DASH标准详解：ISO_23009-1_2014动态流媒体适应性规范

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DASH标准文档，全称为"Dynamic Adaptive Streaming over HTTP (DASH)"，是ISO/IEC国际标准ISO/IEC 23009-1的一个第二版，发布于2014年5月15日。该标准主要关注媒体内容的动态适应性流传输，尤其是在HTTP协议下实现高效的分发和播放。DASH的设计目标是提供一种方法，使网络流媒体内容能够根据用户的网络条件和设备能力进行自适应调整，从而提高视频质量和用户体验。 ISO/IEC 23009-1定义了媒体呈现描述（Media Presentation Description, MPD）和媒体段（Media Segments）的格式。MPD是一种XML文件，它包含了关于媒体内容的元数据，如分辨率、码率、时长、播放列表结构等，以及如何根据用户设备的网络状况动态地选择和组合不同质量的媒体段。这些媒体段则是实际的视频或音频数据，存储在服务器上，可以被客户端按需请求。该标准的关键要点包括： 1. **内容分发**：DASH通过HTTP协议将媒体内容分发到客户端，允许内容提供商将其流化内容分割成多个可独立下载的小片段，便于在网络不稳定的情况下播放。 2. **自适应码率**：根据用户的网络状况动态调整发送的码率，确保视频流畅播放，避免缓冲或停滞。 3. **多屏支持**：DASH设计旨在跨多种设备和平台兼容，包括智能手机、平板电脑、电视和其他智能设备。 4. **媒体描述语言**：MPD的XML格式使得内容可以被解析和理解，确保不同客户端软件能正确处理和渲染。 5. **版权管理**：文档强调版权保护，规定了在复制、利用或通过电子或机械方式（包括打印和互联网传播）使用标准内容之前，必须获得ISO或其成员国组织的书面许可。 DASH标准对现代数字媒体行业的内容分发和播放体验起到了重要作用，推动了在线视频服务的快速发展和优化，使得用户能够在各种网络环境下都能获得高质量的媒体体验。随着技术的进步，DASH不断演进和完善，以适应新的挑战和机遇。

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ISO/IEC 23009-1:2014(E)

DASH is based on a hierarchical data model aligned with the presentation in Figure 3. A DASH Media

Presentation is described by a Media Presentation Description document. This describes the sequence of

Periods (see 5.3.2) in time that make up the Media Presentation. A Period typically represents a media

content period during which a consistent set of encoded versions of the media content is available i.e. the set

of available bitrates, languages, captions, subtitles etc. does not change during a Period.

Within a Period, material is arranged into Adaptation Sets (see 5.3.3). An Adaptation Set represents a set of

interchangeable encoded versions of one or several media content components (see 5.3.4). For example

there may be one Adaptation Set for the main video component and a separate one for the main audio

component. If there is other material available, for example captions or audio descriptions, then these may

each have a separate Adaptation Set. Material may also be provided in multiplexed form, in which case

interchangeable versions of the multiplex may be described as a single Adaptation Set, for example an

Adaptation Set containing both the main audio and main video for a Period. Each of the multiplexed

components may be described individually by a media content component description.

An Adaptation Set contains a set of Representations (see 5.3.5). A Representation describes a deliverable

encoded version of one or several media content components. A Representation includes one or more media

streams (one for each media content component in the multiplex). Any single Representation within an

Adaptation Set is sufficient to render the contained media content components. By collecting different

Representations in one Adaptation Set, the Media Presentation author expresses that the Representations

represent perceptually equivalent content. Typically this means, that clients may switch dynamically from

Representation to Representation within an Adaptation Set in order to adapt to network conditions or other

factors. Switching refers to the presentation of decoded data up to a certain time t, and presentation of

decoded data of another Representation from time t onwards. If Representations are included in one

Adaptation Set, and the client switches properly, the Media Presentation is expected to be perceived

seamless across the switch. Clients may ignore Representations that rely on codecs or other rendering

technologies they do not support or that are otherwise unsuitable.

Within a Representation, the content may be divided in time into Segments (see 5.3.9 and 6) for proper

accessibility and delivery. In order to access a Segment, a URL is provided for each Segment. Consequently,

a Segment is the largest unit of data that can be retrieved with a single HTTP request.

NOTE This is not strictly true, since the MPD may also include a byte range with the URL, meaning that the

Segment is contained in the provided byte range of some larger resource. An intelligent client could in principle

construct a single request for multiple Segments, but this would not be the typical case.

