苹果公司HTTP Live Streaming协议2nd Edition详解

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"HTTP Live Streaming 2nd Edition.pdf 是一份由苹果公司编写的全英文文档，详细解释了HLS（HTTP Live Streaming）协议。这份60多页的文档共分为12章，涵盖了HLS协议的最新版本，旨在替代原有的RFC8216标准。" HTTP Live Streaming (HLS) 是一种互联网上进行实时流媒体传输的协议，由苹果公司开发并广泛应用于iOS、macOS以及Apple TV等平台。这份2nd Edition的文档更新了HLS协议的细节，旨在提供更高效、低延迟的直播体验。 HLS协议的核心在于将连续的多媒体流分割成一系列短小的HTTP可下载的片段，每个片段通常包含几秒的视频内容。这种分段方式允许客户端根据网络条件动态调整播放质量，通过切换不同码率的媒体片段来适应网络带宽的变化，从而实现平滑的流媒体体验。此外，HLS协议还支持加密和授权，以保护内容的安全性。在文档中，可能会详细讨论以下几个关键知识点： 1. **协议结构**：包括M3U8播放列表文件的格式，它定义了可用的媒体片段和它们的URL，以及如何根据网络状况选择合适的片段。 2. **版本8的新特性**：可能涉及更低的延迟优化、增强的错误恢复机制、对自适应码率切换的改进以及其他性能提升。 3. **低延时直播**：HLS协议在降低延迟方面所做的努力，例如通过减少片段长度、使用即时切换技术和改进的缓冲策略来缩短用户看到内容的时间。 4. **加密与安全**：介绍HLS如何使用AES-128加密保护内容，以及如何通过FairPlay Streaming等技术实施更高级别的内容保护。 5. **适应性流媒体**：详细说明如何根据网络状况自动调整视频质量，以及如何处理网络波动以确保流畅播放。 6. **实现与兼容性**：涵盖不同平台和浏览器对HLS的支持情况，以及如何在实际应用中集成HLS。 7. **错误处理和恢复**：描述服务器和客户端如何识别和处理传输错误，以及如何恢复播放。 8. **实时流处理**：针对实时事件直播，如体育赛事或新闻报道，HLS如何提供实时数据流的处理方案。 9. **QoS（服务质量）**：探讨如何通过HLS协议确保良好的用户体验，即使在网络条件不稳定的情况下。这份文档对于开发者、流媒体服务提供商和技术爱好者来说，是一份深入了解HLS协议及其最新进展的重要参考资料。通过阅读和理解这份文档，可以更好地理解和实现基于HLS的流媒体服务，优化用户体验，并跟上流媒体技术的发展步伐。

Internet-Draft HTTP Live Streaming 2nd Edition September 2018

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 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 4.4.2.1). 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 that are attribute-lists. An attribute-list

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

whitespace.

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:

Pantos Expires March 25, 2019 [Page 11]

Internet-Draft HTTP Live Streaming 2nd Edition September 2018

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 AttributeNames.

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

set [0..9] and ’.’ that 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 ’.’ that 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 that is

explicitly defined by the AttributeName. 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.

A given AttributeName MUST NOT appear more than once in a given

attribute-list. Clients SHOULD refuse to parse such Playlists.

4.3. Variable Substitution

The following Playlist elements are subject to variable substitution:

o URI lines

o quoted-string AttributeValues

o hexadecimal-sequence AttributeValues

Pantos Expires March 25, 2019 [Page 12]

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