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Versions: (RFC 8216) 00 01 02 03 04 05 06 07

Informational                                             R. Pantos, Ed.
Internet-Draft                                                Apple Inc.
Obsoletes: 8216 (if approved)                             April 30, 2020
Intended status: Informational
Expires: November 1, 2020


                    HTTP Live Streaming 2nd Edition
                     draft-pantos-hls-rfc8216bis-07

Abstract

   This document obsoletes RFC 8216.  It describes a protocol for
   transferring unbounded streams of multimedia data.  It specifies the
   data format of the files and the actions to be taken by the server
   (sender) and the clients (receivers) of the streams.  It describes
   version 10 of this protocol.


Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 1, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.



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   This Informational Internet Draft is submitted as an RFC Editor
   Contribution and/or non-IETF Document (not as a Contribution, IETF
   Contribution, nor IETF Document) in accordance with BCP 78 and BCP
   79.

Table of Contents

   1.  Introduction to HTTP Live Streaming . . . . . . . . . . . . .   5
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Media Segments  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Supported Media Segment Formats . . . . . . . . . . . . .   7
       3.1.1.  MPEG-2 Transport Streams  . . . . . . . . . . . . . .   8
       3.1.2.  Fragmented MPEG-4 . . . . . . . . . . . . . . . . . .   8
       3.1.3.  Packed Audio  . . . . . . . . . . . . . . . . . . . .   9
       3.1.4.  WebVTT  . . . . . . . . . . . . . . . . . . . . . . .   9
       3.1.5.  IMSC Subtitles  . . . . . . . . . . . . . . . . . . .  10
     3.2.  Partial Segments  . . . . . . . . . . . . . . . . . . . .  11
   4.  Playlists . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Definition of a Playlist  . . . . . . . . . . . . . . . .  12
     4.2.  Attribute Lists . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  Variable Substitution . . . . . . . . . . . . . . . . . .  14
     4.4.  Playlist Tags . . . . . . . . . . . . . . . . . . . . . .  14
       4.4.1.  Basic Tags  . . . . . . . . . . . . . . . . . . . . .  15
         4.4.1.1.  EXTM3U  . . . . . . . . . . . . . . . . . . . . .  15
         4.4.1.2.  EXT-X-VERSION . . . . . . . . . . . . . . . . . .  15
       4.4.2.  Media or Master Playlist Tags . . . . . . . . . . . .  15
         4.4.2.1.  EXT-X-INDEPENDENT-SEGMENTS  . . . . . . . . . . .  16
         4.4.2.2.  EXT-X-START . . . . . . . . . . . . . . . . . . .  16
         4.4.2.3.  EXT-X-DEFINE  . . . . . . . . . . . . . . . . . .  17
       4.4.3.  Media Playlist Tags . . . . . . . . . . . . . . . . .  18
         4.4.3.1.  EXT-X-TARGETDURATION  . . . . . . . . . . . . . .  18
         4.4.3.2.  EXT-X-MEDIA-SEQUENCE  . . . . . . . . . . . . . .  18
         4.4.3.3.  EXT-X-DISCONTINUITY-SEQUENCE  . . . . . . . . . .  19
         4.4.3.4.  EXT-X-ENDLIST . . . . . . . . . . . . . . . . . .  19
         4.4.3.5.  EXT-X-PLAYLIST-TYPE . . . . . . . . . . . . . . .  19
         4.4.3.6.  EXT-X-I-FRAMES-ONLY . . . . . . . . . . . . . . .  20
         4.4.3.7.  EXT-X-PART-INF  . . . . . . . . . . . . . . . . .  20
         4.4.3.8.  EXT-X-SERVER-CONTROL  . . . . . . . . . . . . . .  21
       4.4.4.  Media Segment Tags  . . . . . . . . . . . . . . . . .  22
         4.4.4.1.  EXTINF  . . . . . . . . . . . . . . . . . . . . .  22
         4.4.4.2.  EXT-X-BYTERANGE . . . . . . . . . . . . . . . . .  23
         4.4.4.3.  EXT-X-DISCONTINUITY . . . . . . . . . . . . . . .  23
         4.4.4.4.  EXT-X-KEY . . . . . . . . . . . . . . . . . . . .  24
         4.4.4.5.  EXT-X-MAP . . . . . . . . . . . . . . . . . . . .  26



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         4.4.4.6.  EXT-X-PROGRAM-DATE-TIME . . . . . . . . . . . . .  27
         4.4.4.7.  EXT-X-GAP . . . . . . . . . . . . . . . . . . . .  27
         4.4.4.8.  EXT-X-BITRATE . . . . . . . . . . . . . . . . . .  27
         4.4.4.9.  EXT-X-PART  . . . . . . . . . . . . . . . . . . .  28
       4.4.5.  Media Metadata Tags . . . . . . . . . . . . . . . . .  29
         4.4.5.1.  EXT-X-DATERANGE . . . . . . . . . . . . . . . . .  29
           4.4.5.1.1.  Mapping SCTE-35 into EXT-X-DATERANGE  . . . .  31
           4.4.5.1.2.  EXT-X-SKIP  . . . . . . . . . . . . . . . . .  32
           4.4.5.1.3.  EXT-X-PRELOAD-HINT  . . . . . . . . . . . . .  33
           4.4.5.1.4.  EXT-X-RENDITION-REPORT  . . . . . . . . . . .  34
       4.4.6.  Master Playlist Tags  . . . . . . . . . . . . . . . .  35
         4.4.6.1.  EXT-X-MEDIA . . . . . . . . . . . . . . . . . . .  35
           4.4.6.1.1.  Rendition Groups  . . . . . . . . . . . . . .  38
         4.4.6.2.  EXT-X-STREAM-INF  . . . . . . . . . . . . . . . .  39
           4.4.6.2.1.  Alternative Renditions  . . . . . . . . . . .  44
         4.4.6.3.  EXT-X-I-FRAME-STREAM-INF  . . . . . . . . . . . .  45
         4.4.6.4.  EXT-X-SESSION-DATA  . . . . . . . . . . . . . . .  45
         4.4.6.5.  EXT-X-SESSION-KEY . . . . . . . . . . . . . . . .  46
   5.  Key Files . . . . . . . . . . . . . . . . . . . . . . . . . .  47
     5.1.  Structure of Key Files  . . . . . . . . . . . . . . . . .  47
     5.2.  IV for AES-128  . . . . . . . . . . . . . . . . . . . . .  47
   6.  Client/Server Responsibilities  . . . . . . . . . . . . . . .  47
     6.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  47
     6.2.  Server Responsibilities . . . . . . . . . . . . . . . . .  48
       6.2.1.  General Server Responsibilities . . . . . . . . . . .  48
       6.2.2.  Live Playlists  . . . . . . . . . . . . . . . . . . .  51
       6.2.3.  Encrypting Media Segments . . . . . . . . . . . . . .  52
       6.2.4.  Providing Variant Streams . . . . . . . . . . . . . .  53
       6.2.5.  Delivery Directives Interface . . . . . . . . . . . .  54
         6.2.5.1.  Playlist Delta Updates  . . . . . . . . . . . . .  54
         6.2.5.2.  Blocking Playlist Reload  . . . . . . . . . . . .  55
       6.2.6.  Providing Preload Hints . . . . . . . . . . . . . . .  56
     6.3.  Client Responsibilities . . . . . . . . . . . . . . . . .  57
       6.3.1.  General Client Responsibilities . . . . . . . . . . .  57
       6.3.2.  Loading the Media Playlist File . . . . . . . . . . .  58
       6.3.3.  Playing the Media Playlist File . . . . . . . . . . .  59
       6.3.4.  Reloading the Media Playlist File . . . . . . . . . .  60
       6.3.5.  Determining the Next Segment to Load  . . . . . . . .  61
       6.3.6.  Decrypting Encrypted Media Segments . . . . . . . . .  62
       6.3.7.  Requesting Playlist Delta Updates . . . . . . . . . .  63
       6.3.8.  Issuing Blocking Requests . . . . . . . . . . . . . .  63
   7.  Protocol Version Compatibility  . . . . . . . . . . . . . . .  64
   8.  Playlist Examples . . . . . . . . . . . . . . . . . . . . . .  66
     8.1.  Simple Media Playlist . . . . . . . . . . . . . . . . . .  66
     8.2.  Live Media Playlist Using HTTPS . . . . . . . . . . . . .  66
     8.3.  Playlist with Encrypted Media Segments  . . . . . . . . .  66
     8.4.  Master Playlist . . . . . . . . . . . . . . . . . . . . .  67
     8.5.  Master Playlist with I-Frames . . . . . . . . . . . . . .  67



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     8.6.  Master Playlist with Alternative Audio  . . . . . . . . .  68
     8.7.  Master Playlist with Alternative Video  . . . . . . . . .  68
     8.8.  Session Data in a Master Playlist . . . . . . . . . . . .  69
     8.9.  CHARACTERISTICS Attribute Containing Multiple
           Characteristics . . . . . . . . . . . . . . . . . . . . .  69
     8.10. EXT-X-DATERANGE Carrying SCTE-35 Tags . . . . . . . . . .  70
     8.11. Low-Latency Playlist  . . . . . . . . . . . . . . . . . .  70
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  71
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  71
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  73
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  73
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  73
     12.2.  Informative References . . . . . . . . . . . . . . . . .  77
   Appendix A.  Changes from RFC 8216  . . . . . . . . . . . . . . .  78
   Appendix B.  Server Configuration Profiles  . . . . . . . . . . .  79
     B.1.  Low-Latency Server Configuration Profile  . . . . . . . .  79
   Appendix C.  Low-Latency CDN Tune-in  . . . . . . . . . . . . . .  81
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  82

































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1.  Introduction to HTTP Live Streaming

   HTTP Live Streaming provides a reliable, cost-effective means of
   delivering continuous and long-form video over the Internet.  It
   allows a receiver to adapt the bit rate of the media to the current
   network conditions in order to maintain uninterrupted playback at the
   best possible quality.  It supports interstitial content boundaries.
   It provides a flexible framework for media encryption.  It can
   efficiently offer multiple renditions of the same content, such as
   audio translations.  It offers compatibility with large-scale HTTP
   caching infrastructure to support delivery to large audiences.  It
   supports low-latency playback at scale.