DASH defines different timelines. One of the key features in DASH is that encoded versions of different media

content components share a common timeline. The presentation time of each access unit within the media

content is mapped to the global common presentation timeline for synchronization of different media

components and to enable seamless switching of different coded versions of the same media components.

This timeline is referred as Media Presentation timeline. The Media Segments themselves contain accurate

Media Presentation timing information enabling synchronization of components and seamless switching.

A second timeline is used to signal to clients the availability time of Segments at the specified HTTP-URLs.

These times are referred to as Segment availability times and are provided in wall-clock time. Clients

typically compare the wall-clock time to Segment availability times before accessing the Segments at the

specified HTTP-URLs in order to avoid erroneous HTTP request responses. For static Media Presentations,

the availability times of all Segments are identical. For dynamic Media Presentations, the availability times of

segments depend on the position of the Segment in the Media Presentation timeline, i.e. the Segments get

available over time. Whereas static Media Presentations are suitable to offer On-Demand content, dynamic

Media Presentations are mostly suitable to offer live services.

Segments are assigned a duration, which is the duration of the media contained in the Segment when

presented at normal speed. Typically all Segments in a Representation have the same or roughly similar

duration. However Segment duration may differ from Representation to Representation. A DASH presentation

can be constructed with relative short segments (for example a few seconds), or longer Segments including a

single Segment for the whole Representation.

Short Segments are usually required in the case of live content, where there are restrictions on end-to-end

latency. The duration of a Segment is typically a lower bound on the end-to-end latency. DASH does not

ISO/IEC 23009-1:2014(E)

support the possibility for Segments to be extended over time: a Segment is treated as an object as a

complete and discrete unit that must be made available in its entirety. However, this does not prevent to apply

advanced HTTP transfer modes such as chunked transfer to optimize deployments and reduce end-to-end

latency.

Segments may be further subdivided into Subsegments each of which contains a whole number of complete

access units. There may also be media-format-specific restrictions on Subsegment boundaries, for example in

the ISO Base Media File Format a Subsegment must contain a whole number of complete movie fragments. If

a Segment is divided into Subsegments they are described by a compact Segment index, which provides the

presentation time range in the Representation and corresponding byte range in the Segment occupied by

each Subsegment. Clients may download this index in advance and then issue requests for individual

Subsegments.

Clients may switch from Representation to Representation within an Adaptation Set at any point in the media.

However, switching at arbitrary positions may be complex because of coding dependencies within

Representations and other factors, potentially requiring parallel download and decoding in the DASH client. It

is also desirable to avoid download of 'overlapping' data i.e. media for the same time period from multiple

Representations. Usually, switching is simplest at a Stream Access Point (SAP) in the new stream. In order to

formalize requirements related to switching DASH defines a codec-independent concept of Stream Access

Points and identifies various types of Stream Access Points.

Segmentation and Subsegmentation may be performed in ways that make switching simpler. For example, in

the very simplest cases each Segment or Subsegment begins with a SAP and the boundaries of Segments or

Subsegments are aligned across the Representations of one Adaptation Set. In this case, switching

Representation involves playing to the end of a (Sub)Segment of one Representation and then playing from

the beginning of the next (Sub)Segment of the new Representation. The Media Presentation Description and

Segment Index provide various indications, which describe properties of the Representations that may make

switching simpler. Profiles of this specification may then require these indicators to be set in certain ways,

making implementation of clients for those profiles simpler at the cost of requiring the media data to obey the

indicated constraints.

For On-Demand services, the Media Presentation Description is typically a static document describing the

various aspects of the Media Presentation. All Segments of the Media Presentation are available on the

server once any Segment is available. For live services, however, Segments become available with time as

the content is produced and therefore, dynamic Media Presentations are suitable. The Media Presentation

Description may be updated regularly to reflect changes in the presentation over time, for example Segment

URLs for new segments may be added to the MPD and those for old, no longer available Segments may be

removed. However, if Segment URLs are described using a template, this updating may not be necessary

except for some redundancy/failover cases.

Events may be provided in the MPD or within a Representation in order to signal aperiodic information to the

DASH client or to an application. Events are timed, i.e. each event starts at a specific media presentation time

and typically has duration. Events include DASH specific signalling or application-specific events. Examples

for events are indication of MPD updates on the server, possibly providing the detailed update as part of the

messages. The event mechanisms may also be used to deliver media time related application events, for

example information about ad insertion opportunities, etc.

