . . . . . . . . . .  16
     11.1.  Reducing a Element Data in Size  . . . . . . . . . . . .  16
       11.1.1.  Adding a Void Element  . . . . . . . . . . . . . . .  16
       11.1.2.  Extending the Element Data Size  . . . . . . . . . .  16
       11.1.3.  Terminating Element Data . . . . . . . . . . . . . .  17
     11.2.  Considerations when Updating Elements with CRC . . . . .  17
   12. EBML Document . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  EBML Header  . . . . . . . . . . . . . . . . . . . . . .  18
     12.2.  EBML Body  . . . . . . . . . . . . . . . . . . . . . . .  18
   13. EBML Stream . . . . . . . . . . . . . . . . . . . . . . . . .  19
   14. Elements semantic . . . . . . . . . . . . . . . . . . . . . .  19
     14.1.  EBML Schema  . . . . . . . . . . . . . . . . . . . . . .  19
       14.1.1.  Element  . . . . . . . . . . . . . . . . . . . . . .  20
       14.1.2.  Attributes . . . . . . . . . . . . . . . . . . . . .  20
       14.1.3.  Element  . . . . . . . . . . . . . . . . . . . . . .  20
       14.1.4.  Attributes . . . . . . . . . . . . . . . . . . . . .  21
       14.1.5.  Element  . . . . . . . . . . . . . . . . . . . . . .  25
       14.1.6.  Attributes . . . . . . . . . . . . . . . . . . . . .  25



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       14.1.7.  Element  . . . . . . . . . . . . . . . . . . . . . .  26
       14.1.8.  Element  . . . . . . . . . . . . . . . . . . . . . .  26
       14.1.9.  Attributes . . . . . . . . . . . . . . . . . . . . .  26
       14.1.10. XML Schema for EBML Schema . . . . . . . . . . . . .  26
       14.1.11. EBML Schema Example  . . . . . . . . . . . . . . . .  28
       14.1.12. Identically Recurring Elements . . . . . . . . . . .  29
       14.1.13. Expression of range  . . . . . . . . . . . . . . . .  29
       14.1.14. Textual expression of Floats . . . . . . . . . . . .  30
       14.1.15. Note on the Use of default attributes to define
                Mandatory EBML Elements  . . . . . . . . . . . . . .  31
     14.2.  EBML Header Elements . . . . . . . . . . . . . . . . . .  31
       14.2.1.  EBML Element . . . . . . . . . . . . . . . . . . . .  31
       14.2.2.  EBMLVersion Element  . . . . . . . . . . . . . . . .  32
       14.2.3.  EBMLReadVersion Element  . . . . . . . . . . . . . .  32
       14.2.4.  EBMLMaxIDLength Element  . . . . . . . . . . . . . .  33
       14.2.5.  EBMLMaxSizeLength Element  . . . . . . . . . . . . .  33
       14.2.6.  DocType Element  . . . . . . . . . . . . . . . . . .  34
       14.2.7.  DocTypeVersion Element . . . . . . . . . . . . . . .  34
       14.2.8.  DocTypeReadVersion Element . . . . . . . . . . . . .  35
     14.3.  Global elements (used everywhere in the format)  . . . .  35
       14.3.1.  Void Element . . . . . . . . . . . . . . . . . . . .  36
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     15.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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).

   The goal of this document is to define a generic, binary, space-
   efficient format that can be used to define more complex formats
   (such as containers for multimedia content) using an "EBML Schema".
   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.

   "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.



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2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   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": An "EBML Document Type" is a name provided by
   an "EBML Schema" for a particular implementation of "EBML" for a data
   format (examples: matroska and webm).

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

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

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

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

   "EBML Header": The "EBML Header" is a declaration that provides
   processing instructions and identification of the "EBML Body".  The
   "EBML Header" may be considered as 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" may be considered the "EBML Body".

   "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".



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   "EBML Class": A representation of the octet length of an "Element
   ID".

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

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

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

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

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

   "Root Element": A mandatory, 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 contained.

   "Descendant Element": A "Descendant Element" is 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.

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





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

   "Empty Element": An "Empty Element" is 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.  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 (you can
   derive the presence of "EBML Elements" even with an unknown semantic
   layer with a heuristic approach; not without errors, of course, but
   with a certain degree of confidence).

   Attacks on an "EBML Reader" could include:

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

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

   o  Invalid "Element IDs" comprised of reserved values.

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

   o  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).

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

   o  Missing "EBML Elements" that are mandatory and have no declared
      default value.

   o  Usage of "0x00" octets in "EBML Elements" with a string type.