   Since its first draft publication in 2009, HTTP Live Streaming has
   been implemented and deployed by a wide array of content producers,
   tools vendors, distributors, and device manufacturers.  In the
   subsequent ten years the protocol has been refined by extensive
   review and discussion with a variety of media streaming implementors.

   The purpose of this document is to facilitate interoperability
   between HTTP Live Streaming implementations by describing the media
   transmission protocol.  Using this protocol, a client can receive a
   continuous stream of media from a server for concurrent presentation.

   This document describes version 10 of the protocol.

2.  Overview

   A multimedia presentation is specified by a Uniform Resource
   Identifier (URI) [RFC3986] to a Playlist.

   A Playlist is either a Media Playlist or a Master Playlist.  Both are
   UTF-8 text files containing URIs and descriptive tags.

   A Media Playlist contains a list of Media Segments, which, when
   played sequentially, will play the multimedia presentation.















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   Here is an example of a Media Playlist:

   #EXTM3U
   #EXT-X-TARGETDURATION:10

   #EXTINF:9.009,
   http://media.example.com/first.ts
   #EXTINF:9.009,
   http://media.example.com/second.ts
   #EXTINF:3.003,
   http://media.example.com/third.ts

   The first line is the format identifier tag #EXTM3U.  The line
   containing #EXT-X-TARGETDURATION says that all Media Segments will be
   10 seconds long or less.  Then, three Media Segments are declared.
   The first and second are 9.009 seconds long; the third is 3.003
   seconds.

   To play this Playlist, the client first downloads it and then
   downloads and plays each Media Segment declared within it.  The
   client reloads the Playlist as described in this document to discover
   any added segments.  Data SHOULD be carried over HTTP [RFC7230], but,
   in general, a URI can specify any protocol that can reliably transfer
   the specified resource on demand.

   A more complex presentation can be described by a Master Playlist.  A
   Master Playlist provides a set of Variant Streams, each of which
   describes a different version of the same content.

   A Variant Stream includes a Media Playlist that specifies media
   encoded at a particular bit rate, in a particular format, and at a
   particular resolution for media containing video.

   A Variant Stream can also specify a set of Renditions.  Renditions
   are alternate versions of the content, such as audio produced in
   different languages or video recorded from different camera angles.

   Clients should switch between different Variant Streams to adapt to
   network conditions.  Clients should choose Renditions based on user
   preferences.

   Certain streams can be played in Low-Latency Mode.  Low-Latency Mode
   refers to the combined use of Partial Segments, Blocking Playlist
   Reload and preload hinting to enable playback at a reduced delay from
   live.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and



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   "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.

3.  Media Segments

   A Media Playlist contains a series of Media Segments that make up the
   overall presentation.  A Media Segment is specified by a URI and
   optionally a byte range.

   The duration of each Media Segment is indicated in the Media Playlist
   by its EXTINF tag (Section 4.4.4.1).

   Each segment in a Media Playlist has a unique integer Media Sequence
   Number.  The Media Sequence Number of the first segment in the Media
   Playlist is either 0 or declared in the Playlist (Section 4.4.3.2).
   The Media Sequence Number of every other segment is equal to the
   Media Sequence Number of the segment that precedes it plus one.

   Each Media Segment MUST carry the continuation of the encoded
   bitstream from the end of the segment with the previous Media
   Sequence Number, where values in a series such as timestamps and
   Continuity Counters MUST continue uninterrupted.  The only exceptions
   are the first Media Segment ever to appear in a Media Playlist and
   Media Segments that are explicitly signaled as discontinuities
   (Section 4.4.4.3).  Unmarked media discontinuities can trigger
   playback errors.

   Any Media Segment that contains video SHOULD include enough
   information to initialize a video decoder and decode a continuous set
   of frames that includes the final frame in the Segment; network
   efficiency is optimized if there is enough information in the Segment
   to decode all frames in the Segment.  For example, any Media Segment
   containing H.264 video SHOULD contain an Instantaneous Decoding
   Refresh (IDR); frames prior to the first IDR will be downloaded but
   possibly discarded.

3.1.  Supported Media Segment Formats

   All Media Segments MUST be in a format described in this section.
   Transport of other media file formats is not defined.

   Some media formats require a common sequence of bytes to initialize a
   parser before a Media Segment can be parsed.  This format-specific
   sequence is called the Media Initialization Section.  The Media
   Initialization Section can be specified by an EXT-X-MAP tag
   (Section 4.4.4.5).  The Media Initialization Section MUST NOT contain
   sample data.



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3.1.1.  MPEG-2 Transport Streams

   MPEG-2 Transport Streams are specified by [ISO_13818].

   The Media Initialization Section of an MPEG-2 Transport Stream
   Segment is a Program Association Table (PAT) followed by a Program
   Map Table (PMT).

   Transport Stream Segments MUST contain a single MPEG-2 Program;
   playback of Multi-Program Transport Streams is not defined.  Each
   Transport Stream Segment MUST contain a PAT and a PMT, or have an
   EXT-X-MAP tag (Section 4.4.4.5) applied to it.  The first two
   Transport Stream packets in a Segment without an EXT-X-MAP tag SHOULD
   be a PAT and a PMT.

3.1.2.  Fragmented MPEG-4

   MPEG-4 Fragments are specified by the ISO Base Media File Format
   [ISOBMFF].  Unlike regular MPEG-4 files that have a Movie Box
   ('moov') that contains sample tables and a Media Data Box ('mdat')
   containing the corresponding samples, an MPEG-4 Fragment consists of
   a Movie Fragment Box ('moof') containing a subset of the sample table
   and a Media Data Box containing those samples.  Use of MPEG-4
   Fragments does require a Movie Box for initialization, but that Movie
   Box contains only non-sample-specific information such as track and
   sample descriptions.

   A Fragmented MPEG-4 (fMP4) Segment is a "segment" as defined by
   Section 3 of [ISOBMFF], including the constraints on Media Data Boxes
   in Section 8.16 of [ISOBMFF].

   The Media Initialization Section for an fMP4 Segment is an ISO Base
   Media File that can initialize a parser for that Segment.

   Broadly speaking, fMP4 Segments and Media Initialization Sections are
   [ISOBMFF] files that also satisfy the constraints described in this
   section.

   The Media Initialization Section for an fMP4 Segment MUST contain a
   File Type Box ('ftyp') containing a brand that is compatible with
   'iso6' or higher.  The File Type Box MUST be followed by a Movie Box.
   The Movie Box MUST contain a Track Box ('trak') for every Track
   Fragment Box ('traf') in the fMP4 Segment, with matching track_ID.
   Each Track Box SHOULD contain a sample table, but its sample count
   MUST be zero.  Movie Header Boxes ('mvhd') and Track Header Boxes
   ('tkhd') MUST have durations of zero.  The Movie Box MUST contain a
   Movie Extends Box ('mvex'); it SHOULD follow the last Track Box.




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   Note that a Common Media Application Format [CMAF] Header meets all
   these requirements.

   In an fMP4 Segment, every Track Fragment Box MUST contain a Track
   Fragment Decode Time Box ('tfdt').  fMP4 Segments MUST use movie-
   fragment-relative addressing.  fMP4 Segments MUST NOT use external
   data references.  Note that a CMAF Segment meets these requirements.

   An fMP4 Segment in a Playlist containing the EXT-X-I-FRAMES-ONLY tag
   (Section 4.4.3.6) MAY omit the portion of the Media Data Box
   following the intra-coded frame (I-frame) sample data.

   Each fMP4 Segment in a Media Playlist MUST have an EXT-X-MAP tag
   applied to it.

3.1.3.  Packed Audio

   A Packed Audio Segment contains encoded audio samples and ID3 tags
   that are simply packed together with minimal framing and no per-
   sample timestamps.  Supported Packed Audio formats are Advanced Audio
   Coding (AAC) with Audio Data Transport Stream (ADTS) framing
   [ISO_13818_7], MP3 [ISO_13818_3], AC-3 [AC_3], and Enhanced AC-3
   [AC_3].

   A Packed Audio Segment has no Media Initialization Section.

   Each Packed Audio Segment MUST signal the timestamp of its first
   sample with an ID3 Private frame (PRIV) tag [ID3] at the beginning of
   the segment.  The ID3 PRIV owner identifier MUST be
   "com.apple.streaming.transportStreamTimestamp".  The ID3 payload MUST
   be a 33-bit MPEG-2 Program Elementary Stream timestamp expressed as a
   big-endian eight-octet number, with the upper 31 bits set to zero.
   Clients SHOULD NOT play Packed Audio Segments without this ID3 tag.

