RFC 8794 | EBML | July 2020
Lhomme, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  8794
Category:
  Standards Track
Published:
  July 2020
ISSN:
  2070-1721
Authors:
  S. Lhomme
  D. Rice
  M. Bunkus

# RFC 8794

# Extensible Binary Meta Language

## Abstract

This document defines the Extensible Binary Meta Language (EBML) format as a binary container format designed for audio/video storage. EBML is designed as a binary equivalent to XML and uses a storage-efficient approach to build nested Elements with identifiers, lengths, and values. Similar to how an XML Schema defines the structure and semantics of an XML Document, this document defines how EBML Schemas are created to convey the semantics of an EBML Document.¶

## Status of This Memo

This is an Internet Standards Track document.¶

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

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

## Copyright Notice

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

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

## 1. Introduction

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

The goal of this document is to define a generic, binary, space-efficient format that can be used to define more complex formats using an EBML Schema. EBML is used by the multimedia container, Matroska [Matroska]. The applicability of EBML for other use cases is beyond the scope of this document.¶

The definition of the EBML format recognizes the idea behind HTML and XML as a good one: separate structure and semantics allowing the same structural layer to be used with multiple, possibly widely differing, semantic layers. Except for the EBML Header and a few Global Elements, this specification does not define particular EBML format semantics; however, this specification is intended to define how other EBML-based formats can be defined, such as the audio/video container formats Matroska and WebM [WebM].¶

EBML uses a simple approach of building Elements upon three pieces of data (tag, length, and value), as this approach is well known, easy to parse, and allows selective data parsing. The EBML structure additionally allows for hierarchical arrangement to support complex structural formats in an efficient manner.¶

A typical EBML file has the following structure:¶

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

¶

## 2. Notation and Conventions

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

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

`EBML`:
  Extensible Binary Meta Language¶
`EBML Document Type`:
  A name provided by an `EBML Schema` to designate a particular implementation of `EBML` for a data format (e.g., Matroska and WebM).¶
`EBML Schema`:
  A standardized definition for the structure of an `EBML Document Type`.¶
`EBML Document`:
  A datastream comprised of only two components, an `EBML Header` and an `EBML Body`.¶
`EBML Reader`:
  A data parser that interprets the semantics of an `EBML Document` and creates a way for programs to use `EBML`.¶
`EBML Stream`:
  A file that consists of one or more `EBML Documents` that are concatenated together.¶
`EBML Header`:
  A declaration that provides processing instructions and identification of the `EBML Body`. The `EBML Header` is analogous to an XML Declaration [XML] (see Section 2.8 on "Prolog and Document Type Declaration").¶
`EBML Body`:
  All data of an `EBML Document` following the `EBML Header`.¶
`Variable-Size Integer`:
  A compact variable-length binary value that defines its own length.¶
`VINT`:
  Also known as `Variable-Size Integer`.¶
`EBML Element`:
  A foundation block of data that contains three parts: an `Element ID`, an `Element Data Size`, and `Element Data`.¶
`Element ID`:
  A binary value, encoded as a `Variable-Size Integer`, used to uniquely identify a defined `EBML Element` within a specific `EBML Schema`.¶
`Element Data Size`:
  An expression, encoded as a `Variable-Size Integer`, of the length in octets of `Element Data`.¶
`VINTMAX`:
  The maximum possible value that can be stored as `Element Data Size`.¶
`Unknown-Sized Element`:
  An `Element` with an unknown `Element Data Size`.¶
`Element Data`:
  The value(s) of the `EBML Element`, which is identified by its `Element ID` and `Element Data Size`. The form of the `Element Data` is defined by this document and the corresponding `EBML Schema` of the Element's `EBML Document Type`.¶
`Root Level`:
  The starting level in the hierarchy of an `EBML Document`.¶
`Root Element`:
  A mandatory, nonrepeating `EBML Element` that occurs at the top level of the path hierarchy within an `EBML Body` and contains all other `EBML Elements` of the `EBML Body`, excepting optional `Void Elements`.¶
`Top-Level Element`:
  An `EBML Element` defined to only occur as a `Child Element` of the `Root Element`.¶
`Master Element`:
  The `Master Element` contains zero, one, or many other `EBML Elements`.¶
`Child Element`:
  A `Child Element` is a relative term to describe the `EBML Elements` immediately contained within a `Master Element`.¶
`Parent Element`:
  A relative term to describe the `Master Element` that contains a specified element. For any specified `EBML Element` that is not at `Root Level`, the `Parent Element` refers to the `Master Element` in which that `EBML Element` is directly contained.¶
`Descendant Element`:
  A relative term to describe any `EBML Elements` contained within a `Master Element`, including any of the `Child Elements` of its `Child Elements`, and so on.¶
`Void Element`:
  An `Element` used to overwrite data or reserve space within a `Master Element` for later use.¶
`Element Name`:
  The human-readable name of the `EBML Element`.¶
`Element Path`:
  The hierarchy of `Parent Element` where the `EBML Element` is expected to be found in the `EBML Body`.¶
`Empty Element`:
  An `EBML Element` that has an `Element Data Size` with all `VINT_DATA` bits set to zero, which indicates that the `Element Data` of the `Element` is zero octets in length.¶