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   o  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).

   o  Usage of invalid data in "EBML Elements" with a date type.

   Side channel attacks could exploit:

   o  The semantic equivalence of the same string stored in a "String
      Element" or "UTF-8 Element" with and without zero-bit padding.

   o  The semantic equivalence of "VINT_DATA" within "Element Data Size"
      with to different lengths due to left-padding zero bits.

   o  Data contained within a "Master Element" which is not itself part
      of an "EBML Element".

   o  Extraneous copies of "Identically Recurring Element".

   o  Copies of "Identically Recurring Element" within a "Parent
      Element" that contain invalid "CRC-32 Elements".

   o  Use of "Void Elements".

4.  IANA Considerations

   This document has no IANA actions.

5.  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
   many other "EBML Elements", or both.

6.  Variable Size Integer

   The "Element ID" and "Element Data Size" are both encoded as a
   "Variable Size Integer", developed according to a UTF-8 like system.
   The "Variable Size Integer" is composed of a "VINT_WIDTH",
   "VINT_MARKER", and "VINT_DATA", in that order.  "Variable Size
   Integers" SHALL left-pad the "VINT_DATA" value with zero bits so that
   the whole "Variable Size Integer" is octet-aligned.  "Variable Size
   Integers" SHALL be referred to as "VINT" for shorthand.






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6.1.  VINT_WIDTH

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

   Within the "EBML Header" the "VINT_WIDTH" MUST NOT exceed three bits
   in length (meaning that the "Variable Size Integer" MUST NOT exceed
   four octets in length).  Within the "EBML Body", when "VINTs" are
   used to express an "Element ID", the maximum length allowed for the
   "VINT_WIDTH" is one less than the value set in the "EBMLMaxIDLength
   Element".  Within the "EBML Body", when "VINTs" are used to express
   an "Element Data Size", the maximum length allowed for the
   "VINT_WIDTH" is one less than the value set in the "EBMLMaxSizeLength
   Element".

6.2.  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" MUST be 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".

6.3.  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" combined use one out of 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" SHOULD 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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6.4.  VINT Examples

   This table shows examples of "Variable Size Integers" with lengths
   from 1 to 5 octets.  The Size column refers to the size of the
   "VINT_DATA" in bits.  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    | Size | Representation                               |
   |    Length   |      |                                              |
   +-------------+------+----------------------------------------------+
   |      1      | 2^7  | 1xxx xxxx                                    |
   |      2      | 2^14 | 01xx xxxx xxxx xxxx                          |
   |      3      | 2^21 | 001x xxxx xxxx xxxx xxxx xxxx                |
   |      4      | 2^28 | 0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx      |
   |      5      | 2^35 | 0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx |
   |             |      | xxxx                                         |
   +-------------+------+----------------------------------------------+

   Data encoded as a "Variable Size Integer" MAY be rendered at octet
   lengths larger than needed to store the data.  In this table a binary
   value of "0b10" is shown encoded as different "Variable Size
   Integers" with lengths from one octet to four octet.  All four
   encoded examples have identical semantic meaning though the
   "VINT_WIDTH" and the padding of the "VINT_DATA" vary.

   +--------------+--------------+-------------------------------------+
   | Binary Value | Octet Length | As Represented in Variable Size     |
   |              |              | Integer                             |
   +--------------+--------------+-------------------------------------+
   |      10      |      1       | 1000 0010                           |
   |      10      |      2       | 0100 0000 0000 0010                 |
   |      10      |      3       | 0010 0000 0000 0000 0000 0010       |
   |      10      |      4       | 0001 0000 0000 0000 0000 0000 0000  |
   |              |              | 0010                                |
   +--------------+--------------+-------------------------------------+

7.  Element ID

   The "Element ID" MUST be encoded as a "Variable Size Integer".  By
   default, "Element IDs" are encoded in lengths from one octet to four
   octets, although "Element IDs" of greater lengths are used if the
   octet length of the longest "Element ID" of the "EBML Document" is
   declared in the "EBMLMaxIDLength Element" of the "EBML Header" (see
   Section 14.2.4).  The "VINT_DATA" component of the "Element ID" MUST
   NOT be either defined or written as either all zero values or all one
   values.  Any "Element ID" with the "VINT_DATA" component set as all



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   zero values or all one values MUST be ignored and MUST NOT be
   considered an error in the "EBML Document".  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.

   The following table details these specific examples further:

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

   The octet length of an "Element ID" determines its "EBML Class".

