Introduction

Matroska is an audiovisual data container format. It was derived from a project called [@?MCF], but diverges from it significantly because it is based on EBML (Extensible Binary Meta Language) [@!RFC8794], a binary derivative of XML. EBML provides significant advantages in terms of future format extensibility, without breaking file support in old parsers.

First, it is essential to clarify exactly “What an Audio/Video container is”, to avoid any misunderstandings:

It is NOT a video or audio compression format (codec)
It is an envelope in which there can be many audio, video, and subtitles streams, allowing the user to store a complete movie or CD in a single file.

Matroska is designed with the future in mind. It incorporates features such as:

Fast seeking in the file
Chapter entries
Full metadata (tags) support
Selectable subtitle/audio/video streams
Modularly expandable
Error resilience (can recover playback even when the stream is damaged)
Streamable over the internet and local networks (HTTP [@?RFC9110], FTP [@?RFC0959], SMB [@?SMB-CIFS], etc.)
Menus (like DVDs have [@?DVD-Video])

Status of this document

This document covers Matroska versions 1, 2, 3 and 4. Matroska v4 is the current version. Matroska 1 to 3 are no longer maintained. No new elements are expected in files with version numbers 1, 2, or 3.

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 Matroska. Specific terms are defined below:

Matroska:: A multimedia container format based on EBML (Extensible Binary Meta Language).
Matroska Reader:: A data parser that interprets the semantics of a Matroska document and creates a way for programs to use Matroska.
Matroska Player:: A Matroska Reader with a primary purpose of playing audiovisual files, including Matroska documents.
Matroska Writer:: A data writer that creates Matroska documents.

Matroska Overview

Principles

Matroska is a Document Type of EBML (Extensible Binary Meta Language). This specification is dependent on the EBML Specification [@!RFC8794]. For an understanding of Matroska’s EBML Schema, see in particular the sections of the EBML Specification covering EBML Element Types (Section 7), EBML Schema (Section 11.1), and EBML Structure (Section 3).

Added EBML Constraints

As an EBML Document Type, Matroska adds the following constraints to the EBML specification.

The docType of the EBML Header MUST be “matroska”.
The EBMLMaxIDLength of the EBML Header MUST be 4.
The EBMLMaxSizeLength of the EBML Header MUST be between 1 and 8 inclusive.

Design Rules

The Root Element and all Top-Levels Elements MUST use 4 octets for their EBML Element ID – i.e. Segment and direct children of Segment.

Legacy EBML/Matroska parsers did not handle Empty Elements properly, elements present in the file but with a length of zero. They always assumed the value was 0 for integers/dates or 0x0p+0, the textual expression of floats using the [@!ISO9899] format, no matter the default value of the element which should have been used instead. Therefore Matroska writers MUST NOT use EBML Empty Elements, if the element has a default value that is not 0 for integers/dates and 0x0p+0 for floats.

When adding new elements to Matroska, these rules apply:

A non-mandatory integer/date Element MUST NOT have a default value other than 0.
A non-mandatory float Element MUST NOT have a default value other than 0x0p+0.
A non-mandatory string Element MUST NOT have a default value, as empty string cannot be defined in the XML Schema.