## 3. Structure

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

## 4. Variable-Size Integer

The Element ID and Element Data Size are both encoded as a Variable-Size Integer. The Variable-Size Integer is composed of a VINT_WIDTH, VINT_MARKER, and VINT_DATA, in that order. Variable-Size Integers MUST left-pad the VINT_DATA value with zero bits so that the whole Variable-Size Integer is octet aligned. The Variable-Size Integer will be referred to as VINT for shorthand.¶

### 4.1. VINT_WIDTH

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

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

### 4.2. VINT_MARKER

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

### 4.3. VINT_DATA

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

### 4.4. VINT Examples

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

Table 1: VINT examples depicting usable bits
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

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.¶

Table 2: VINT examples depicting the same integer value rendered at different VINT lengths
Integer | Octet Length | As Represented in VINT (binary) | As Represented in VINT (hexadecimal)
--- | --- | --- | ---
2 | 1 | 1000 0010 | 0x82
2 | 2 | 0100 0000 0000 0010 | 0x4002
2 | 3 | 0010 0000 0000 0000 0000 0010 | 0x200002
2 | 4 | 0001 0000 0000 0000 0000 0000 0000 0010 | 0x10000002

## 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:¶

Table 3: Examples of valid and invalid Element IDs
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 range and count of possible Element IDs are determined by their octet length. Examples of this are provided in Table 4.¶

Table 4: Examples of count and range for Element IDs at various octet lengths
Element ID Octet Length | Range of Valid Element IDs | Number of Valid Element IDs
--- | --- | ---
1 | 0x81 - 0xFE | 126
2 | 0x407F - 0x7FFE | 16,256
3 | 0x203FFF - 0x3FFFFE | 2,080,768
4 | 0x101FFFFF - 0x1FFFFFFE | 268,338,304

## 6. Element Data Size

### 6.1. Data Size Format

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

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

### 6.2. Unknown Data Size

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

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

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

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

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

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

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

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

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

### 6.3. Data Size Values

For Element Data Sizes encoded at octet lengths from one to eight, Table 6 depicts the range of possible values that can be encoded as an Element Data Size. An Element Data Size with an octet length of 8 is able to express a size of 256-2 or 72,057,594,037,927,934 octets (or about 72 petabytes). The maximum possible value that can be stored as Element Data Size is referred to as VINTMAX.¶

Table 6: Possible range of values that can be stored in VINTs, by octet length
Octet Length | Possible Value Range
--- | ---
1 | 0 to 27 - 2
2 | 0 to 214 - 2
3 | 0 to 221 - 2
4 | 0 to 228 - 2
5 | 0 to 235 - 2
6 | 0 to 242 - 2
7 | 0 to 249 - 2
8 | 0 to 256 - 2

If the length of Element Data equals 2n*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 21*7-1) and 16,383 (which is equal to 22*7-1).¶

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

## 7. EBML Element Types

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

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

### 7.1. Signed Integer Element

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

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

### 7.2. Unsigned Integer Element

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

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

### 7.3. Float Element

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

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

### 7.4. String Element

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

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

### 7.5. UTF-8 Element

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

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

### 7.6. Date Element

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

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

### 7.7. Master Element

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

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

### 7.8. Binary Element

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

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

## 8. EBML Document

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

### 8.1. EBML Header

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

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

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

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

### 8.2. EBML Body

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

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

## 9. EBML Stream

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

## 10. EBML Versioning

An EBML Document handles 2 different versions: the version of the EBML Header and the version of the EBML Body. Both versions are meant to be backward compatible.¶

