2019年ISO/IEC 23009-1：MPEG-DASH动态自适应流媒体标准

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"ISO/IEC 23009-1是国际标准化组织与国际电工委员会于2019年12月发布的第四版MPEG-DASH标准规范文本，全称为‘信息技术——基于HTTP的动态自适应流媒体（DASH）——第1部分：媒体呈现描述和分段格式’。该标准提供了多媒体内容在互联网上高效、灵活的传输方法，旨在确保不同网络条件下的流畅播放体验。" MPEG-DASH（Dynamic Adaptive Streaming over HTTP）是一种用于在互联网上传输高质量视频和其他多媒体内容的国际标准。它允许内容提供商根据用户的网络状况动态调整流的质量，从而优化播放体验，减少缓冲和卡顿。2019年12月发布的第四版标准对之前的标准进行了更新和完善，确保了更广泛的应用兼容性和更高的性能。标准的核心组成部分包括媒体呈现描述（Media Presentation Description, MPD）和分段格式。MPD文件提供了多媒体内容的元数据，包括可用的比特率、分辨率、时间轴等信息，使得客户端可以根据网络条件选择合适的数据流。分段格式则将大文件分割成小段，便于快速加载和适应性切换。此标准还涵盖了错误恢复机制、加密和版权保护措施，以及多码率适应策略。通过支持不同的编码标准如H.264、HEVC等，MPEG-DASH能够处理多种类型的多媒体内容。此外，该标准与HTTP协议兼容，利用了现有互联网基础设施的广泛普及，降低了部署成本。 ISO/IEC 23009-1的第四版可能包括了以下方面的改进： 1. **增强的自适应策略**：可能包含了更智能的算法，以更好地适应网络波动，提供更平滑的播放体验。 2. **更好的安全特性**：可能增加了对内容保护的新机制，以防止非法复制和分发。 3. **扩展的元数据支持**：可能扩展了MPD的元数据能力，支持更多内容属性的描述，如字幕、3D视频等。 4. **提高的效率**：可能优化了分段格式，减少了传输开销，提高了带宽利用率。 5. **兼容性提升**：可能增强了与其他标准（如HLS）的互操作性，以扩大生态系统。这个275页的标准文本对于开发者、服务提供商和研究人员来说是一份宝贵的资源，它详细阐述了MPEG-DASH的实施细节和技术要求，有助于实现高效、高质量的流媒体服务。虽然获取可能有一定难度，但其内容的价值对于理解和应用DASH技术至关重要。

ISO/IEC 23009-1:2019(E)

3.1.40

Segment availability start time

latest time instant in wall-clock time (3.1.52) at which a Segment (3.1.39) becomes an available Segment

(3.1.6)

3.1.41

adjusted Segment availability start time

time instant in wall-clock time (3.1.52) at which a Segment (3.1.39) becomes an available Segment

(3.1.6)

3.1.42

Segment availability end time

time instant in wall-clock time (3.1.52) at which a Segment (3.1.39) ceases to be an available Segment

(3.1.6)

3.1.43

Segment Index

compact index of the time range to byte range mapping within a Media Segment (3.1.28) separately

from the MPD (3.1.26)

3.1.44

Segment Sequence

sequence of Segments (3.1.39) that are sharing a common number address

3.1.45

Segment Track

concatenation of Segments (3.1.39) forming a track with potential conformance properties

3.1.46

Spatial Object

media content component (3.1.22) corresponding to a region in a coordinate system associated to this

media content component

3.1.47

stream access point

SAP

position in a Representation (3.1.38) enabling playback of a media stream (3.1.29) to be started using

only the information contained in Representation data starting from that position onwards [preceded

by initializing data in the Initialization Segment (3.1.19), if any]

3.1.48

sub-asset

media content component (3.1.22) (or part thereof) identified as corresponding to a part of an asset

(3.1.4)

3.1.49

Sub-Representation

part of a Representation (3.1.38) described in the MPD (3.1.26) that is present in the entire Period

(3.1.34)

3.1.50

Subsegment

unit within Media Segments (3.1.28) that is indexed by a Segment Index (3.1.43)

ISO/IEC 23009-1:2019(E)

3.3 Conventions

— Elements in an XML document are identified by an upper-case first letter and in bold face as

Element. To express that an element Element1 is contained in another element Element2, the

following format is used: Element2.Element1. If an element's name consists of two or more

combined words, camel-casing is typically used, e.g. ImportantElement. Elements may be present

either exactly once, or the minimum and maximum occurrence is defined by <minOccurs> ...