      +------------+--------------+--------------------------------+
      | EBML Class | Octet Length | Number of Possible Element IDs |
      +------------+--------------+--------------------------------+
      |  Class A   |      1       | 2^7  - 2        =         126  |
      |  Class B   |      2       | 2^14 - 2^7  - 1 =      16,255  |
      |  Class C   |      3       | 2^21 - 2^14 - 1 =   2,080,767  |
      |  Class D   |      4       | 2^28 - 2^21 - 1 = 266,338,303  |
      +------------+--------------+--------------------------------+



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8.  Element Data Size

   The "Element Data Size" expresses the length in octets of "Element
   Data".  The "Element Data Size" itself MUST be 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 14.2.5).  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 9).  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 interpret the value of the
   "Empty Element" as defined as part of the definition of the
   corresponding "EBML Element Type" associated with the "Element ID".

   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 "Master Elements" SHALL be "Unknown-Sized Elements".  "Master
   Elements" MUST NOT use an unknown size unless the
   "unknownsizeallowed" attribute of their "EBML Schema" is set to true
   (see Section 14.1.4.10).  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 Element" MUST NOT be used or
   defined unnecessarily; however if the "Element Data Size" is not
   known before the "Element Data" is written, such as in some cases of
   data streaming, then "Unknown-Sized Elements" MAY be used.  The end
   of an "Unknown-Sized Element" is determined by whichever comes first:
   the end of the file or the beginning of the next "EBML Element",
   defined by this document or the corresponding "EBML Schema", that is



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   not independently valid as "Descendant Element" of the "Unknown-Sized
   Element".

   For "Element Data Sizes" encoded at octet lengths from one to eight,
   this table 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      |
                  +--------------+----------------------+

   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 a reserved
   value.  For example, an "EBML Element" with an octet length of 127
   MUST NOT be encoded in an "Element Data Size" encoding with a one
   octet length.  The following table 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).

   +------------+-------------+----------------+-----------------------+
   | VINT_WIDTH | VINT_MARKER |      VINT_DATA |   Element Data Size   |
   |            |             |                |         Status        |
   +------------+-------------+----------------+-----------------------+
   |            |           1 |        1111111 |   Reserved (meaning   |
   |            |             |                |        Unknown)       |
   |          0 |           1 | 00000001111111 |   Valid (meaning 127  |
   |            |             |                |        octets)        |
   +------------+-------------+----------------+-----------------------+

9.  EBML Element Types

   "EBML Elements" are defined by an "EBML Schema" 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



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   "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.

9.1.  Signed Integer Element

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

   A "Signed Integer Element" stores an integer (meaning that it can be
   written without a fractional component) which could be negative,
   positive, or zero.  Signed Integers MUST be 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.

9.2.  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.

9.3.  Float Element

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

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






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9.4.  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 10).

9.5.  UTF-8 Element

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

   A "UTF-8 Element" contains only a valid Unicode string as defined in
   [RFC3629], with an exception made for termination (see Section 10).

9.6.  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.

9.7.  Master Element

   A "Master Element" MUST declare a length in octets from zero to
   "VINTMAX".  The "Master Element" MAY also use an unknown length.  See
   Section 8 for rules that apply to elements of unknown length.

   The "Master Element" contains zero, one, or many other elements.
   "EBML Elements" contained within a "Master Element" MUST have the
   "EBMLParentPath" of their "Element Path" equals to the
   "EBMLReferencePath" of the "Master Element" "Element Path" (see
   Section 14.1.4.2).  "Element Data" stored within "Master Elements"
   SHOULD only consist of "EBML Elements" and SHOULD NOT contain any



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   data that is not part of an "EBML Element".  When "EBML" is used in
   transmission or streaming, data that is not part of an "EBML Element"
   is permitted to be present within a "Master Element" if
   "unknownsizeallowed" is enabled within the definition for that
   "Master Element".  In this case, the "EBML Reader" should skip data
   until a valid "Element ID" of the same "EBMLParentPath" or the next
   upper level "Element Path" of the "Master Element" is found.  What
   "Element IDs" are considered valid within a "Master Element" is
   identified by the "EBML Schema" 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.

9.8.  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".

10.  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.  An
   "EBML Reader" MUST consider the value of the "String Element" or
   "UTF-8 Element" to be terminated upon the first read "Null Octet" and
   MUST ignore any data following the first "Null Octet" within that
   "Element".  A string value and a copy of that string value terminated
   by one or more "Null Octets" are semantically equal.