### 10.1. EBML Header Version

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

### 10.2. EBML Document Version

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

## 11. Elements semantics

### 11.1. EBML Schema

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

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

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

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

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

#### 11.1.1. EBML Schema Example

    <?xml version="1.0" encoding="utf-8"?>
    <EBMLSchema xmlns="urn:ietf:rfc:8794"
      docType="files-in-ebml-demo" version="1">
     <!-- constraints to the range of two EBML Header Elements -->
     <element name="EBMLReadVersion" path="\EBML\EBMLReadVersion"
       id="0x42F7" minOccurs="1" maxOccurs="1" range="1" default="1"
       type="uinteger"/>
     <element name="EBMLMaxSizeLength"
       path="\EBML\EBMLMaxSizeLength" id="0x42F3" minOccurs="1"
       maxOccurs="1" range="8" default="8" type="uinteger"/>
     <!-- Root Element-->
     <element name="Files" path="\Files" id="0x1946696C"
       type="master">
      <documentation lang="en"
        purpose="definition">Container of data and
      attributes representing one or many files.</documentation>
     </element>
     <element name="File" path="\Files\File" id="0x6146"
       type="master" minOccurs="1">
      <documentation lang="en" purpose="definition">
        An attached file.
      </documentation>
     </element>
     <element name="FileName" path="\Files\File\FileName"
       id="0x614E" type="utf-8"
       minOccurs="1">
      <documentation lang="en" purpose="definition">
        Filename of the attached file.
      </documentation>
     </element>
     <element name="MimeType" path="\Files\File\MimeType"
       id="0x464D" type="string"
         minOccurs="1">
      <documentation lang="en" purpose="definition">
        MIME type of the file.
      </documentation>
     </element>
     <element name="ModificationTimestamp"
       path="\Files\File\ModificationTimestamp" id="0x4654"
       type="date" minOccurs="1">
      <documentation lang="en" purpose="definition">
        Modification timestamp of the file.
      </documentation>
     </element>
     <element name="Data" path="\Files\File\Data" id="0x4664"
       type="binary" minOccurs="1">
      <documentation lang="en" purpose="definition">
        The data of the file.
      </documentation>
     </element>
    </EBMLSchema>

¶

#### 11.1.2. `<EBMLSchema>` Element

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

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

#### 11.1.3. `<EBMLSchema>` Namespace

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

#### 11.1.4. `<EBMLSchema>` Attributes

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

##### 11.1.4.1. docType

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

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

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

##### 11.1.4.2. version

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

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

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

##### 11.1.4.3. ebml

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

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

#### 11.1.5. `<element>` Element

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

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

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

#### 11.1.6. `<element>` Attributes

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

##### 11.1.6.1. name

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

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

The `name` attribute is REQUIRED.¶

##### 11.1.6.2. path

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

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

The `path` attribute is REQUIRED.¶

    EBMLFullPath           = EBMLParentPath EBMLElement

    EBMLParentPath         = PathDelimiter [EBMLParents]

    EBMLParents            = 0*IntermediatePathAtom EBMLLastParent
    IntermediatePathAtom   = EBMLPathAtom / GlobalPlaceholder
    EBMLLastParent         = EBMLPathAtom / GlobalPlaceholder

    EBMLPathAtom           = [IsRecursive] EBMLAtomName PathDelimiter
    EBMLElement            = [IsRecursive] EBMLAtomName

    PathDelimiter          = "\"
    IsRecursive            = "+"
    EBMLAtomName           = ALPHA / DIGIT 0*EBMLNameChar
    EBMLNameChar           = ALPHA / DIGIT / "-" / "."

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

¶

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

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

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

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

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

The GlobalParentOccurrence part is interpreted as the number of valid EBMLPathAtom parts that can replace the GlobalPlaceholder in the path. PathMinOccurrence represents the minimum number of EBMLPathAtoms required to replace the GlobalPlaceholder. PathMaxOccurrence represents the maximum number of EBMLPathAtoms possible to replace the GlobalPlaceholder.¶

If PathMinOccurrence is not present, then that GlobalParentOccurrence has a PathMinOccurrence value of 0. If PathMaxOccurrence is not present, then there is no upper bound for the permitted number of EBMLPathAtoms possible to replace the GlobalPlaceholder. PathMaxOccurrence MUST NOT have the value 0, as it would mean no EBMLPathAtom can replace the GlobalPlaceholder, and the EBMLFullPath would be the same without that GlobalPlaceholder part. PathMaxOccurrence MUST be bigger than, or equal to, PathMinOccurrence.¶

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

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

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

##### 11.1.6.3. id

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

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

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

The `id` attribute is REQUIRED.¶

##### 11.1.6.4. minOccurs

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

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

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

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

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

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

##### 11.1.6.5. maxOccurs

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

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

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

The `maxOccurs` attribute is OPTIONAL. If the `maxOccurs` attribute is not present, then there is no upper bound for the permitted number of occurrences of this EBML Element within its Parent Element or within the EBML Document, depending on whether or not the EBMLParentPath of the EBML Element is empty.¶

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

##### 11.1.6.6. range

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

A numerical range for EBML Elements that are of numerical types (Unsigned Integer, Signed Integer, Float, and Date). If specified, the value of the EBML Element MUST be within the defined range. See Section 11.1.6.6.1 for rules applied to expression of range values.¶