4.4 Protocols

This Part of ISO/IEC 23009 may be deployed in a system according to Figure 1 for which

 The DASH Client includes a client as specified in RFC 2616 and,

 The HTTP Server hosting the DASH Segments complies with a server as specified in RFC 2616.

DASH Clients typically use the HTTP GET method or the HTTP partial GET method, as specified in RFC 2616,

Clause 9.3, to access Segments or parts thereof.

ISO/IEC 23009-1:2014(E)

The use of HTTP as a transport protocol inherently provides many advanced features such as caching,

redirection or authentication. As another example, transport security in HTTP-based delivery may be achieved

by using HTTP over TLS as specified in RFC 2818. Yet another example is the use of HTTP state

management mechanisms (also known as Cookies) as defined in RFC 6265.

However, the formats defined in this part of ISO/IEC 23009 may also be used with other protocols. In

particular, the objects may be delivered with any object delivery protocol that provides a binding between an

HTTP-URL and the delivered object.

4.5 Media Stream and Representation properties

4.5.1 Switching and Random Access Support

The formats defined in this part of the standard are designed for providing good user experience even in case

the access bandwidth of the DASH segment delivery or the cache varies. A key functionality is the ability that

the DASH client can seamlessly switch across different Representations of the same media component.

DASH clients may use the common timeline across different Representation representing the same media

component to present one Representation up to a certain time t and continue presentation of another

Representation from time t onwards. However, in practical implementations, this operation may be complex,

as switching at time t may require parallel download and decoding of two Representations. Therefore,

providing suitable switching opportunities in regular time intervals simplifies client implementations. The

standard provides means for providing suitable switching opportunities and in addition provides abilities to

signal the position and media time of the switching opportunities.

For this purpose this subsection defines three relevant concepts to support seamless switching, namely

 Media stream access points in 4.5.2 to signal positions where to easily switch to a Representation,

and in addition where to suitable access a Representation at start-up or seek.

 Non-overlapping Segments and Subsegments in 4.5.3 to signal that at the signalled stream access

points, no overlap decoding of Representations is necessary in order to provide a continuous switch

 Bitstream concatenation in 4.5.4 to signal that the concatenation of two Representations at a switch

point results in a conforming bitstream

These three properties are neither sufficient nor necessary for seamless switching, but certain implementation

or profiles may use these properties in order to simplify practical implementations.

4.5.2 Media stream access points

To be able to

access a Representation, each of the media streams that are contained in the Representation

requires Media Stream Access Points (SAPs). SAPs in the context of this part of ISO/IEC 23009 refer to the

SAP definition in ISO/IEC 14496-12 Annex I. ISO/IEC 14496-12 Annex I.3 defines different types of SAPs that

provide a relationship between the position where a stream can be accessed, relative to the start of a

Segment or Subsegment, its presentation time and the presentation times and position of other access unit in

the stream. The same SAP type definitions shall apply for this part of ISO/IEC 23009.

A SAP is a position in a Representation that enables playback of a media stream to be started using only the

information contained in Representation data starting from that position onwards (preceded by initializing data

in the Initialization Segment, if any).

For each SAP the properties, I

SAP

, T

SAP

, I

, T

DEC

, T

EPT

, and T

PTF

are identified and defined in ISO/IEC

14496-12 Annex I.2.

In particular, T

SAP

is defined to be earliest presentation time of any access unit of the media stream such that

all access units of the media stream with presentation time greater than or equal to T

SAP

can be correctly

decoded using data in the Representation starting at byte position I

SAP

and no data before I

SAP

ISO/IEC 23009-1:2014(E)

NOTE The type of SAP is dependent only on which Access Units are correctly decodable and their

arrangement in presentation order. The types informally correspond with some common terms:

- Type 1 corresponds to what is known in some coding schemes as a “Closed GoP random access point”

(in which all access units, in decoding order, starting from I

SAP

can be correctly decoded, resulting in a

continuous time sequence of correctly decoded access units with no gaps) and in addition the access

unit in decoding order is also the first access unit in presentation order.

- Type 2 corresponds to what is known in some coding schemes as a “Closed GoP random access point”,

for which the first access unit in decoding order in the media stream starting from I

is not the first

access unit in presentation order.