   The following table shows examples of semantics and validation for
   the use of "Null Octets".  Values to represent "Stored Values" and
   the "Semantic Meaning" as represented as hexidecimal values.

               +---------------------+---------------------+
               | Stored Value        | Semantic Meaning    |
               +---------------------+---------------------+
               | 0x65 0x62 0x6d 0x6c | 0x65 0x62 0x6d 0x6c |
               | 0x65 0x62 0x00 0x6c | 0x65 0x62           |
               | 0x65 0x62 0x00 0x00 | 0x65 0x62           |
               | 0x65 0x62           | 0x65 0x62           |
               +---------------------+---------------------+



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11.  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".

11.1.  Reducing a Element Data in Size

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

11.1.1.  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".

11.1.2.  Extending the Element Data Size

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

   For example, the first row of the following table depicts a "String
   Element" that stores an "Element ID" (3 octets), "Element Data Size"
   (1 octet), and "Element Data" (4 octets).  If the "Element Data" is
   changed to reduce the length by one octet and if the current length
   of the "Element Data Size" is less than its maximum permitted length,
   then the "Element Data Size" of that "Element" MAY be rewritten to
   increase its length by one octet.  Thus before and after the change
   the "EBML Element" maintains the same length of 8 octets and data
   around the "Element" does not need to be moved.

      +-------------+------------+-------------------+--------------+
      |    Status   | Element ID | Element Data Size | Element Data |
      +-------------+------------+-------------------+--------------+
      | Before edit |  0x3B4040  |        0x84       |  0x65626d6c  |
      |  After edit |  0x3B4040  |       0x4003      |   0x6d6b76   |
      +-------------+------------+-------------------+--------------+

   This method is only RECOMMENDED for reducing "Element Data" 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



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   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,
   the following table 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 octent "Void Element".

   +--------+------------+-----------------+-------------+-------------+
   | Status | Element ID |   Element Data  |   Element   |     Void    |
   |        |            |       Size      |     Data    |   Element   |
   +--------+------------+-----------------+-------------+-------------+
   | Before |  0x3B4040  |      0x4003     |   0x6d6b76  |             |
   | After  |  0x3B4040  |       0x82      |    0x6869   |    0xEC80   |
   +--------+------------+-----------------+-------------+-------------+

11.1.3.  Terminating Element Data

   For "String Elements" and "UTF-8 Elements" the length of "Element
   Data" MAY be reduced by adding "Null Octets" to terminate the
   "Element Data" (see Section 10).

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

   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.

11.2.  Considerations when Updating Elements with CRC

   If the "Element" to be changed is a "Descendant Element" of any
   "Master Element" that contains an "CRC-32 Element" 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.



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12.  EBML Document

   An "EBML Document" is comprised 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".

12.1.  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 14.2.1) and any number of additional "EBML Elements" within
   it.  The "EBML Header" MUST only contain "EBML Elements" that are
   defined as part of this document.

   All "EBML Elements" within the "EBML Header" MUST NOT use any
   "Element ID" with a length greater than 4 octets.  All "EBML
   Elements" within the "EBML Header" MUST NOT use any "Element Data
   Size" with a length greater than 4 octets.

12.2.  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 considered as
   whichever comes first: the beginning of a new "EBML Header" at the
   "Root Level" or the end of the file.  The "EBML Body" MUST consist
   only of "EBML Elements" and MUST NOT contain any data that is not
   part of an "EBML Element".  This document defines precisely what
   "EBML Elements" are to be used within the "EBML Header", but does not
   name or define what "EBML Elements" are to be used within the "EBML
   Body".  The definition of what "EBML Elements" are to be used within
   the "EBML Body" is defined by an "EBML Schema".





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13.  EBML Stream

   An "EBML Stream" is a file that consists of one or many "EBML
   Documents" that are concatenated together.  An occurrence of a "EBML
   Header" at the "Root Level" marks the beginning of an "EBML
   Document".

14.  Elements semantic

14.1.  EBML Schema

   An "EBML Schema" is an XML Document 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" may be considered analogous to the relationship of an
   XML Schema [W3C.REC-xmlschema-0-20010502] to an XML Document
   [W3C.REC-xml-20081126].  An "EBML Schema" MUST be clearly associated
   with one or many "EBML Document Types".  An "EBML Schema" must be
   expressed as well-formed XML.  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 14.2.6).  The "DocType"
   value for an "EBML Document Type" SHOULD be unique and persistent.