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

###### 11.1.6.6.1. Expression of range

The `range` attribute MUST only be used with EBML Elements that are either signed integer, unsigned integer, float, or date. The expression defines the upper, lower, exact, or excluded value of the EBML Element and optionally an upper boundary value combined with a lower boundary. The range expression may contain whitespace (using the ASCII 0x20 character) for readability, but whitespace within a range expression MUST NOT convey meaning.¶

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

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

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

The lower and upper bounds can be combined into an expression to form a closed boundary. The lower boundary comes first, followed by the upper boundary, separated by a comma. For example, `>3,<= 20` means the Element value MUST be greater than 3 and less than or equal to 20.¶

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

##### 11.1.6.7. length

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

The `length` attribute is a value to express the valid length of the Element Data as written, measured in octets. The length provides a constraint in addition to the Length value of the definition of the corresponding EBML Element Type. This length MUST be expressed as either a nonnegative integer or a range (see Section 11.1.6.6.1) that consists of only nonnegative integers and valid operators.¶

The `length` attribute is OPTIONAL. If the `length` attribute is not present for that EBML Element, then that EBML Element is only limited in length by the definition of the associated EBML Element Type.¶

##### 11.1.6.8. default

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

If an Element is mandatory (has a `minOccurs` value greater than zero) but not written within its Parent Element or stored as an Empty Element, then the EBML Reader of the EBML Document MUST semantically interpret the EBML Element as present with this specified default value for the EBML Element. An unwritten mandatory Element with a declared default value is semantically equivalent to that Element if written with the default value stored as the Element Data. EBML Elements that are Master Elements MUST NOT declare a default value. EBML Elements with a `minOccurs` value greater than 1 MUST NOT declare a default value.¶

The default attribute is OPTIONAL.¶

##### 11.1.6.9. type

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

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

The `type` attribute is REQUIRED.¶

##### 11.1.6.10. unknownsizeallowed

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

This attribute is a boolean to express whether an EBML Element is permitted to be an Unknown-Sized Element (having all VINT_DATA bits of Element Data Size set to 1). EBML Elements that are not Master Elements MUST NOT set `unknownsizeallowed` to true. An EBML Element that is defined with an `unknownsizeallowed` attribute set to 1 MUST also have the `unknownsizeallowed` attribute of its Parent Element set to 1.¶

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

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

##### 11.1.6.11. recursive

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

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

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

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

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

##### 11.1.6.12. recurring

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

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

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

##### 11.1.6.13. minver

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

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

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

##### 11.1.6.14. maxver

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

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

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

#### 11.1.7. `<documentation>` Element

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

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

#### 11.1.8. `<documentation>` Attributes

##### 11.1.8.1. lang

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

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

The `lang` attribute is OPTIONAL.¶

##### 11.1.8.2. purpose

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

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

Table 8: Definitions of the permitted values for the `purpose` attribute of the documentation Element
value of `purpose` attribute | definition
--- | ---
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 notes | Recommended practices or guidelines for both reading, writing, or interpreting the Element.
references | Informational references to support the contextualization and understanding of the value of the Element.

The `purpose` attribute is REQUIRED.¶

#### 11.1.9. `<implementation_note>` Element

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

In some cases within an EBML Document Type, the attributes of the `<element>` element are not sufficient to clearly communicate how the defined EBML Element is intended to be implemented. For instance, one EBML Element might only be mandatory if another EBML Element is present. As another example, the default value of an EBML Element might be derived from a related Element's content. In these cases where the Element's definition is conditional or advanced implementation notes are needed, one or many `<implementation_note>` elements can be used to store that information. The `<implementation_note>` refers to a specific attribute of the parent `<element>` as expressed by the `note_attribute` attribute (see Section 11.1.10.1).¶

#### 11.1.10. `<implementation_note>` Attributes

##### 11.1.10.1. note_attribute

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

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

The `note_attribute` attribute is REQUIRED.¶

##### 11.1.10.2. `<implementation_note>` Example

The following fragment of an EBML Schema demonstrates how an `<implementation_note>` is used. In this case, an EBML Schema documents a list of items that are described with an optional cost. The Currency Element uses an `<implementation_note>` to say that the Currency Element is REQUIRED if the Cost Element is set, otherwise not.¶