3.1.4.  WebVTT

   A WebVTT Segment is a section of a WebVTT [WebVTT] file.  WebVTT
   Segments carry subtitles.

   The Media Initialization Section of a WebVTT Segment is the WebVTT
   header.

   Each WebVTT Segment MUST contain all subtitle cues that are intended
   to be displayed during the period indicated by the segment EXTINF
   duration.  The start time offset and end time offset of each cue MUST
   indicate the total display time for that cue, even if part of the cue
   time range is outside the Segment period.  A WebVTT Segment MAY




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   contain no cues; this indicates that no subtitles are to be displayed
   during that period.

   Each WebVTT Segment MUST either start with a WebVTT header or have an
   EXT-X-MAP tag applied to it.

   In order to synchronize timestamps between audio/video and subtitles,
   an X-TIMESTAMP-MAP metadata header SHOULD be added to each WebVTT
   header.  This header maps WebVTT cue timestamps to media timestamps
   in other Renditions of the Variant Stream.  Its format is:

   X-TIMESTAMP-MAP=LOCAL:<cue time>,MPEGTS:<media time>
   e.g., X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000

   indicating the media time to which the cue time MUST be mapped.  The
   cue timestamp in the LOCAL attribute MAY fall outside the range of
   time covered by the segment.

   The MPEGTS media timestamp MUST use a 90KHz timescale, even when non-
   WebVTT Media Segments use a different timescale.

   If a WebVTT segment does not have the X-TIMESTAMP-MAP, the client
   MUST assume that the WebVTT cue time of 0 maps to an media timestamp
   of 0.

   When synchronizing WebVTT with PES timestamps, clients SHOULD account
   for cases where the 33-bit PES timestamps have wrapped and the WebVTT
   cue times have not.  When the PES timestamp wraps, the WebVTT segment
   SHOULD have a X-TIMESTAMP-MAP header that maps the current WebVTT
   time to the new (low valued) PES timestamp.

3.1.5.  IMSC Subtitles

   An IMSC Segment is a Fragmented MPEG-4 (Section 3.1.2) Media Segment
   that carries subtitle media according to MPEG-4 Part 30
   [MP4_TIMED_TEXT].  This subtitle media MUST comply with the Text
   Profile of IMSC1 [IMSC1].

   The Media Initialization Section of an IMSC Segment is specified in
   Section 3.1.2.

   Each IMSC Segment MUST contain all subtitle samples that are intended
   to be displayed during the period indicated by the segment EXTINF
   duration.  Each Segment MUST contain definitions for all styles which
   are applied to any part of any sample in the Segment.






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3.2.  Partial Segments

   One component of viewer delay in a live stream is publishing latency:
   a Segment cannot be distributed until it has been completely encoded
   and packaged.  A long Segment encoded in real-time introduces a delay
   equal to its duration.  Partial Segments provide a parallel channel
   for distributing media at the live edge of the Media Playlist, where
   the media is divided into a larger number of smaller pieces, such as
   CMAF Chunks.  These subsets are called Partial Segments.  Because
   each Partial Segment has a short duration, it can be packaged,
   published, and added to the Media Playlist much earlier than its
   Parent Segment.

   A Partial Segment MUST be in one of the Supported Media Segment
   Formats described in Section 3.1.  A Partial Segment is associated
   with a regular Media Segment, called its Parent Segment, by appearing
   before it in the Media Playlist, and after the previous Media
   Segment.  Partial Segments are identified by the EXT-X-PART tag
   (Section 4.4.4.9).

   A Partial Segment MUST contain a subset of the media samples in its
   Parent Segment.  A Parent Segment and its entire set of Partial
   Segments MUST contain the same set of media samples, with the same
   timing and metadata.

   Each Partial Segment has a Part Index, which is an integer indicating
   the position of the Partial Segment within its Parent Segment.  The
   first Partial Segment has a Part Index of zero.

   Each Partial Segment also has a Media Sequence Number, which is equal
   to the Media Sequence Number of its Parent Segment.

4.  Playlists

   This section describes the Playlist files used by HTTP Live
   Streaming.  In this section, "MUST" and "MUST NOT" specify the rules
   for the syntax and structure of legal Playlist files.  Playlists that
   violate these rules are invalid; clients MUST fail to parse them.
   See Section 6.3.2.

   The format of the Playlist files is derived from the M3U [M3U]
   playlist file format and inherits two tags from that earlier file
   format: EXTM3U (Section 4.4.1.1) and EXTINF (Section 4.4.4.1).

   In the specification of tag syntax, a string enclosed by <>
   identifies a tag parameter; its specific format is described in its
   tag definition.  If a parameter is further surrounded by [], it is
   optional; otherwise, it is required.



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   Each Playlist file MUST be identifiable either by the path component
   of its URI or by HTTP Content-Type.  In the first case, the path MUST
   end with either .m3u8 or .m3u.  In the second, the HTTP Content-Type
   MUST be "application/vnd.apple.mpegurl" or "audio/mpegurl".  Clients
   SHOULD refuse to parse Playlists that are not so identified.

4.1.  Definition of a Playlist

   Playlist files MUST be encoded in UTF-8 [RFC3629].  They MUST NOT
   contain any Byte Order Mark (BOM); clients SHOULD fail to parse
   Playlists that contain a BOM or do not parse as UTF-8.  Playlist
   files MUST NOT contain UTF-8 control characters (U+0000 to U+001F and
   U+007F to U+009F), with the exceptions of CR (U+000D) and LF
   (U+000A).  All character sequences MUST be normalized according to
   Unicode normalization form "NFC" [UNICODE].  Note that US-ASCII
   [US_ASCII] conforms to these rules.

   Lines in a Playlist file are terminated by either a single line feed
   character or a carriage return character followed by a line feed
   character.  Each line is a URI, is blank, or starts with the
   character '#'.  Blank lines are ignored.  Whitespace 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 4.4.6.2).  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 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 4.4.4.1).  Note that
   this includes container overhead but does not include overhead
   imposed by the delivery system, such as HTTP, TCP, or IP headers.



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   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
   times the Target Duration and 1.5 times the Target Duration plus 0.5
   seconds (since media segments may exceed the Target Duration by up to
   0.5 seconds).  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:

   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.




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   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

   A Variable Reference is a string of the form "{$" (0x7B,0x24)
   followed by a Variable Name followed by "}" (0x7D).  Variable Names
   are defined by the EXT-X-DEFINE tag (Section 4.4.2.3).

   See Section 6.3.1 for more information about variable substitution.

4.4.  Playlist Tags

   Playlist tags specify either global parameters of the Playlist or
   information about the Media Segments or Media Playlists that appear
   after them.








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4.4.1.  Basic Tags

   These tags are allowed in both Media Playlists and Master Playlists.

4.4.1.1.  EXTM3U

   The EXTM3U tag indicates that the file is an Extended M3U [M3U]
   Playlist file.  It MUST be the first line of every Media Playlist and
   every Master Playlist.  Its format is:

   #EXTM3U

4.4.1.2.  EXT-X-VERSION

   The EXT-X-VERSION tag indicates the compatibility version of the
   Playlist file, its associated media, and its server.

   The EXT-X-VERSION tag applies to the entire Playlist file.  Its
   format is:

   #EXT-X-VERSION:<n>

   where n is an integer indicating the protocol compatibility version
   number.

   It MUST appear in all Playlists containing tags or attributes that
   are not compatible with protocol version 1 to support
   interoperability with older clients.  Section 7 specifies the minimum
   value of the compatibility version number for any given Playlist
   file.

   A Playlist file MUST NOT contain more than one EXT-X-VERSION tag.  If
   a client encounters a Playlist with multiple EXT-X-VERSION tags, it
   MUST fail to parse it.

4.4.2.  Media or Master Playlist Tags

   The tags in this section can appear in either Master Playlists or
   Media Playlists.  If one of these tags appears in a Master Playlist,
   it SHOULD NOT appear in any Media Playlist referenced by that Master
   Playlist.  A tag that appears in both MUST have the same value;
   otherwise, clients SHOULD ignore the value in the Media Playlist(s).

   Tags in this section MUST NOT appear more than once in a Playlist.
   If one does, clients MUST fail to parse the Playlist.  The only
   exception to this rule is EXT-X-DEFINE, which MAY appear more than
   once.




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4.4.2.1.  EXT-X-INDEPENDENT-SEGMENTS

   The EXT-X-INDEPENDENT-SEGMENTS tag indicates that all media samples
   in a Media Segment can be decoded without information from other
   segments.  It applies to every Media Segment in the Playlist.

   Its format is:

   #EXT-X-INDEPENDENT-SEGMENTS

   If the EXT-X-INDEPENDENT-SEGMENTS tag appears in a Master Playlist,
   it applies to every Media Segment in every Media Playlist in the
   Master Playlist.

4.4.2.2.  EXT-X-START

   The EXT-X-START tag indicates a preferred point at which to start
   playing a Playlist.  By default, clients SHOULD start playback at
   this point when beginning a playback session.  This tag is OPTIONAL.

   Its format is:

   #EXT-X-START:<attribute-list>

   The following attributes are defined:

      TIME-OFFSET

      The value of TIME-OFFSET is a signed-decimal-floating-point number
      of seconds.  A positive number indicates a time offset from the
      beginning of the Playlist.  A negative number indicates a negative
      time offset from the end of the last Media Segment in the
      Playlist.  This attribute is REQUIRED.