   An "EBML Schema" MUST declare exactly one "EBML Element" at "Root
   Level" (referred to as the "Root Element") that MUST occur exactly
   once within an "EBML Document".  The "Void Element" MAY also occur at
   "Root Level" but is not considered to be "Root Elements" (see
   Section 14.3.1).

   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").

   An "EBML Schema" MAY constrain the use of "EBML Header Elements" (see
   Section 14.2) 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 constain 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 14.1.11.







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14.1.1.  Element

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

14.1.2.  Attributes

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

14.1.2.1.  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.

14.1.2.2.  version

   The "version" lists an incremental 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.

14.1.3.  Element

   Each "<element>" defines one "EBML Element" through the use of
   several attributes that are defined in Section 14.1.2.  "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 many "<documentation>" sub-
   elements and zero or one "<restriction>" sub-element.  All
   "<element>" nodes MUST be sub-elements of the "<EBMLSchema>".






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14.1.4.  Attributes

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

14.1.4.1.  name

   The "name" provides the official 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 "name" attribute is REQUIRED.

14.1.4.2.  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           = EBMLElementOccurrence "(" EBMLReferencePath ")"
EBMLReferencePath        = [EBMLParentPath] EBMLElementPath
EBMLParentPath           = EBMLFixedParent EBMLLastParent
EBMLFixedParent          = *(EBMLPathAtom)
EBMLElementPath          = EBMLPathAtom / EBMLPathAtomRecursive
EBMLPathAtom             = PathDelimiter EBMLAtomName
EBMLPathAtomRecursive    = "(1*(" EBMLPathAtom "))"
EBMLLastParent           = EBMLPathAtom / EBMLVariableParent
EBMLVariableParent       = "(" VariableParentOccurrence "\)"
EBMLAtomName             = 1*(EBMLNameChar)
EBMLNameChar             = ALPHA / DIGIT / "-" / "."
PathDelimiter            = "\"
EBMLElementOccurrence    = [EBMLMinOccurrence] "*" [EBMLMaxOccurrence]
EBMLMinOccurrence        = 1*DIGIT
EBMLMaxOccurrence        = 1*DIGIT
VariableParentOccurrence = [PathMinOccurrence] "*" [PathMaxOccurrence]
PathMinOccurrence        = 1*DIGIT
PathMaxOccurrence        = 1*DIGIT

   The ""*"", ""("" and "")"" symbols MUST be interpreted as they are
   defined in the ABNF.





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   The "EBMLPathAtom" part of the "EBMLElementPath" MUST be equal to the
   "name" attribute of the "EBML Schema".

   The starting "PathDelimiter" of the path corresponds to the root of
   the "EBML Document".

   The "EBMLElementOccurrence" part is interpreted as an ABNF Variable
   Repetition.  The repetition amounts correspond to how many times the
   "EBML Element" can be found in its "Parent Element".

   The "EBMLMinOccurrence" represents the minimum number of occurrences
   of this "EBML Element" within its "Parent Element".  Each instance of
   the "Parent Element" MUST contain at least this many instances of
   this "EBML Element".  If the "EBML Element" has an empty
   "EBMLParentPath" then "EBMLMinOccurrence" refers to constraints on
   the occurrence of the "EBML Element" within the "EBML Document".  If
   "EBMLMinOccurrence" is not present then that "EBML Element" is
   considered to have a "EBMLMinOccurrence" value of 0.  The semantic
   meaning of "EBMLMinOccurrence" within an "EBML Schema" is considered
   analogous to the meaning of "minOccurs" within an "XML Schema".
   "EBML Elements" with "EBMLMinOccurrence" set to "1" that also have a
   "default" value (see Section 14.1.4.8) declared are not REQUIRED to
   be stored but are REQUIRED to be interpreted, see Section 14.1.15.
   An "EBML Element" defined with a "EBMLMinOccurrence" value greater
   than zero is called a "Mandatory EBML Element".

   The "EBMLMaxOccurrence" represents the maximum 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".  If the "EBML Element" has an empty "EBMLParentPath"
   then "EBMLMaxOccurrence" refers to constraints on the occurrence of
   the "EBML Element" within the "EBML Document".  If
   "EBMLMaxOccurrence" is not present then that "EBML Element" is
   considered to have no maximum occurrence.  The semantic meaning of
   "EBMLMaxOccurrence" within an "EBML Schema path" is considered
   analogous to the meaning of "maxOccurs" within an "XML Schema".

   The "VariableParentOccurrence" part is interpreted as an ABNF
   Variable Repetition.  The repetition amounts correspond to the amount
   of unspecified "Parent Element" levels there can be between the
   "EBMLFixedParent" and the actual "EBMLElementPath".