    <element name="Items" path="\Items" id="0x4025" type="master"
      minOccurs="1" maxOccurs="1">
      <documentation lang="en" purpose="definition">
        A set of items.
      </documentation>
    </element>
    <element name="Item" path="\Items\Item" id="0x4026"
      type="master">
      <documentation lang="en" purpose="definition">
        An item.
      </documentation>
    </element>
    <element name="Cost" path="\Items\Item\Cost" id="0x4024"
      type="float" maxOccurs="1">
      <documentation lang="en" purpose="definition">
        The cost of the item, if any.
      </documentation>
    </element>
    <element name="Currency" path="\Items\Item\Currency" id="0x403F"
      type="string" maxOccurs="1">
      <documentation lang="en" purpose="definition">
        The currency of the item's cost.
      </documentation>
      <implementation_note note_attribute="minOccurs">
        Currency MUST be set (minOccurs=1) if the associated Item stores
        a Cost, else Currency MAY be unset (minOccurs=0).
      </implementation_note>
    </element>

¶

#### 11.1.11. `<restriction>` Element

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

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

#### 11.1.12. `<enum>` Element

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

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

#### 11.1.13. `<enum>` Attributes

##### 11.1.13.1. label

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

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

The `label` attribute is OPTIONAL.¶

##### 11.1.13.2. value

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

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

The `value` attribute is REQUIRED.¶

#### 11.1.14. `<extension>` Element

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

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

#### 11.1.15. `<extension>` Attributes

##### 11.1.15.1. type

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

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

The `type` attribute is REQUIRED.¶

#### 11.1.16. XML Schema for EBML Schema

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

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

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

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

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

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

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

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

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

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

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

¶

#### 11.1.17. Identically Recurring Elements

An Identically Recurring Element is an EBML Element that MAY occur within its Parent Element more than once, but each recurrence of it within that Parent Element MUST be identical both in storage and semantics. Identically Recurring Elements are permitted to be stored multiple times within the same Parent Element in order to increase data resilience and optimize the use of EBML in transmission. For instance, a pertinent Top-Level Element could be periodically resent within a datastream so that an EBML Reader that starts reading the stream from the middle could better interpret the contents. Identically Recurring Elements SHOULD include a CRC-32 Element as a Child Element; this is especially recommended when EBML is used for long-term storage or transmission. If a Parent Element contains more than one copy of an Identically Recurring Element that includes a CRC-32 Element as a Child Element, then the first instance of the Identically Recurring Element with a valid CRC-32 value should be used for interpretation. If a Parent Element contains more than one copy of an Identically Recurring Element that does not contain a CRC-32 Element, or if CRC-32 Elements are present but none are valid, then the first instance of the Identically Recurring Element should be used for interpretation.¶

#### 11.1.18. Textual expression of floats

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

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

Within an expression of a float range, as in an integer range, the - (hyphen) character is the separator between the minimum and maximum values permitted by the range. Hexadecimal Floating-Point Constants also use a - (hyphen) when indicating a negative binary power. Within a float range, when a - (hyphen) is immediately preceded by a letter p, then the - (hyphen) is a part of the Hexadecimal Floating-Point Constant that notes negative binary power. Within a float range, when a - (hyphen) is not immediately preceded by a letter p, then the - (hyphen) represents the separator between the minimum and maximum values permitted by the range.¶

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

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

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

Table 10 clarifies whether a Mandatory EBML Element MUST be written, according to whether the default value is declared, the value of the EBML Element is equal to the declared default value, and/or the Parent Element is used.¶

Table 10: Demonstration of the conditional requirements of VINT Storage
Is the default value declared? | Is the value equal to default? | Is the Parent Element present? | Then is storing the EBML Element 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

### 11.2. EBML Header Elements

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

#### 11.2.1. EBML Element

name:
  EBML¶
path:
  `\EBML`¶
id:
  0x1A45DFA3¶
minOccurs:
  1¶
maxOccurs:
  1¶
type:
  Master Element¶
description:
  Set the EBML characteristics of the data to follow. Each EBML Document has to start with this.¶

#### 11.2.2. EBMLVersion Element

name:
  EBMLVersion¶
path:
  `\EBML\EBMLVersion`¶
id:
  0x4286¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  not 0¶
default:
  1¶
type:
  Unsigned Integer¶
description:
  The version of EBML specifications used to create the EBML Document. The version of EBML defined in this document is 1, so EBMLVersion SHOULD be 1.¶

#### 11.2.3. EBMLReadVersion Element

name:
  EBMLReadVersion¶
path:
  `\EBML\EBMLReadVersion`¶
id:
  0x42F7¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  1¶
default:
  1¶
type:
  Unsigned Integer¶
description:
  The minimum EBML version an EBML Reader has to support to read this EBML Document. The EBMLReadVersion Element MUST be less than or equal to EBMLVersion.¶

#### 11.2.4. EBMLMaxIDLength Element

name:
  EBMLMaxIDLength¶
path:
  `\EBML\EBMLMaxIDLength`¶
id:
  0x42F2¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  >=4¶
default:
  4¶
type:
  Unsigned Integer¶
description:
  The EBMLMaxIDLength Element stores the maximum permitted length in octets of the Element IDs to be found within the EBML Body. An EBMLMaxIDLength Element value of four is RECOMMENDED, though larger values are allowed.¶