<maxOccurs>.

— Attributes in an XML document are identified by a lower-case first letter as well as they are preceded

by an '@'-sign, e.g. @attribute. To point to a specific attribute @attribute contained in an

element Element, one may write Element@attribute. If an attribute's name consists of two or

more combined words, camel-casing is typically used after the first word, e.g.

@veryImportantAttribute. Attributes may have assigned a status in the XML as mandatory

(M), optional (O), optional with default value (OD) and conditionally mandatory (CM).

— Namespace qualification of elements and attributes is used as per XML standards, in the form of

namespace:Element or @namespace:attribute. The fully qualified namespace is provided in

the schema fragment associated with the declaration. External specifications extending the

namespace of DASH are expected to document the element name in the semantic table with an

extension namespace prefix.

— Variables defined in the context of this document are specifically highlighted with italics, e.g.

InternalVariable.

— Structures that are defined as part of the hierarchical data model are identified by an upper-case first

letter, e.g. Period, Adaptation Set, Representation, Segment, etc.

— The term "this clause" refers to the entire clause included within the same first heading number. The

term "this subclause" refers to all text contained in the subclause with the lowest hierarchy heading.

— For improved interoperability, this document uses ABNF notation according to IETF RFC 5234 to

specify certain underdefined values of XML attributes and elements.

4 Overview

4.1 System description

Dynamic adaptive streaming over HTTP (DASH) specifies XML and binary formats that enable delivery

of continuous media content from standard HTTP servers to HTTP clients and enable caching of content

by standard HTTP caches.

This document primarily defines two formats:

— The Media Presentation Description (MPD) describes a Media Presentation, i.e. a bounded or

unbounded presentation of continuous media content. In particular, it defines formats to announce

resource identifiers for Segments and to provide the context for these identified resources within a

Media Presentation. These resource identifiers are HTTP-URLs possibly combined with a byte range,

or with a data URL.

ISO/IEC 23009-1:2019(E)

— The Segment formats specify the formats of the entity body of the HTTP response to an HTTP GET

request or a partial HTTP GET with the indicated byte range using HTTP/1.1 as defined in

IETF RFC 7233 to a resource identified in the MPD. Segments typically contain efficiently coded

media data and metadata conforming to or at least closely aligned with common media formats.

The MPD provides sufficient information for a client to provide a streaming service to the user by

accessing the Segments through the protocol specified in the scheme of the defined resources. In the

context of this document, the assumed protocol is HTTP/1.1. Such a client is referred to as a DASH Client

in the remainder of this document. However, this document does not provide a normative specification

for such a client.

Figure 1 shows a possible deployment architecture in which the formats defined in this document may

be used. Boxes with solid lines indicate devices that are mentioned in this document as they host or

process the formats defined in this document whereas dashed boxes are conceptual or transparent. This

document deals with the definition of formats that are accessible on the interface to the DASH Client,

indicated by the solid lines. Any other formats or interfaces are outside the scope of this document. In the

considered deployment scenario, it is assumed that the DASH Client has access to an MPD. The MPD

provides sufficient information for the DASH Client to provide a streaming service to the user by

requesting Segments from an HTTP server and demultiplexing, decoding and rendering the included

media streams.

Figure 1 — Example system for DASH formats

Although the formats are initially designed to be used in the above deployment scenario, their application

is obviously not restricted to this scenario. The particular aspect on "HTTP" in DASH is the usage of HTTP-

URLs in the MPD for the purpose to refer to Segments. The usage of HTTP-URLs enables unique location

information and it provides well-defined methods to access the resources, in particular HTTP GET and

HTTP partial GET.

4.2 DASH Client model

The design of the formats defined in this document is based on the informative client model as shown in

Figure 2. The figure illustrates the logical components of a conceptual DASH Client model and the relation

to other components in a media streaming application. In this figure, the DASH access engine receives the

Media Presentation Description (MPD), constructs and issues requests and receives Segments or parts of

Segments. The DASH Client may use metadata provided in the MPD for the selection of media components

by communication with the media streaming application. Such metadata may for example include codec

capability information, language codes, accessibility information and other information for the selection

of media components. In the context of this document, the output of the DASH access engine consists of

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