   If the path contains a "EBMLPathAtomRecursive" part, the "EBML
   Element" can occur within itself recursively (see the
   Section 14.1.4.11).






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14.1.4.3.  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 "id" attribute is REQUIRED.

14.1.4.4.  minOccurs

   An integer expressing the minimum number of occurrences of this "EBML
   Element" within its "Parent Element".  The "minOccurs" value MUST be
   equal to the "EBMLMinOccurrence" value of the "path".

   The "minOccurs" attribute is OPTIONAL.  If the "minOccurs" attribute
   is not present then that "EBML Element" is considered to have a
   "minOccurs" value of 0.

14.1.4.5.  maxOccurs

   An integer expressing the maximum number of occurrences of this "EBML
   Element" within its "Parent Element".  The "maxOccurs" value MUST be
   equal to the "EBMLMaxOccurrence" value of the "path".

   The "maxOccurs" attribute is OPTIONAL.  If the "maxOccurs" attribute
   is not present then that "EBML Element" is considered to have no
   maximum occurrence, similar to "unbounded" in the XML world.

14.1.4.6.  range

   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 14.1.13 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.







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14.1.4.7.  size

   A value to express the valid length of the "Element Data" as written
   measured in octets.  The "size" provides a constraint in addition to
   the Length value of the definition of the corresponding "EBML Element
   Type".  This "size" MUST be expressed as either a non-negative
   integer or a range (see Section 14.1.13) that consists of only non-
   negative integers and valid operators.

   The "size" attribute is OPTIONAL.  If the "size" attribute is not
   present for that "EBML Element" then that "EBML Element" is only
   limited in size by the definition of the associated "EBML Element
   Type".

14.1.4.8.  default

   If an Element is mandatory (has a "EBMLMinOccurrence" 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".  "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.

14.1.4.9.  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 9.

   The "type" attribute is REQUIRED.

14.1.4.10.  unknownsizeallowed

   A boolean to express if an "EBML Element" MAY be used as an "Unknown-
   Sized Element" (having all "VINT_DATA" bits of "Element Data Size"
   set to 1).  "EBML Elements" that are not "Master Elements" MUST NOT
   set "unknownsizeallowed" to true.  An "EBML Element" that is defined
   with an "unknownsizeallowed" attribute set to 1 MUST also have the
   "unknownsizeallowed" attribute of its "Parent Element" set to 1.

   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".



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14.1.4.11.  recursive

   A boolean to express if an "EBML Element" MAY 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 "path" contains a "EBMLPathAtomRecursive" part then the
   "recursive" value MUST be true and false otherwise.

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

14.1.4.12.  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".

14.1.4.13.  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>".

14.1.5.  Element

   The "<documentation>" element provides additional information about
   the "EBML Element".

14.1.6.  Attributes

14.1.6.1.  lang

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

   The "lang" attribute is OPTIONAL.



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14.1.6.2.  type

   A "type" attribute distinguishes the meaning of the documentation.
   Values for the "<documentation>" sub-element's "type" attribute MUST
   include one of the following: "definition", "rationale", "usage
   notes", and "references".

   The "type" attribute is OPTIONAL.

14.1.7.  Element

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

14.1.8.  Element

   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">" can not be a valid value
   for a "EBML Element" that is defined as an unsigned integer).  An
   "<enum>" element MAY also store "<documentation>" elements to further
   describe the "<enum>".

14.1.9.  Attributes

14.1.9.1.  label

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

   The "label" attribute is OPTIONAL.

14.1.9.2.  value

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

   The "value" attribute is REQUIRED.

14.1.10.  XML Schema for EBML Schema

  <?xml version="1.0" encoding="UTF-8"?>
  <xsd:schema xmlns="https://ietf.org/cellar/ebml"
    targetNamespace="https://ietf.org/cellar/ebml"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="qualified" version="01">
    <xsd:element name="EBMLSchema" type="EBMLSchemaType"/>