#### 11.2.5. EBMLMaxSizeLength Element

name:
  EBMLMaxSizeLength¶
path:
  `\EBML\EBMLMaxSizeLength`¶
id:
  0x42F3¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  not 0¶
default:
  8¶
type:
  Unsigned Integer¶
description:
  The EBMLMaxSizeLength Element stores the maximum permitted length in octets of the expressions of all Element Data Sizes to be found within the EBML Body. The EBMLMaxSizeLength Element documents an upper bound for the `length` of all Element Data Size expressions within the EBML Body and not an upper bound for the `value` of all Element Data Size expressions within the EBML Body. EBML Elements that have an Element Data Size expression that is larger in octets than what is expressed by EBMLMaxSizeLength Element are invalid.¶

#### 11.2.6. DocType Element

name:
  DocType¶
path:
  `\EBML\DocType`¶
id:
  0x4282¶
minOccurs:
  1¶
maxOccurs:
  1¶
length:
  >0¶
type:
  String¶
description:
  A string that describes and identifies the content of the EBML Body that follows this EBML Header.¶

#### 11.2.7. DocTypeVersion Element

name:
  DocTypeVersion¶
path:
  `\EBML\DocTypeVersion`¶
id:
  0x4287¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  not 0¶
default:
  1¶
type:
  Unsigned Integer¶
description:
  The version of DocType interpreter used to create the EBML Document.¶

#### 11.2.8. DocTypeReadVersion Element

name:
  DocTypeReadVersion¶
path:
  `\EBML\DocTypeReadVersion`¶
id:
  0x4285¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  not 0¶
default:
  1¶
type:
  Unsigned Integer¶
description:
  The minimum DocType version an EBML Reader has to support to read this EBML Document. The value of the DocTypeReadVersion Element MUST be less than or equal to the value of the DocTypeVersion Element.¶

#### 11.2.9. DocTypeExtension Element

name:
  DocTypeExtension¶
path:
  `\EBML\DocTypeExtension`¶
id:
  0x4281¶
minOccurs:
  0¶
type:
  Master Element¶
description:
  A DocTypeExtension adds extra Elements to the main DocType+DocTypeVersion tuple it's attached to. An EBML Reader MAY know these extra Elements and how to use them. A DocTypeExtension MAY be used to iterate between experimental Elements before they are integrated into a regular DocTypeVersion. Reading one DocTypeExtension version of a DocType+DocTypeVersion tuple doesn't imply one should be able to read upper versions of this DocTypeExtension.¶

#### 11.2.10. DocTypeExtensionName Element

name:
  DocTypeExtensionName¶
path:
  `\EBML\DocTypeExtension\DocTypeExtensionName`¶
id:
  0x4283¶
minOccurs:
  1¶
maxOccurs:
  1¶
length:
  >0¶
type:
  String¶
description:
  The name of the DocTypeExtension to differentiate it from other DocTypeExtensions of the same DocType+DocTypeVersion tuple. A DocTypeExtensionName value MUST be unique within the EBML Header.¶

#### 11.2.11. DocTypeExtensionVersion Element

name:
  DocTypeExtensionVersion¶
path:
  `\EBML\DocTypeExtension\DocTypeExtensionVersion`¶
id:
  0x4284¶
minOccurs:
  1¶
maxOccurs:
  1¶
range:
  not 0¶
type:
  Unsigned Integer¶
description:
  The version of the DocTypeExtension. Different DocTypeExtensionVersion values of the same DocType + DocTypeVersion + DocTypeExtensionName tuple MAY contain completely different sets of extra Elements. An EBML Reader MAY support multiple versions of the same tuple, only one version of the tuple, or not support the tuple at all.¶

### 11.3. Global Elements

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

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

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

#### 11.3.1. CRC-32 Element

name:
  CRC-32¶
path:
  `\(1-\)CRC-32`¶
id:
  0xBF¶
minOccurs:
  0¶
maxOccurs:
  1¶
length:
  4¶
type:
  Binary¶
description:
  The CRC-32 Element contains a 32-bit Cyclic Redundancy Check value of all the Element Data of the Parent Element as stored except for the CRC-32 Element itself. When the CRC-32 Element is present, the CRC-32 Element MUST be the first ordered EBML Element within its Parent Element for easier reading. All Top-Level Elements of an EBML Document that are Master Elements SHOULD include a CRC-32 Element as a Child Element. The CRC in use is the IEEE-CRC-32 algorithm as used in the [ISO3309] standard and in Section 8.1.1.6.2 of [ITU.V42], with initial value of 0xFFFFFFFF. The CRC value MUST be computed on a little-endian bytestream and MUST use little-endian storage.¶