      The absolute value of TIME-OFFSET SHOULD NOT be larger than the
      Playlist duration.  If the absolute value of TIME-OFFSET exceeds
      the duration of the Playlist, it indicates either the end of the
      Playlist (if positive) or the beginning of the Playlist (if
      negative).

      If the Playlist does not contain the EXT-X-ENDLIST tag, the TIME-
      OFFSET SHOULD NOT be within three Target Durations of the end of
      the Playlist file.

      PRECISE

      The value is an enumerated-string; valid strings are YES and NO.
      If the value is YES, clients SHOULD start playback at the Media



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      Segment containing the TIME-OFFSET, but SHOULD NOT render media
      samples in that segment whose presentation times are prior to the
      TIME-OFFSET.  If the value is NO, clients SHOULD attempt to render
      every media sample in that segment.  This attribute is OPTIONAL.
      If it is missing, its value should be treated as NO.

4.4.2.3.  EXT-X-DEFINE

   The EXT-X-DEFINE tag provides a Playlist variable definition or
   declaration.  This tag is OPTIONAL.

   Its format is:

   #EXT-X-DEFINE:<attribute-list>

   The following attributes are defined:

      NAME

      The value is a quoted-string which specifies the Variable Name.
      All characters in the quoted-string MUST be from the following
      set: [a..z], [A..Z], [0..9], '-', and '_'.

      VALUE

      The value is a quoted-string which specifies the Variable Value.
      This attribute is REQUIRED if the EXT-X-DEFINE tag has a NAME
      attribute.

      IMPORT

      The value is a quoted-string which specifies the Variable Name and
      indicates that its value is that of the variable of the same name
      in the Master Playlist.  EXT-X-DEFINE tags containing the IMPORT
      attribute MUST NOT occur in Master Playlists; they are only
      allowed in Media Playlists.

      If the IMPORT attribute value does not match any Variable Name
      declared in the Master Playlist, or if the Media Playlist was not
      loaded from a Master Playlist, the parser MUST fail to parse the
      Playlist.

   An EXT-X-DEFINE tag MUST contain either a NAME or an IMPORT
   attribute, but not both.

   An EXT-X-DEFINE tag MUST NOT specify the same Variable Name as any
   other EXT-X-DEFINE tag in the same Playlist.  Parsers that encounter
   duplicate Variable Name declarations MUST fail to parse the Playlist.



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   EXT-X-DEFINE tags do NOT implicitly persist across Playlist reloads.

4.4.3.  Media Playlist Tags

   Media Playlist tags describe global parameters of the Media Playlist.
   There MUST NOT be more than one Media Playlist tag of each type in
   any Media Playlist.

   A Media Playlist tag MUST NOT appear in a Master Playlist

4.4.3.1.  EXT-X-TARGETDURATION

   The EXT-X-TARGETDURATION tag specifies the Target Duration, an upper
   bound on the duration of all Media Segments in the Playlist.  The
   EXTINF duration of each Media Segment in a Playlist file, when
   rounded to the nearest integer, MUST be less than or equal to the
   Target Duration.  Longer segments can trigger playback stalls or
   other errors.  It applies to the entire Playlist file.  Its format
   is:

   #EXT-X-TARGETDURATION:<s>

   where s is a decimal-integer indicating the Target Duration in
   seconds.  The EXT-X-TARGETDURATION tag is REQUIRED.

4.4.3.2.  EXT-X-MEDIA-SEQUENCE

   The EXT-X-MEDIA-SEQUENCE tag indicates the Media Sequence Number of
   the first Media Segment that appears in a Playlist file.  Its format
   is:

   #EXT-X-MEDIA-SEQUENCE:<number>

   where number is a decimal-integer.

   If the Media Playlist file does not contain an EXT-X-MEDIA-SEQUENCE
   tag, then the Media Sequence Number of the first Media Segment in the
   Media Playlist SHALL be considered to be 0.  A client MUST NOT assume
   that segments with the same Media Sequence Number in different Media
   Playlists contain matching content (see Section 6.3.2).

   A URI for a Media Segment is not required to contain its Media
   Sequence Number.

   See Section 6.2.1 and Section 6.3.5 for more information on setting
   the EXT-X-MEDIA-SEQUENCE tag.





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   The EXT-X-MEDIA-SEQUENCE tag MUST appear before the first Media
   Segment in the Playlist.

4.4.3.3.  EXT-X-DISCONTINUITY-SEQUENCE

   The EXT-X-DISCONTINUITY-SEQUENCE tag allows synchronization between
   different Renditions of the same Variant Stream or different Variant
   Streams that have EXT-X-DISCONTINUITY tags in their Media Playlists.

   Its format is:

   #EXT-X-DISCONTINUITY-SEQUENCE:<number>

   where number is a decimal-integer.

   If the Media Playlist does not contain an EXT-X-DISCONTINUITY-
   SEQUENCE tag, then the Discontinuity Sequence Number of the first
   Media Segment in the Playlist SHALL be considered to be 0.

   The EXT-X-DISCONTINUITY-SEQUENCE tag MUST appear before the first
   Media Segment in the Playlist.

   The EXT-X-DISCONTINUITY-SEQUENCE tag MUST appear before any EXT-
   X-DISCONTINUITY tag.

   See Section 6.2.1 and Section 6.2.2 for more information about
   setting the value of the EXT-X-DISCONTINUITY-SEQUENCE tag.

4.4.3.4.  EXT-X-ENDLIST

   The EXT-X-ENDLIST tag indicates that no more Media Segments will be
   added to the Media Playlist file.  It MAY occur anywhere in the Media
   Playlist file.  Its format is:

   #EXT-X-ENDLIST

4.4.3.5.  EXT-X-PLAYLIST-TYPE

   The EXT-X-PLAYLIST-TYPE tag provides mutability information about the
   Media Playlist file.  It applies to the entire Media Playlist file.
   It is OPTIONAL.  Its format is:

   #EXT-X-PLAYLIST-TYPE:<type-enum>

   where type-enum is either EVENT or VOD.

   Section 6.2.1 defines the implications of the EXT-X-PLAYLIST-TYPE
   tag.



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   If the EXT-X-PLAYLIST-TYPE value is EVENT, Media Segments can only be
   added to the end of the Media Playlist.  If the EXT-X-PLAYLIST-TYPE
   value is Video On Demand (VOD), the Media Playlist cannot change.

   If the EXT-X-PLAYLIST-TYPE tag is omitted from a Media Playlist, the
   Playlist can be updated according to the rules in Section 6.2.1 with
   no additional restrictions.  For example, a live Playlist
   (Section 6.2.2) MAY be updated to remove Media Segments in the order
   that they appeared.

4.4.3.6.  EXT-X-I-FRAMES-ONLY

   The EXT-X-I-FRAMES-ONLY tag indicates that each Media Segment in the
   Playlist describes a single I-frame.  I-frames are encoded video
   frames whose decoding does not depend on any other frame.  I-frame
   Playlists can be used for trick play, such as fast forward, rapid
   reverse, and scrubbing.

   The EXT-X-I-FRAMES-ONLY tag applies to the entire Playlist.  Its
   format is:

   #EXT-X-I-FRAMES-ONLY

   In a Playlist with the EXT-X-I-FRAMES-ONLY tag, the Media Segment
   duration (EXTINF tag value) is the time between the presentation time
   of the I-frame in the Media Segment and the presentation time of the
   next I-frame in the Playlist, or the end of the presentation if it is
   the last I-frame in the Playlist.

   Media resources containing I-frame segments MUST begin with either a
   Media Initialization Section (Section 3) or be accompanied by an EXT-
   X-MAP tag indicating the Media Initialization Section so that clients
   can load and decode I-frame segments in any order.  The byte range of
   an I-frame segment with an EXT-X-BYTERANGE tag applied to it
   (Section 4.4.4.2) MUST NOT include its Media Initialization Section;
   clients can assume that the Media Initialization Section is defined
   by the EXT-X-MAP tag, or is located between the start of the resource
   and the offset of the first I-frame segment in that resource.

   Use of the EXT-X-I-FRAMES-ONLY REQUIRES a compatibility version
   number of 4 or greater.

4.4.3.7.  EXT-X-PART-INF

   The EXT-X-PART-INF tag provides information about the Partial
   Segments in the Playlist.  It is REQUIRED if a Playlist contains one
   or more EXT-X-PART tags.  Its format is:




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   #EXT-X-PART-INF:<attribute-list>

   The following attributes are defined:

      PART-TARGET

      The value is a decimal-floating-point number of seconds indicating
      the Part Target Duration.  This attribute is REQUIRED.

4.4.3.8.  EXT-X-SERVER-CONTROL

   The EXT-X-SERVER-CONTROL tag allows the Server to indicate support
   for Delivery Directives (Section 6.2.5).  Its format is:.

   #EXT-X-SERVER-CONTROL:<attribute-list>

   The following attributes are defined:

      CAN-SKIP-UNTIL

      Indicates that the Server can produce Playlist Delta Updates in
      response to the _HLS_skip Delivery Directive.  Its value is the
      Skip Boundary, a decimal-floating-point number of seconds.  The
      Skip Boundary MUST be at least six times the Target Duration.

      This attribute is OPTIONAL.  It MAY appear in any Media Playlist.

      CAN-SKIP-DATERANGES

      A value of YES indicates that the Server can produce Playlist
      Delta Updates that skip older EXT-X-DATERANGE tags in addition to
      Media Segments.

      This attribute is OPTIONAL.  It REQUIRES the presence of the CAN-
      SKIP-UNTIL attribute.