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    <xsd:complexType name="EBMLSchemaType">
      <xsd:sequence>
        <xsd:element name="element" type="elementType" minOccurs="0"
          maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:attribute name="docType" use="required"/>
      <xsd:attribute name="version" use="required"/>
    </xsd:complexType>
    <xsd:complexType name="elementType">
      <xsd:sequence>
        <xsd:element name="documentation" type="documentationType"
          minOccurs="0" maxOccurs="unbounded"/>
        <xsd:element name="restriction" type="restrictionType"
          minOccurs="0" maxOccurs="1"/>
      </xsd:sequence>
      <xsd:attribute name="name" use="required"/>
      <xsd:attribute name="path" use="required"/>
      <xsd:attribute name="id" use="required"/>
      <xsd:attribute name="minOccurs" default="0"/>
      <xsd:attribute name="maxOccurs" default="1"/>
      <xsd:attribute name="range"/>
      <xsd:attribute name="size"/>
      <xsd:attribute name="default"/>
      <xsd:attribute name="type" use="required"/>
      <xsd:attribute name="unknownsizeallowed"/>
      <xsd:attribute name="recursive"/>
      <xsd:attribute name="minver" default="1"/>
      <xsd:attribute name="maxver"/>
    </xsd:complexType>
    <xsd:complexType name="restrictionType">
      <xsd:sequence>
        <xsd:element name="enum" type="enumType" minOccurs="0"
          maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
    <xsd:complexType name="enumType">
      <xsd:sequence>
        <xsd:element name="documentation" type="documentationType"
          minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:attribute name="label"/>
      <xsd:attribute name="value" use="required"/>
    </xsd:complexType>
    <xsd:complexType name="documentationType" mixed="true">
      <xsd:sequence>
        <xsd:any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:attribute name="lang"/>



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      <xsd:attribute name="type"/>
    </xsd:complexType>
  </xsd:schema>

14.1.11.  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="1*1(\EBML\EBMLReadVersion)"
    id="0x42F7" minOccurs="1" maxOccurs="1" range="1" default="1"
    type="uinteger"/>
  <element name="EBMLMaxSizeLength" path="1*1(\EBML\EBMLMaxSizeLength)"
    id="0x42F3" minOccurs="1" maxOccurs="1" range="8" default="8"
    type="uinteger"/>
  <!-- Root Element-->
  <element name="Files" path="*1(\Files)" id="0x1946696C" type="master">
   <documentation lang="en" type="definition">Container of data and
   attributes representing one or many files.</documentation>
  </element>
  <element name="File" path="1*(\Files\File)" id="0x6146" type="master"
    minOccurs="1">
   <documentation lang="en" type="definition">
     An attached file.
   </documentation>
  </element>
  <element name="FileName" path="1*1(\Files\File\FileName)"
    id="0x614E" type="utf-8"
    minOccurs="1">
   <documentation lang="en" type="definition">
     Filename of the attached file.
   </documentation>
  </element>
  <element name="MimeType" path="1*1(\Files\File\MimeType)"
    id="0x464D" type="string"
      minOccurs="1">
   <documentation lang="en" type="definition">
     MIME type of the file.
   </documentation>
  </element>
  <element name="ModificationTimestamp"
    path="1*1(\Files\File\ModificationTimestamp)" id="0x4654"
    type="date" minOccurs="1">
   <documentation lang="en" type="definition">
     Modification timestamp of the file.
   </documentation>
  </element>



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  <element name="Data" path="1*1(\Files\File\Data)" id="0x4664"
    type="binary" minOccurs="1">
   <documentation lang="en" type="definition">
     The data of the file.
   </documentation>
  </element>
 </EBMLSchema>

14.1.12.  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 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.

14.1.13.  Expression of range

   The "range" attribute MUST only be used with "EBML Elements" that are
   either "signed integer", "unsigned integer", "float", or "date".  The
   "range" expression may contain whitespace for readability but
   whitespace within a "range" expression MUST NOT convey meaning.  The
   expression of the "range" MUST adhere to one of the following forms:

   o  "x-y" where x and y are integers or floats and "y" MUST be greater
      than "x", meaning that the value MUST be greater than or equal to
      "x" and less than or equal to "y". "x" MUST be less than "y".

   o  ">x" where "x" is an integer or float, meaning that the value MUST
      be greater than "x".





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   o  ">=x" where "x" is an integer or float, meaning that the value
      MUST be greater than or equal to "x".

   o  "<x" where "x" is an integer or float, meaning that the value MUST
      be less than "x".

   o  "<=x" where "x" is an integer or float, meaning that the value
      MUST be less than or equal to "x".

   o  "x" where "x" is an integer or float, meaning that the value MUST
      be equal "x".

   The "range" may use the prefix "not" to indicate that the expressed
   range is negated.  Please also see Section 14.1.14.

14.1.14.  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).  The following table provides examples of expressions of
   float ranges.

      +-------------------+-----------------------------------------+
      | as decimal        | as Hexadecimal Floating-Point Constants |
      +-------------------+-----------------------------------------+
      | 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"                    |
      +-------------------+-----------------------------------------+

   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-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.