#### 11.3.2. Void Element

name:
  Void¶
path:
  `\(-\)Void`¶
id:
  0xEC¶
minOccurs:
  0¶
type:
  Binary¶
description:
  Used to void data or to avoid unexpected behaviors when using damaged data. The content is discarded. Also used to reserve space in a subelement for later use.¶

## 12. Considerations for Reading EBML Data

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

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

In the following XML representation of a simple, hypothetical EBML fragment, a Master Element called CONTACT contains two Child Elements, NAME and ADDRESS. In this example, some data within the NAME Element had been altered so that the CRC-32 of the NAME Element does not validate, and thus any Ancestor Element with a CRC-32 would therefore also no longer validate. However, even though the CONTACT Element has a CRC-32 that does not validate (because of the changed data within the NAME Element), the CRC-32 of the ADDRESS Element does validate, and thus the contents and semantics of the ADDRESS Element MAY be used.¶

    <CONTACT>
        <CRC-32>c119a69b</CRC-32><!-- does not validate -->
        <NAME>
            <CRC-32>1f59ee2b</CRC-32><!-- does not validate -->
            <FIRST-NAME>invalid data</FIRST-NAME>
            <LAST-NAME>invalid data</LAST-NAME>
        </NAME>
        <ADDRESS>
            <CRC-32>df941cc9</CRC-32><!-- validates -->
            <STREET>valid data</STREET>
            <CITY>valid data</CITY>
        </ADDRESS>
    </CONTACT>

¶

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

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

## 13. Terminating Elements

Null Octets, which are octets with all bits set to zero, MAY follow the value of a String Element or UTF-8 Element to serve as a terminator. An EBML Writer MAY terminate a String Element or UTF-8 Element with Null Octets in order to overwrite a stored value with a new value of lesser length while maintaining the same Element Data Size; this can prevent the need to rewrite large portions of an EBML Document. Otherwise, the use of Null Octets within a String Element or UTF-8 Element is NOT RECOMMENDED. The Element Data of a UTF-8 Element MUST be a valid UTF-8 string up to whichever comes first: the end of the Element or the first occurring Null octet. Within the Element Data of a String or UTF-8 Element, any Null octet itself and any following data within that Element SHOULD be ignored. A string value and a copy of that string value terminated by one or more Null Octets are semantically equal.¶

Table 11 shows examples of semantics and validation for the use of Null Octets. Values to represent Stored Values and the Semantic Meaning as represented as hexadecimal values.¶

Table 11: Examples of semantics for Null Octets in VINT_DATA
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

## 14. Guidelines for Updating Elements

An EBML Document can be updated without requiring that the entire EBML Document be rewritten. These recommendations describe strategies for changing the Element Data of a written EBML Element with minimal disruption to the rest of the EBML Document.¶

### 14.1. Reducing Element Data in Size

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

#### 14.1.1. Adding a Void Element

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

#### 14.1.2. Extending the Element Data Size

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

For example, the first row of Table 12 depicts a String Element that stores an Element ID (3 octets), Element Data Size (1 octet), and Element Data (4 octets). If the Element Data is changed to reduce the length by one octet, and if the current length of the Element Data Size is less than its maximum permitted length, then the Element Data Size of that Element MAY be rewritten to increase its length by one octet. Thus, before and after the change, the EBML Element maintains the same length of 8 octets, and data around the Element does not need to be moved.¶

Table 12: Example of editing a VINT to reduce VINT_DATA length by one octet
Status | Element ID | Element Data Size | Element Data
--- | --- | --- | ---
Before edit | 0x3B4040 | 0x84 | 0x65626D6C
After edit | 0x3B4040 | 0x4003 | 0x6D6B76

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.¶

Table 13: Example of editing a VINT to reduce VINT_DATA length by more than one octet
Status | Element ID | Element Data Size | Element Data | Void Element
--- | --- | --- | --- | ---
Before | 0x3B4040 | 0x4003 | 0x6D6B76 |
After | 0x3B4040 | 0x82 | 0x6869 | 0xEC80

#### 14.1.3. Terminating Element Data

For String Elements and UTF-8 Elements, the length of Element Data could be reduced by adding Null Octets to terminate the Element Data (see Section 13).¶

In Table 14, Element Data four octets long is changed to a value three octets long, followed by a Null Octet; the Element Data Size includes any Null Octets used to terminate Element Data and therefore remains unchanged.¶