- Type 3 corresponds to what is known in some coding schemes as an “Open GoP random access point”,

in which there are some access units in decoding order following I

that cannot be correctly decoded

and have presentation times less than T

SAP

- Type 4 corresponds to what is known in some coding schemes as an "Gradual Decoding Refresh (GDR)

random access point”, in which there are some access units in decoding order starting from and

following I

that cannot be correctly decoded and have presentation times less than T

SAP

- Type 5 corresponds to the case for which there is at least one access unit in decoding order starting

from I

SAP

that cannot be correctly decoded and has presentation time greater than T

DEC

and where T

DEC

is the earliest presentation time of any access unit starting from I

- Type 6 corresponds to the case for which there is at least one access unit in decoding order starting

from I

SAP

that cannot be correctly decoded and has presentation time greater than T

DEC

and where T

DEC

is not the earliest presentation time of any access unit starting from I

SAPs are mostly relevant for two purposes in this part of ISO/IEC 23009:

1. For randomly accessing a Media Presentation, for example at the startup of the Media Presentation,

after a seeking operation or after an error event especially in live cases.

2. To permit switching between two Representations whereby for seamless switching each media

stream i in the switch-from Representation is presented up to T

SAP

(i) and each media stream i in the

switch-to Representation is presented from the media Stream Access Point starting from T

SAP

(i).

There are obvious benefits for the client to be able to identify SAPs and one or several of their properties, in

particular I

SAP

and T

SAP

for each media stream without requiring to access data at positions following I

SAP

DASH provides functionalities to explicitly signal such information by using signals in the MPD or the Segment

Index or combinations of the two.

4.5.3 Non-overlapping Segments and Subsegments

Segment

s and Subsegments represent units for which the client has an exact map on how to access and

download the unit using HTTP GET or HTTP partial GET methods.

Segments (respectively Subsegments) are typically generated by segmenting encoded media streams into

appropriate units. If the generation of Segments (respectively Subsegments) adheres to certain rules, then the

sequential decoding and presentation of Media Segments (respectively Subsegments) results in a correct

presentation of all contained media streams. To define such rules the notion of “non-overlapping” segments

(respectively Subsegments) is defined as follows.

Let

 T

(S,i) be the earliest presentation time of any access unit in stream i of a Segment or Subsegment

 T

(S,i) be the latest presentation time of any access unit in stream i of a Segment or Subsegment S.

ISO/IEC 23009-1:2014(E)

Then two segments (respectively Subsegments), A and B, which may or may not be of different

Representations, are non-overlapping if T

(A,i) < T

(B,i) for all media streams i in A and B or if T

(B,i) < T

(A,i)

for all streams i in A and B where i refers to the same media component.

The property of “non-overlapping” segments (respectively Subsegments) is used to define the terms Segment

alignment and Subsegment alignment.

4.5.4 Bitstream concatenation

A sequ

ence of Segments (respectively Subsegments) is a “conforming Segment (respectively Subsegment)

sequence” if the concatenation of all Segments (respectively Subsegments) in the sequence of Segments

(respectively Subsegments) results in a bitstream that conforms to the media formats in use (including

container and codecs).

NOTE This implies that a player conforming to the media format can play the resulting bitstream.

4.6 Brands

The ISO Base Media File Format, ISO/IEC 14496-12, defines the concept of brands; brand values identify

specifications or conformance points. This part of ISO/IEC 23009 specifies several brands, as listed in Table 1.

Table 1 — Brands defined in this part of ISO/IEC 23009

Brand

Identifier

Clause in this part of

ISO/IEC 23009

Informative description

emsg

5.10.3.3 Event message box

msdh

6.3.4.2 Media Segment conforming to the general format type for ISO

base media file format.

msix

6.3.4.3 Media Segment conforming to the Indexed Media Segment

format type for ISO base media file format.

sims

6.3.4.4 Media Segment conforming to the Sub-Indexed Media Segment

format type for ISO base media file format.

dsms

6.3.5.1 Media Segment conforming to the DASH Self-Initializing Media

Segment format type for ISO base media file format.

dash

6.3.5.2 ISO base media file format file specifically designed for DASH

including movie fragments and Segment Index.

sisx

6.4.6.2 Single Index Segment used to index MPEG-2 TS based Media

Segments.

risx

6.4.6.3 Representation Index Segment used to index MPEG-2 TS

based Media Segments.

ssss

6.4.6.4 Subsegment Index Segment used to index MPEG-2 TS based

Media Segments.

lmsg

7.3.1 last Media Segment indicator for ISO base media file format.

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