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14.1.15.  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 interpreted 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.

   This table 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    |  Is the value | Is the Parent |  Then is storing  |
   | default value |    equal to   |    Element    |  the EBML Element |
   |   declared?   |    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        |
   +---------------+---------------+---------------+-------------------+

14.2.  EBML Header Elements

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

14.2.1.  EBML Element

   name: "EBML"

   path: "1*1(\EBML)"

   id: "0x1A45DFA3"



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   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.

14.2.2.  EBMLVersion Element

   name: "EBMLVersion"

   path: "1*1(\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.

14.2.3.  EBMLReadVersion Element

   name: "EBMLReadVersion"

   path: "1*1(\EBML\EBMLReadVersion)"

   id: "0x42F7"

   minOccurs: 1

   maxOccurs: 1

   range: 1

   default: 1

   type: Unsigned Integer



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   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".

14.2.4.  EBMLMaxIDLength Element

   name: "EBMLMaxIDLength"

   path: "1*1(\EBML\EBMLMaxIDLength)"

   id "0x42F2"

   minOccurs: 1

   maxOccurs: 1

   range: >=4

   default: 4

   type: Unsigned Integer

   description: The "EBMLMaxIDLength Element" stores the maximum 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.

14.2.5.  EBMLMaxSizeLength Element

   name: "EBMLMaxSizeLength"

   path: "1*1(\EBML\EBMLMaxSizeLength)"

   id "0x42F3"

   minOccurs: 1

   maxOccurs: 1

   range: not 0

   default: 8

   type: Unsigned Integer

   description: The "EBMLMaxSizeLength Element" stores the maximum
   length in octets of the expression of all "Element Data Sizes" to be
   found within the "EBML Body".  To be clear the "EBMLMaxSizeLength



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   Element" documents the maximum 'length' of all "Element Data Size"
   expressions within the "EBML Body" and not the maximum '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" SHALL
   be considered invalid.

14.2.6.  DocType Element

   name: "DocType"

   path: "1*1(\EBML\DocType)"

   id "0x4282"

   minOccurs: 1

   maxOccurs: 1

   size: >0

   type: String

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

14.2.7.  DocTypeVersion Element

   name: "DocTypeVersion"

   path: "1*1(\EBML\DocTypeVersion)"

   id "0x4287"

   minOccurs: 1

   maxOccurs: 1

   default: 1

   type: Unsigned Integer

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







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14.2.8.  DocTypeReadVersion Element

   name: DocTypeReadVersion

   path: "1*1(\EBML\DocTypeReadVersion)"

   id "0x4285"

   minOccurs: 1

   maxOccurs: 1

   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".

14.3.  Global elements (used everywhere in the format)

   name: CRC-32

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

   id: "0xBF"

   minOccurs: 0

   maxOccurs: 1

   size: 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 bitstream and MUST use little endian storage.



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14.3.1.  Void Element

   name: Void

   path: "*((*\)\Void)"

   id: "0xEC"

   minOccurs: 0

   type: Binary

   description: Used to void damaged data, to avoid unexpected behaviors
   when using damaged data.  The content is discarded.  Also used to
   reserve space in a sub-element for later use.

15.  References

15.1.  Normative References

   [IEEE.754.1985]
              Institute of Electrical and Electronics Engineers,
              "Standard for Binary Floating-Point Arithmetic",
              IEEE Standard 754, August 1985.

   [ISO.3309.1979]
              International Organization for Standardization, "Data
              communication - High-level data link control procedures -
              Frame structure", ISO Standard 3309, 1979.

   [ISO.9899.2011]
              International Organization for Standardization,
              "Programming languages - C", ISO Standard 9899, 2011.

   [ITU.V42.1994]
              International Telecommunications Union, "Error-correcting
              Procedures for DCEs Using Asynchronous-to-Synchronous
              Conversion", ITU-T Recommendation V.42, 1994.

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

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




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

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

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

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

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

   [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, November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

15.2.  Informative References

   [W3C.REC-xmlschema-0-20010502]
              Fallside, D., "XML Schema Part 0: Primer", World Wide Web
              Consortium Recommendation REC-xmlschema-0-20010502, May
              2001,
              <http://www.w3.org/TR/2001/REC-xmlschema-0-20010502>.

Authors' Addresses

   Steve Lhomme

   Email: slhomme@matroska.org


   Dave Rice

   Email: dave@dericed.com





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   Moritz Bunkus

   Email: moritz@bunkus.org
















































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