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

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

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

## 15. Backward and Forward Compatibility

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

### 15.1. Backward Compatibility

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

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

### 15.2. Forward Compatibility

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

## 16. Security Considerations

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

Even if the semantic layer offers any kind of encryption, EBML itself could leak information at both the semantic layer (as declared via the DocType Element) and within the EBML structure (the presence of EBML Elements can be derived even with an unknown semantic layer using a heuristic approach -- not without errors, of course, but with a certain degree of confidence).¶

An EBML Document that has the following issues may still be handled by the EBML Reader and the data accepted as such, depending on how strict the EBML Reader wants to be:¶

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

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

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

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

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

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

- Invalid Element Data Size values (e.g., extending the length of the EBML Element beyond the scope of the Parent Element, possibly triggering access-out-of-bounds issues).¶

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

- Missing EBML Elements that are mandatory in a Master Element and have no declared default value, making the semantic invalid at that Master Element level.¶

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

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

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

Side-channel attacks could exploit:¶

- The semantic equivalence of the same string stored in a String Element or UTF-8 Element with and without zero-bit padding, making comparison at the semantic level invalid.¶

- The semantic equivalence of VINT_DATA within Element Data Size with two different lengths due to left-padding zero bits, making comparison at the semantic level invalid.¶

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

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

- Copies of Identically Recurring Element within a Parent Element that contain invalid CRC-32 Elements. EBML Readers not checking the CRC-32 might use the version of the element with mismatching CRC-32s.¶

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

## 17. IANA Considerations

### 17.1. EBML Element IDs Registry

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

### 17.2. EBML DocTypes Registry

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

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

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

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

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

## 18. Normative References

[IEEE.754]
  IEEE, "IEEE Standard for Binary Floating-Point Arithmetic", 13 June 2019, <https://standards.ieee.org/standard/754-2019.html>.
[ISO3309]
  International Organization for Standardization, "Data communication -- High-level data link control procedures -- Frame structure", ISO 3309, 3rd Edition, October 1984, <https://www.iso.org/standard/8558.html>.
[ISO9899]
  International Organization for Standardization, "Information technology -- Programming languages -- C", ISO/IEC 9899:2011, 2011, <https://www.iso.org/standard/57853.html>.
[ITU.V42]
  International Telecommunications Union, "Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion", ITU-T Recommendation V.42, March 2002, <https://www.itu.int/rec/T-REC-V.42>.
[RFC0020]
  Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, <https://www.rfc-editor.org/info/rfc20>.
[RFC2119]
  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2648]
  Moats, R., "A URN Namespace for IETF Documents", RFC 2648, DOI 10.17487/RFC2648, August 1999, <https://www.rfc-editor.org/info/rfc2648>.
[RFC3339]
  Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, <https://www.rfc-editor.org/info/rfc3339>.
[RFC3629]
  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3688]
  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004, <https://www.rfc-editor.org/info/rfc3688>.
[RFC5234]
  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>.
[RFC5646]
  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC7405]
  Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, <https://www.rfc-editor.org/info/rfc7405>.
[RFC8126]
  Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8141]
  Saint-Andre, P. and J. Klensin, "Uniform Resource Names (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017, <https://www.rfc-editor.org/info/rfc8141>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XHTML]
  McCarron, S., "XHTML(tm) Basic 1.1 -- Second Edition", Latest version available at https://www.w3.org/TR/xhtml-basic, 27 March 2018, <https://www.w3.org/TR/2018/SPSD-xhtml-basic-20180327/>.
[XML]
  Bray, T., Ed., Paoli, J., Ed., Sperberg-McQueen, C.M., Ed., Maler, E., Ed., and F. Yergeau, Ed., "Extensible Markup Language (XML) 1.0 (Fifth Edition)", Latest version available at https://www.w3.org/TR/xml/, 26 November 2008, <https://www.w3.org/TR/2008/REC-xml-20081126/>.
[XML-SCHEMA]
  Fallside, D.C. and P. Walmsley, "XML Schema Part 0: Primer Second Edition", Latest version available at http://www.w3.org/TR/xmlschema-0/, 28 October 2004, <https://www.w3.org/TR/2004/REC-xmlschema-0-20041028/>.

## 19. Informative References

[Matroska]
  Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media Container Format Specifications", Work in Progress, Internet-Draft, draft-ietf-cellar-matroska-05, 17 April 2020, <https://tools.ietf.org/html/draft-ietf-cellar-matroska-05>.
[WebM]
  The WebM Project, "WebM Container Guidelines", 28 November 2017, <https://www.webmproject.org/docs/container/>.
[XPath]
  Clark, J., Ed. and S. DeRose, "XML Path Language (XPath) Version 1.0", Latest version available at https://www.w3.org/TR/xpath, 16 November 1999, <https://www.w3.org/TR/1999/REC-xpath-19991116>.

## Authors' Addresses