      HOLD-BACK

      The value is a decimal-floating-point number of seconds that
      indicates the server-recommended minimum distance from the end of
      the Playlist at which clients should begin to play or to which
      they should seek, unless PART-HOLD-BACK applies.  Its value MUST
      be at least three times the Target Duration.

      This attribute is OPTIONAL.  Its absence implies a value of three
      times the Target Duration.  It MAY appear in any Media Playlist.

      PART-HOLD-BACK



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      The value is a decimal-floating-point number of seconds that
      indicates the server-recommended minimum distance from the end of
      the Playlist at which clients should begin to play or to which
      they should seek when playing in Low-Latency Mode.  Its value MUST
      be at least twice the Part Target Duration.  Its value SHOULD be
      at least three times the Part Target Duration.  If different
      Renditions have different Part Target Durations then PART-HOLD-
      BACK SHOULD be at least three times the maximum Part Target
      Duration.

      PART-HOLD-BACK is REQUIRED if the Playlist contains the EXT-X-
      PART-INF tag.

      CAN-BLOCK-RELOAD

      The value is an enumerated-string whose value is YES if the server
      supports Blocking Playlist Reload (Section 6.2.5.2).  This
      attribute is OPTIONAL; its absence implies no support.

4.4.4.  Media Segment Tags

   Each Media Segment is specified by a series of Media Segment tags
   followed by a URI.  Some Media Segment tags apply to just the next
   segment; others apply to all subsequent segments until another
   instance of the same tag.

   A Media Segment tag MUST NOT appear in a Master Playlist.  Clients
   MUST fail to parse Playlists that contain both Media Segment tags and
   Master Playlist tags (Section 4.4.6).

4.4.4.1.  EXTINF

   The EXTINF tag specifies the duration of a Media Segment.  It applies
   only to the next Media Segment.  This tag is REQUIRED for each Media
   Segment.  Its format is:

   #EXTINF:<duration>,[<title>]

   where duration is a decimal-floating-point or decimal-integer number
   (as described in Section 4.2) that specifies the duration of the
   Media Segment in seconds.  Durations SHOULD be decimal-floating-
   point, with enough accuracy to avoid perceptible error when segment
   durations are accumulated.  However, if the compatibility version
   number is less than 3, durations MUST be integers.  Durations that
   are reported as integers SHOULD be rounded to the nearest integer.
   The remainder of the line following the comma is an optional human-
   readable informative title of the Media Segment expressed as UTF-8
   text.



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4.4.4.2.  EXT-X-BYTERANGE

   The EXT-X-BYTERANGE tag indicates that a Media Segment is a sub-range
   of the resource identified by its URI.  It applies only to the next
   URI line that follows it in the Playlist.  Its format is:

   #EXT-X-BYTERANGE:<n>[@<o>]

   where n is a decimal-integer indicating the length of the sub-range
   in bytes.  If present, o is a decimal-integer indicating the start of
   the sub-range, as a byte offset from the beginning of the resource.
   If o is not present, the sub-range begins at the next byte following
   the sub-range of the previous Media Segment.

   If o is not present, a previous Media Segment MUST appear in the
   Playlist file and MUST be a sub-range of the same media resource, or
   the Media Segment is undefined and the client MUST fail to parse the
   Playlist.

   A Media Segment without an EXT-X-BYTERANGE tag consists of the entire
   resource identified by its URI.

   Use of the EXT-X-BYTERANGE tag REQUIRES a compatibility version
   number of 4 or greater.

4.4.4.3.  EXT-X-DISCONTINUITY

   The EXT-X-DISCONTINUITY tag indicates a discontinuity between the
   Media Segment that follows it and the one that preceded it.

   Its format is:

   #EXT-X-DISCONTINUITY

   The EXT-X-DISCONTINUITY tag MUST be present if there is a change in
   any of the following characteristics:

   o  file format

   o  number, type, and identifiers of tracks

   o  timestamp sequence

   The EXT-X-DISCONTINUITY tag SHOULD be present if there is a change in
   any of the following characteristics:

   o  encoding parameters




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   o  encoding sequence

   See Section 3, Section 6.2.1, and Section 6.3.3 for more information
   about the EXT-X-DISCONTINUITY tag.

4.4.4.4.  EXT-X-KEY

   Media Segments MAY be encrypted.  The EXT-X-KEY tag specifies how to
   decrypt them.  It applies to every Media Segment and to every Media
   Initialization Section declared by an EXT-X-MAP tag that appears
   between it and the next EXT-X-KEY tag in the Playlist file with the
   same KEYFORMAT attribute (or the end of the Playlist file).  Two or
   more EXT-X-KEY tags with different KEYFORMAT attributes MAY apply to
   the same Media Segment if they ultimately produce the same decryption
   key.  The format is:

   #EXT-X-KEY:<attribute-list>

   The following attributes are defined:

      METHOD

      The value is an enumerated-string that specifies the encryption
      method.  This attribute is REQUIRED.

      The methods defined are: NONE, AES-128, and SAMPLE-AES.

      An encryption method of NONE means that Media Segments are not
      encrypted.  If the encryption method is NONE, other attributes
      MUST NOT be present.

      An encryption method of AES-128 signals that Media Segments are
      completely encrypted using the Advanced Encryption Standard (AES)
      [AES_128] with a 128-bit key, Cipher Block Chaining (CBC), and
      Public-Key Cryptography Standards #7 (PKCS7) padding [RFC5652].
      CBC is restarted on each segment boundary, using either the
      Initialization Vector (IV) attribute value or the Media Sequence
      Number as the IV; see Section 5.2.

      An alternative to whole-segment encryption is Sample Encryption.
      With Sample Encryption, only media sample data - such as audio
      packets or video frames - is encrypted.  The rest of the Media
      Segment is unencrypted.  Sample Encryption allows parts of the
      Segment to be processed without (or before) decrypting the media
      itself.

      An encryption method of SAMPLE-AES means that the Media Segments
      are Sample Encrypted using the Advanced Encryption Standard



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      [AES_128].  How these media streams are encrypted and encapsulated
      in a segment depends on the media encoding and the media format of
      the segment.  fMP4 Media Segments are encrypted using the 'cbcs'
      scheme of Common Encryption [COMMON_ENC].  Encryption of other
      Media Segment formats containing H.264 [H_264], AAC [ISO_14496],
      AC-3 [AC_3], and Enhanced AC-3 [AC_3] media streams is described
      in the HTTP Live Streaming (HLS) Sample Encryption specification
      [SampleEnc].  The IV attribute MAY be present; see Section 5.2.

      URI

      The value is a quoted-string containing a URI that specifies how
      to obtain the key.  This attribute is REQUIRED unless the METHOD
      is NONE.

      IV

      The value is a hexadecimal-sequence that specifies a 128-bit
      unsigned integer Initialization Vector to be used with the key.
      Use of the IV attribute REQUIRES a compatibility version number of
      2 or greater.  See Section 5.2 for when the IV attribute is used.

      KEYFORMAT

      The value is a quoted-string that specifies how the key is
      represented in the resource identified by the URI; see Section 5
      for more detail.  This attribute is OPTIONAL; its absence
      indicates an implicit value of "identity".  Use of the KEYFORMAT
      attribute REQUIRES a compatibility version number of 5 or greater.

      KEYFORMATVERSIONS

      The value is a quoted-string containing one or more positive
      integers separated by the "/" character (for example, "1", "1/2",
      or "1/2/5").  If more than one version of a particular KEYFORMAT
      is defined, this attribute can be used to indicate which
      version(s) this instance complies with.  This attribute is
      OPTIONAL; if it is not present, its value is considered to be "1".
      Use of the KEYFORMATVERSIONS attribute REQUIRES a compatibility
      version number of 5 or greater.

   If the Media Playlist file does not contain an EXT-X-KEY tag, then
   Media Segments are not encrypted.

   See Section 5 for the format of the Key file, and Section 5.2,
   Section 6.2.3, and Section 6.3.6 for additional information on Media
   Segment encryption.




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4.4.4.5.  EXT-X-MAP

   The EXT-X-MAP tag specifies how to obtain the Media Initialization
   Section (Section 3) required to parse the applicable Media Segments.
   It applies to every Media Segment that appears after it in the
   Playlist until the next EXT-X-MAP tag or until the end of the
   Playlist.

   Its format is:

   #EXT-X-MAP:<attribute-list>

   The following attributes are defined:

      URI

      The value is a quoted-string containing a URI that identifies a
      resource that contains the Media Initialization Section.  This
      attribute is REQUIRED.

      BYTERANGE

      The value is a quoted-string specifying a byte range into the
      resource identified by the URI attribute.  This range SHOULD
      contain only the Media Initialization Section.  The format of the
      byte range is described in Section 4.4.4.2.  This attribute is
      OPTIONAL; if it is not present, the byte range is the entire
      resource indicated by the URI.

   An EXT-X-MAP tag SHOULD be supplied for Media Segments in Playlists
   with the EXT-X-I-FRAMES-ONLY tag when the first Media Segment (i.e.,
   I-frame) in the Playlist (or the first segment following an EXT-
   X-DISCONTINUITY tag) does not immediately follow the Media
   Initialization Section at the beginning of its resource.

   Use of the EXT-X-MAP tag in a Media Playlist that contains the EXT-
   X-I-FRAMES-ONLY tag REQUIRES a compatibility version number of 5 or
   greater.  Use of the EXT-X-MAP tag in a Media Playlist that DOES NOT
   contain the EXT-X-I-FRAMES-ONLY tag REQUIRES a compatibility version
   number of 6 or greater.

