HTTP Live Streaming (HLS) 协议规范

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"HTTP Live Streaming (HLS) 规范" HLS（HTTP Live Streaming）是一种由苹果公司开发的流媒体协议，主要用于通过HTTP传输实时或近实时的音视频数据。该协议被广泛应用于互联网电视、在线直播、移动设备等场景。`draft-pantos-http-live-streaming-23.pdf` 文件描述的是HLS协议的第7版规范。 HLS协议的核心特点包括以下几个方面： 1. **基于HTTP**: HLS协议利用了HTTP协议进行数据传输，这意味着它可以利用现有的广泛部署的HTTP基础设施，如缓存和代理服务器，从而提高了可伸缩性和可靠性。 2. **分段媒体文件**: 媒体文件被分割成一系列短小的片段，每个片段都是一个独立的HTTP请求可以获取的文件。这种设计允许客户端根据网络条件动态调整播放质量，即自适应比特率流。 3. **M3U8播放列表**: 每个HLS流都由一个M3U8格式的播放列表文件来描述，其中包含了媒体片段的URL以及它们的顺序、时长和编码参数。M3U8是扩展的M3U元数据格式，用于定义多媒体播放列表。 4. **版本更新**: 文档提到的版本7代表了HLS协议的一个特定迭代，可能包含对之前版本的改进和新特性，如更灵活的比特率切换策略、增强的安全性支持、对新格式的支持等。 5. **状态与生命周期**: 作为Internet-Draft，这个文档遵循IETF的标准流程，可能存在更新、替换或废弃的情况。其有效期为6个月，意味着它可能随着时间的推移而变化。 6. **应用范围**: HLS不仅适用于桌面和移动设备，还被用在各种网络环境，包括固定宽带、蜂窝网络，以及对延迟敏感的应用，如体育赛事直播。 7. **兼容性**: 由于其基于HTTP，HLS协议在各种平台和设备上具有良好的兼容性，尤其是苹果的iOS和Mac OS系统，但也逐渐被其他操作系统和浏览器支持。 8. **错误恢复与适应性**: HLS协议允许客户端检测和处理网络波动，通过选择不同比特率的媒体片段，实现平滑的播放体验，避免因网络状况变化导致的卡顿或缓冲。 9. **加密与版权保护**: HLS支持对媒体内容进行加密，可以集成DRM系统，以保护内容不被非法复制和分发，满足内容提供商对版权保护的需求。 HLS协议提供了一种高效、灵活和可靠的实时音视频流传输机制，确保了内容能够适应各种网络条件和终端设备。随着流媒体服务的普及，HLS已成为互联网音视频传输领域的重要标准之一。

Internet-Draft HTTP Live Streaming May 2017

character. Each line is a URI, blank, or starts with the character

’#’. Blank lines are ignored. White space MUST NOT be present,

except for elements in which it is explicitly specified.

Lines that start with the character ’#’ are either comments or tags.

Tags begin with #EXT. They are case-sensitive. All other lines that

begin with ’#’ are comments and SHOULD be ignored.

A URI line identifies a Media Segment or a Playlist file (see

Section 4.3.4.2). Each Media Segment is specified by a URI and the

tags that apply to it.

A Playlist is a Media Playlist if all URI lines in the Playlist

identify Media Segments. A Playlist is a Master Playlist if all URI

lines in the Playlist identify Media Playlists. A Playlist MUST be

either a Media Playlist or a Master Playlist; all other Playlists are

invalid.

A URI in a Playlist, whether it is a URI line or part of a tag, MAY

be relative. Any relative URI is considered to be relative to the

URI of the Playlist that contains it.

The duration of a Media Playlist file is the sum of the durations of

the Media Segments within it.

The segment bit rate of a Media Segment is the size of the Media

Segment divided by its EXTINF duration (

Section 4.3.2.1). Note that

this includes container overhead but does not include overhead

imposed by the delivery system, such as HTTP, TCP or IP headers.

The peak segment bit rate of a Media Playlist is the largest bit rate

of any contiguous set of segments whose total duration is between 0.5

and 1.5 times the target duration. The bit rate of a set is

calculated by dividing the sum of the segment sizes by the sum of the

segment durations.

The average segment bit rate of a Media Playlist is the sum of the

sizes (in bits) of every Media Segment in the Media Playlist, divided

by the Media Playlist duration. Note that this includes container

overhead, but not HTTP or other overhead imposed by the delivery

system.

4.2. Attribute Lists

Certain tags have values which are Attribute Lists. An attribute-

list is a comma-separated list of attribute/value pairs with no

whitespace.

Pantos & May Expires November 23, 2017 [Page 10]

Internet-Draft HTTP Live Streaming May 2017

An attribute/value pair has the following syntax:

AttributeName=AttributeValue

An AttributeName is an unquoted string containing characters from the

set [A..Z], [0..9] and ’-’. Therefore, AttributeNames contain only

uppercase letters, not lowercase. There MUST NOT be any whitespace

between the AttributeName and the ’=’ character, nor between the ’=’

character and the AttributeValue.

An AttributeValue is one of the following:

o decimal-integer: an unquoted string of characters from the set

[0..9] expressing an integer in base-10 arithmetic in the range

from 0 to 2^64-1 (18446744073709551615). A decimal-integer may be

from 1 to 20 characters long.

o hexadecimal-sequence: an unquoted string of characters from the

set [0..9] and [A..F] that is prefixed with 0x or 0X. The maximum

length of a hexadecimal-sequence depends on its AttributeName.

o decimal-floating-point: an unquoted string of characters from the

set [0..9] and ’.’ which expresses a non-negative floating-point

number in decimal positional notation.

o signed-decimal-floating-point: an unquoted string of characters

from the set [0..9], ’-’ and ’.’ which expresses a signed

floating-point number in decimal positional notation.

o quoted-string: a string of characters within a pair of double-

quotes (0x22). The following characters MUST NOT appear in a

quoted-string: line feed (0xA), carriage return (0xD), or double

quote (0x22). Quoted-string AttributeValues SHOULD be constructed

so that byte-wise comparison is sufficient to test two quoted-

string AttributeValues for equality. Note that this implies case-

sensitive comparison.

o enumerated-string: an unquoted character string from a set which

is explicitly defined by the Attribute. An enumerated-string will

never contain double-quotes ("), commas (,), or whitespace.

o decimal-resolution: two decimal-integers separated by the "x"

character. The first integer is a horizontal pixel dimension

(width); the second is a vertical pixel dimension (height).

The type of the AttributeValue for a given AttributeName is specified

by the Attribute definition.

Pantos & May Expires November 23, 2017 [Page 11]

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