   If the Media Initialization Section declared by an EXT-X-MAP tag is
   encrypted with a METHOD of AES-128, the IV attribute of the EXT-X-KEY
   tag that applies to the EXT-X-MAP is REQUIRED.







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4.4.4.6.  EXT-X-PROGRAM-DATE-TIME

   The EXT-X-PROGRAM-DATE-TIME tag associates the first sample of a
   Media Segment with an absolute date and/or time.  It applies only to
   the next Media Segment.  Its format is:

   #EXT-X-PROGRAM-DATE-TIME:<date-time-msec>

   where date-time-msec is an ISO/IEC 8601:2004 [ISO_8601] date/time
   representation, such as YYYY-MM-DDThh:mm:ss.SSSZ.  It SHOULD indicate
   a time zone and fractional parts of seconds, to millisecond accuracy.

   For example:

   #EXT-X-PROGRAM-DATE-TIME:2010-02-19T14:54:23.031+08:00

   See Section 6.2.1 and Section 6.3.3 for more information on the EXT-
   X-PROGRAM-DATE-TIME tag.

4.4.4.7.  EXT-X-GAP

   The EXT-X-GAP tag indicates that the segment URL to which it applies
   does not contain media data and SHOULD NOT be loaded by clients.  It
   applies only to the next Media Segment.

   Its format is:

   #EXT-X-GAP

   See Section 6.2.1 and Section 6.3.3 for more information on the EXT-
   X-GAP tag.

4.4.4.8.  EXT-X-BITRATE

   The EXT-X-BITRATE tag identifies the approximate segment bit rate of
   the Media Segment(s) to which it applies.  It applies to every Media
   Segment between it and the next EXT-X-BITRATE tag in the Playlist
   file (or the end of the Playlist file) that does not have an EXT-
   X-BYTERANGE tag applied to it.  Its format is:

   #EXT-X-BITRATE:<rate>

   where rate is a decimal-integer of kilobits per second.

   This tag is OPTIONAL.  If it is present then its value MUST be no
   less than 90% of the segment bit rate of each Media Segment to which
   it is applied and no greater than 110% of the segment bit rate of
   each Media Segment to which it is applied.



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4.4.4.9.  EXT-X-PART

   The EXT-X-PART tag identifies a Partial Segment.  It is OPTIONAL.
   Its format is:

   #EXT-X-PART:<attribute-list>

   The following attributes are defined:

      URI

      The value is the URI for the Partial Segment.  This attribute is
      REQUIRED.

      DURATION

      The value is the duration of the Partial Segment as a decimal-
      floating-point number of seconds.  This attribute is REQUIRED.

      INDEPENDENT

      The value is an enumerated-string whose value is YES if the
      Partial Segment contains an independent frame.  This attribute is
      OPTIONAL; however every Partial Segment containing an independent
      frame SHOULD carry it to increase the efficiency with which
      clients can join and switch Renditions.

      BYTERANGE

      Indicates that the Partial Segment is a subrange of the resource
      specified by the URI attribute.  The value has the same format as
      the EXT-X-BYTERANGE tag: <n>[@<o>].

      GAP

      The value is an enumerated-string whose value is YES if the
      Partial Segment is not available.  It is REQUIRED for such Partial
      Segments.

   All Media Segment Tags (Section 4.4.4) except for EXT-X-BYTERANGE and
   EXT-X-GAP that are applied to a Parent Segment MUST appear before the
   first EXT-X-PART tag of that Parent Segment.

   The duration of a Partial Segment MUST be less than or equal to the
   Part Target Duration.  The duration of each Partial Segment MUST be
   at least 85% of the Part Target Duration, with the exception of
   Partial Segments with the INDEPENDENT=YES attribute and the final
   Partial Segment of any Parent Segment.



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4.4.5.  Media Metadata Tags

   Media Metadata tags provide information about the playlist that is
   not associated with specific Media Segments.  There MAY be more than
   one Media Metadata tag of each type in any Media Playlist.

4.4.5.1.  EXT-X-DATERANGE

   The EXT-X-DATERANGE tag associates a Date Range (i.e., a range of
   time defined by a starting and ending date) with a set of attribute/
   value pairs.  Its format is:

   #EXT-X-DATERANGE:<attribute-list>

   where the defined attributes are:

      ID

      A quoted-string that uniquely identifies a Date Range in the
      Playlist.  This attribute is REQUIRED.

      CLASS

      A client-defined quoted-string that specifies some set of
      attributes and their associated value semantics.  All Date Ranges
      with the same CLASS attribute value MUST adhere to these
      semantics.  This attribute is OPTIONAL.

      START-DATE

      A quoted-string containing the [ISO_8601] date/time at which the
      Date Range begins.  This attribute is REQUIRED.

      END-DATE

      A quoted-string containing the [ISO_8601] date/time at which the
      Date Range ends.  It MUST be equal to or later than the value of
      the START-DATE attribute.  This attribute is OPTIONAL.

      DURATION

      The duration of the Date Range expressed as a decimal-floating-
      point number of seconds.  It MUST NOT be negative.  A single
      instant in time (e.g., crossing a finish line) SHOULD be
      represented with a duration of 0.  This attribute is OPTIONAL.

      PLANNED-DURATION




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      The expected duration of the Date Range expressed as a decimal-
      floating-point number of seconds.  It MUST NOT be negative.  This
      attribute SHOULD be used to indicate the expected duration of a
      Date Range whose actual duration is not yet known.  It is
      OPTIONAL.

      X-<client-attribute>

      The "X-" prefix defines a namespace reserved for client-defined
      attributes.  The client-attribute MUST be a legal AttributeName.
      Clients SHOULD use a reverse-DNS syntax when defining their own
      attribute names to avoid collisions.  The attribute value MUST be
      a quoted-string, a hexadecimal-sequence, or a decimal-floating-
      point.  An example of a client-defined attribute is X-COM-EXAMPLE-
      AD-ID="XYZ123".  These attributes are OPTIONAL.

      SCTE35-CMD, SCTE35-OUT, SCTE35-IN

      Used to carry SCTE-35 data; see Section 4.4.5.1.1 for more
      information.  These attributes are OPTIONAL.

      END-ON-NEXT

      An enumerated-string whose value MUST be YES.  This attribute
      indicates that the end of the range containing it is equal to the
      START-DATE of its Following Range.  The Following Range is the
      Date Range of the same CLASS that has the earliest START-DATE
      after the START-DATE of the range in question.  This attribute is
      OPTIONAL.

   An EXT-X-DATERANGE tag with an END-ON-NEXT=YES attribute MUST have a
   CLASS attribute.  Other EXT-X-DATERANGE tags with the same CLASS
   attribute MUST NOT specify Date Ranges that overlap.

   An EXT-X-DATERANGE tag with an END-ON-NEXT=YES attribute MUST NOT
   contain DURATION or END-DATE attributes.

   A Date Range with neither a DURATION, an END-DATE, nor an END-ON-
   NEXT=YES attribute has an unknown duration, even if it has a PLANNED-
   DURATION.

   If a Playlist contains an EXT-X-DATERANGE tag, it MUST also contain
   at least one EXT-X-PROGRAM-DATE-TIME tag.

   If a Playlist contains two EXT-X-DATERANGE tags with the same ID
   attribute value, then any AttributeName that appears in both tags
   MUST have the same AttributeValue.




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   If a Date Range contains both a DURATION attribute and an END-DATE
   attribute, the value of the END-DATE attribute MUST be equal to the
   value of the START-DATE attribute plus the value of the DURATION
   attribute.

   Clients SHOULD ignore EXT-X-DATERANGE tags with illegal syntax.

4.4.5.1.1.  Mapping SCTE-35 into EXT-X-DATERANGE

   Splice information carried in source media according to the SCTE-35
   specification [SCTE35] MAY be represented in a Media Playlist using
   EXT-X-DATERANGE tags.

   Each SCTE-35 splice_info_section() containing a splice_null(),
   splice_schedule(), bandwidth_reservation(), or private_cmd() SHOULD
   be represented by an EXT-X-DATERANGE tag with an SCTE35-CMD attribute
   whose value is the big-endian binary representation of the
   splice_info_section(), expressed as a hexadecimal-sequence.

   An SCTE-35 splice out/in pair signaled by a pair of splice_insert()
   commands SHOULD be represented by one or more EXT-X-DATERANGE tags
   carrying the same ID attribute, which MUST be unique to that splice
   out/in pair.  The "out" splice_info_section() (with
   out_of_network_indicator set to 1) MUST be placed in an SCTE35-OUT
   attribute, with the same formatting as SCTE35-CMD.  The "in"
   splice_info_section() (with out_of_network_indicator set to 0) MUST
   be placed in an SCTE35-IN attribute, with the same formatting as
   SCTE35-CMD.

   An SCTE-35 splice out/in pair signaled by a pair of time_signal()
   commands, each carrying a single segmentation_descriptor(), SHOULD be
   represented by one or more EXT-X-DATERANGE tags carrying the same ID
   attribute, which MUST be unique to that splice out/in pair.  The
   "out" splice_info_section() MUST be placed in an SCTE35-OUT
   attribute; the "in" splice_info_section() MUST be placed in an
   SCTE35-IN attribute.

   Different types of segmentation, as indicated by the
   segmentation_type_id in the segmentation_descriptor(), SHOULD be
   represented by separate EXT-X-DATERANGE tags, even if two or more
   segmentation_descriptor()s arrive in the same splice_info_section().
   In that case, each EXT-X-DATERANGE tag will have an SCTE35-OUT,
   SCTE35-IN, or SCTE35-CMD attribute whose value is the entire
   splice_info_section().

   An SCTE-35 time_signal() command that does not signal a splice out or
   in point SHOULD be represented by an EXT-X-DATERANGE tag with an
   SCTE35-CMD attribute.



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   The START-DATE of an EXT-X-DATERANGE tag containing an SCTE35-OUT
   attribute MUST be the date and time that corresponds to the program
   time of that splice.

   The START-DATE of an EXT-X-DATERANGE tag containing an SCTE35-CMD
   MUST be the date and time specified by the splice_time() in the
   command or the program time at which the command appeared in the
   source stream if the command does not specify a splice_time().

   An EXT-X-DATERANGE tag containing an SCTE35-OUT attribute MAY contain
   a PLANNED-DURATION attribute.  Its value MUST be the planned duration
   of the splice.

   The DURATION of an EXT-X-DATERANGE tag containing an SCTE35-IN
   attribute MUST be the actual (not planned) program duration between
   the corresponding out-point and that in-point.

   The END-DATE of an EXT-X-DATERANGE tag containing an SCTE35-IN
   attribute MUST be the actual (not planned) program date and time of
   that in-point.

   If the actual end date and time is not known when an SCTE35-OUT
   attribute is added to the Playlist, the DURATION attribute and the
   END-TIME attribute MUST NOT be present; the actual end date of the
   splice SHOULD be signaled by another EXT-X-DATERANGE tag once it has
   been established.

   A canceled splice SHOULD NOT appear in the Playlist as an EXT-
   X-DATERANGE tag.

   An EXT-X-DATERANGE tag announcing a splice SHOULD be added to a
   Playlist at the same time as the last pre-splice Media Segment, or
   earlier if possible.

   The ID attribute of an EXT-X-DATERANGE tag MAY contain a
   splice_event_id and/or a segmentation_event_id, but it MUST be unique
   in the Playlist.  If there is a possibility that an SCTE-35 id will
   be reused, the ID attribute value MUST include disambiguation, such
   as a date or sequence number.

4.4.5.1.2.  EXT-X-SKIP

   A server produces a Playlist Delta Update (Section 6.2.5.1), by
   replacing tags earlier than the Skip Boundary with an EXT-X-SKIP tag.

   When replacing Media Segments, the EXT-X-SKIP tag replaces the
   segment URI lines and all Media Segment Tags tags that are applied to
   those segments.



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   Its format is:

   #EXT-X-SKIP:<attribute-list>

   The following attributes are defined:

      SKIPPED-SEGMENTS

      The value is the count of Media Segments were replaced by the EXT-
      X-SKIP tag.  This attribute is REQUIRED.

      RECENTLY-REMOVED-DATERANGES

      The value is a quoted-string consisting of a tab (0x9) delimited
      list of EXT-X-DATERANGE IDs that have been removed from the
      Playlist recently.  See Section 6.2.5.1 for more information.
      This attribute is REQUIRED if the Client requested an update that
      skips EXT-X-DATERANGE tags.

4.4.5.1.3.  EXT-X-PRELOAD-HINT

   The EXT-X-PRELOAD-HINT tag allows a Client loading media from a live
   stream to reduce the time to obtain a resource from the Server by
   issuing its request before the resource is available to be delivered.
   The server will hold onto the request ("block") until it can respond.

   Its format is:

   #EXT-X-PRELOAD-HINT:<attribute-list>

   The following attributes are defined:

      TYPE

      The value is an enumerated-string that specifies the type of the
      hinted resource.  If the value is PART, the resource is a Partial
      Segment.  If the value is MAP, the resource is a Media
      Initialization Section.  This attribute is REQUIRED.

      URI

      The value is a URI identifying the hinted resource.  It MUST match
      the URI that will be subsequently added to the Playlist as a non-
      hinted resource (for example, the URI of an EXT-X-PART tag).  The
      URI MAY be relative to the URI of the Playlist or it MAY be
      absolute.  The hostname MAY differ from the hostname of the
      Playlist URI.  This attribute is REQUIRED.




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      BYTERANGE-START

      The value is the byte offset of the first byte of the hinted
      resource, from the beginning of the resource identified by the URI
      attribute.  This attribute is OPTIONAL.  Its absence implies a
      value of 0.

      BYTERANGE-LENGTH

      The value is the length of the hinted resource.  This attribute is
      OPTIONAL.  Its absence indicates that the last byte of the hinted
      resource is the last byte of the resource identified by the URI
      attribute.

   Note that when a hinted Partial Segment eventually appears in the
   Playlist as an EXT-X-PART tag, it MAY have a different Discontinuity
   Sequence Number, Media Initialization Section, or encryption
   configuration.  In other words, the Partial Segment can be preceded
   by an EXTINF tag indicating the end of the previous Parent Segment
   and an EXT-X-DISCONTINUITY, EXT-X-MAP, or EXT-X-KEY tag.

   A Playlist containing an EXT-X-ENDLIST tag MUST NOT contain an EXT-X-
   PRELOAD-HINT tag.

4.4.5.1.4.  EXT-X-RENDITION-REPORT

   The EXT-X-RENDITION-REPORT tag carries information about an
   associated Rendition that is as up-to-date as the Playlist that
   contains it.  Its format is:

   #EXT-X-RENDITION-REPORT:<attribute-list>

   The following attributes are defined:

      URI

      The value is the URI for the Media Playlist of the specified
      Rendition.  It MUST be relative to the URI of the Media Playlist
      containing the EXT-X-RENDITION-REPORT tag.  This attribute is
      REQUIRED.

      LAST-MSN

      The value is a decimal-integer specifying the Media Sequence
      Number of the last Media Segment currently in the specified
      Rendition.  If the Rendition contains Partial Segments then this
      value is the Media Sequence Number of the last Partial Segment.
      This attribute is REQUIRED.



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      LAST-PART

      The value is a decimal-integer that indicates the Part Index of
      the last Partial Segment currently in the specified Rendition
      whose Media Sequence Number is equal to the LAST-MSN attribute
      value.  This attribute is REQUIRED if the Rendition contains a
      Partial Segment.

   A server MAY omit adding an attribute to an EXT-X-RENDITION-REPORT
   tag - even a mandatory attribute - if its value is the same as that
   of the Rendition Report of the Media Playlist to which the EXT-X-
   RENDITION-REPORT tag is being added.  Doing so reduces the size of
   the Rendition Report.

4.4.6.  Master Playlist Tags

   Master Playlist tags define the Variant Streams, Renditions, and
   other global parameters of the presentation.

   Master Playlist tags MUST NOT appear in a Media Playlist; clients
   MUST fail to parse any Playlist that contains both a Master Playlist
   tag and either a Media Playlist tag or a Media Segment tag.

4.4.6.1.  EXT-X-MEDIA

   The EXT-X-MEDIA tag is used to relate Media Playlists that contain
   alternative Renditions (Section 4.4.6.2.1) of the same content.  For
   example, three EXT-X-MEDIA tags can be used to identify audio-only
   Media Playlists that contain English, French, and Spanish Renditions
   of the same presentation.  Or, two EXT-X-MEDIA tags can be used to
   identify video-only Media Playlists that show two different camera
   angles.

   Its format is:

   #EXT-X-MEDIA:<attribute-list>

   The following attributes are defined:

      TYPE

      The value is an enumerated-string; valid strings are AUDIO, VIDEO,
      SUBTITLES, and CLOSED-CAPTIONS.  This attribute is REQUIRED.

      Typically, closed-caption [CEA608] media is carried in the video
      stream.  Therefore, an EXT-X-MEDIA tag with TYPE of CLOSED-
      CAPTIONS does not specify a Rendition; the closed-caption media is
      present in the Media Segments of every video Rendition.



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      URI

      The value is a quoted-string containing a URI that identifies the
      Media Playlist file.  This attribute is OPTIONAL; see
      Section 4.4.6.2.1.  If the TYPE is CLOSED-CAPTIONS, the URI
      attribute MUST NOT be present.

      GROUP-ID

      The value is a quoted-string that specifies the group to which the
      Rendition belongs.  See Section 4.4.6.1.1.  This attribute is
      REQUIRED.

      LANGUAGE

      The value is a quoted-string containing one of the standard Tags
      for Identifying Languages [RFC5646], which identifies the primary
      language used in the Rendition.  This attribute is OPTIONAL.

      ASSOC-LANGUAGE

      The value is a quoted-string containing a language tag [RFC5646]
      that identifies a language that is associated with the Rendition.
      An associated language is often used in a different role than the
      language specified by the LANGUAGE attribute (e.g., written versus
      spoken, or a fallback dialect).  This attribute is OPTIONAL.

      The LANGUAGE and ASSOC-LANGUAGE attributes can be used, for
      example, to link Norwegian Renditions that use different spoken
      and written languages.

      NAME

      The value is a quoted-string containing a human-readable
      description of the Rendition.  If the LANGUAGE attribute is
      present, then this description SHOULD be in that language.  This
      attribute is REQUIRED.

      DEFAULT

      The value is an enumerated-string; valid strings are YES and NO.
      If the value is YES, then the client SHOULD play this Rendition of
      the content in the absence of information from the user indicating
      a different choice.  This attribute is OPTIONAL.  Its absence
      indicates an implicit value of NO.

      AUTOSELECT




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      The value is an enumerated-string; valid strings are YES and NO.
      This attribute is OPTIONAL.  Its absence indicates an implicit
      value of NO.  If the value is YES, then the client MAY choose to
      play this Rendition in the absence of explicit user preference
      because it matches the current playback environment, such as
      chosen system language.

      If the AUTOSELECT attribute is present, its value MUST be YES if
      the value of the DEFAULT attribute is YES.

      FORCED

      The value is an enumerated-string; valid strings are YES and NO.
      This attribute is OPTIONAL.  Its absence indicates an implicit
      value of NO.  The FORCED attribute MUST NOT be present unless the
      TYPE is SUBTITLES.

      A value of YES indicates that the Rendition contains content that
      is considered essential to play.  When selecting a FORCED
      Rendition, a client SHOULD choose the one that best matches the
      current playback environment (e.g., language).

      A value of NO indicates that the Rendition contains content that
      is intended to be played in response to explicit user request.

      INSTREAM-ID

      The value is a quoted-string that specifies a Rendition within the
      segments in the Media Playlist.  This attribute is REQUIRED if the
      TYPE attribute is CLOSED-CAPTIONS, in which case it MUST have one
      of the values: "CC1", "CC2", "CC3", "CC4", or "SERVICEn" where n
      MUST be an integer between 1 and 63 (e.g., "SERVICE9" or
      "SERVICE42").

      The values "CC1", "CC2", "CC3", and "CC4" identify a Line 21 Data
      Services channel [CEA608].  The "SERVICE" values identify a
      Digital Television Closed Captioning [CEA708] service block
      number.

      For all other TYPE values, the INSTREAM-ID MUST NOT be specified.

      CHARACTERISTICS

      The value is a quoted-string containing one or more Uniform Type
      Identifiers [UTI] separated by comma (,) characters.  This
      attribute is OPTIONAL.  Each UTI indicates an individual
      characteristic of the Rendition.




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      A SUBTITLES Rendition MAY include the following characteristics:
      "public.accessibility.transcribes-spoken-dialog",
      "public.accessibility.describes-music-and-sound", and
      "public.easy-to-read" (which indicates that the subtitles have
      been edited for ease of reading).

      An AUDIO Rendition MAY include the following characteristic:
      "public.accessibility.describes-video".

      The CHARACTERISTICS attribute MAY include private UTIs.

      CHANNELS

      The value is a quoted-string that specifies an ordered, slash-
      separated ("/") list of parameters.

      If the TYPE attribute is AUDIO, then the first parameter is a
      count of audio channels expressed as a decimal-integer, indicating
      the maximum number of independent, simultaneous audio channels
      present in any Media Segment in the Rendition.  For example, an
      AC-3 5.1 Rendition would have a CHANNELS="6" attribute.

      If the TYPE attribute is AUDIO, then the second parameter
      identifies the encoding of object-based audio used by the
      Rendition.  This parameter is a comma-separated list of Audio
      Object Coding Identifiers.  It is optional.  An Audio Object
      Coding Identifier is a string containing characters from the set
      [A..Z], [0..9], and '-'.  They are codec-specific.  A parameter
      value of consisting solely of the dash character (0x2D) indicates
      that the audio is not object-based.

      No other CHANNELS parameters are currently defined.

      All audio EXT-X-MEDIA tags SHOULD have a CHANNELS attribute.  If a
      Master Playlist contains two Renditions with the same NAME encoded
      with the same codec but a different number of channels, then the
      CHANNELS attribute is REQUIRED; otherwise, it is OPTIONAL.

4.4.6.1.1.  Rendition Groups

   A set of one or more EXT-X-MEDIA tags with the same GROUP-ID value
   and the same TYPE value defines a Group of Renditions.  Each member
   of the Group MUST be an alternative Rendition of the same content;
   otherwise, playback errors can occur.

   All EXT-X-MEDIA tags in a Playlist MUST meet the following
   constraints:




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   o  All EXT-X-MEDIA tags in the same Group MUST have different NAME
      attributes.

   o  A Group MUST NOT have more than one member with a DEFAULT
      attribute of YES.

   o  Each EXT-X-MEDIA tag with an AUTOSELECT=YES attribute SHOULD have
      a combination of LANGUAGE [RFC5646], ASSOC-LANGUAGE, FORCED, and
      CHARACTERISTICS attributes that is distinct from those of other
      AUTOSELECT=YES members of its Group.

   A Playlist MAY contain multiple Groups of the same TYPE in order to
   provide multiple encodings of that media type.  If it does so, each
   Group of the same TYPE MUST have the same set of members, and each
   corresponding member MUST have identical attributes with the
   exception of the URI and CHANNELS attributes.

   Each member in a Group of Renditions MAY have a different sample
   format.  For example, an English Rendition can be encoded with AC-3
   5.1 while a Spanish Rendition is encoded with AAC stereo.  However,
   any EXT-X-STREAM-INF tag (Section 4.4.6.2) or EXT-X-I-FRAME-STREAM-
   INF tag (Section 4.4.6.3) that references such a Group MUST have a
   CODECS attribute that lists every sample format present in any
   Rendition in the Group, or client playback failures can occur.  In
   the example above, the CODECS attribute would include
   "ac-3,mp4a.40.2".

4.4.6.2.  EXT-X-STREAM-INF

   The EXT-X-STREAM-INF tag specifies a Variant Stream, which is a set
   of Renditions that can be combined to play the presentation.  The
   attributes of the tag provide information about the Variant Stream.

   The URI line that follows the EXT-X-STREAM-INF tag specifies a Media
   Playlist that carries a Rendition of the Variant Stream.  The URI
   line is REQUIRED.  Clients that do not support multiple video
   Renditions SHOULD play this Rendition.

   Its format is:

   #EXT-X-STREAM-INF:<attribute-list>
   <URI>

   The following attributes are defined:

      BANDWIDTH





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      The value is a decimal-integer of bits per second.  It represents
      the peak segment bit rate of the Variant Stream.

      If all the Media Segments in a Variant Stream have already been
      created, the BANDWIDTH value MUST be the largest sum of peak
      segment bit rates that is produced by any playable combination of
      Renditions.  (For a Variant Stream with a single Media Playlist,
      this is just the peak segment bit rate of that Media Playlist.)
      An inaccurate value can cause playback stalls or prevent clients
      from playing the variant.

      If the Master Playlist is to be made available before all Media
      Segments in the presentation have been encoded, the BANDWIDTH
      value SHOULD be the BANDWIDTH value of a representative period of
      similar content, encoded using the same settings.

      Every EXT-X-STREAM-INF tag MUST include the BANDWIDTH attribute.

      AVERAGE-BANDWIDTH

      The value is a decimal-integer of bits per second.  It represents
      the average segment bit rate of the Variant Stream.

      If all the Media Segments in a Variant Stream have already been
      created, the AVERAGE-BANDWIDTH value MUST be the largest sum of
      average segment bit rates that is produced by any playable
      combination of Renditions.  (For a Variant Stream with a single
      Media Playlist, this is just the average segment bit rate of that
      Media Playlist.)  An inaccurate value can cause playback stalls or
      prevent clients from playing the variant.

      If the Master Playlist is to be made available before all Media
      Segments in the presentation have been encoded, the AVERAGE-
      BANDWIDTH value SHOULD be the AVERAGE-BANDWIDTH value of a
      representative period of similar content, encoded using the same
      settings.

      The AVERAGE-BANDWIDTH attribute is OPTIONAL.

      CODECS

      The value is a quoted-string containing a comma-separated list of
      formats, where each format specifies a media sample type that is
      present in one or more Renditions specified by the Variant Stream.
      Valid format identifiers are those in the ISO Base Media File
      Format Name Space defined by "The 'Codecs' and 'Profiles'
      Parameters for "Bucket" Media Types" [RFC6381].




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      For example, a stream containing AAC low complexity (AAC-LC) audio
      and H.264 Main Profile Level 3.0 video would have a CODECS value
      of "mp4a.40.2,avc1.4d401e".

      Note that if a Variant Stream specifies one or more Renditions
      that include IMSC subtitles, the CODECS attribute MUST indicate
      this with a format identifier such as "stpp.ttml.im1t".

      Every EXT-X-STREAM-INF tag SHOULD include a CODECS attribute.

      RESOLUTION

      The value is a decimal-resolution describing the optimal pixel
      resolution at which to display all the video in the Variant
      Stream.

      The RESOLUTION attribute is OPTIONAL but is recommended if the
      Variant Stream includes video.

      FRAME-RATE

      The value is a decimal-floating-point describing the maximum frame
      rate for all the video in the Variant Stream, rounded to three
      decimal places.

      The FRAME-RATE attribute is OPTIONAL but is recommended if the
      Variant Stream includes video.  The FRAME-RATE attribute SHOULD be
      included if any video in a Variant Stream exceeds 30 frames per
      second.

      HDCP-LEVEL

      The value is an enumerated-string; valid strings are TYPE-0, TYPE-
      1, and NONE.  This attribute is advisory.  A value of TYPE-0
      indicates that the Variant Stream could fail to play unless the
      output is protected by High-bandwidth Digital Content Protection
      (HDCP) Type 0 [HDCP] or equivalent.  A value of TYPE-1 indicates
      that the Variant Stream could fail to play unless the output is
      protected by HDCP Type 1 or equivalent.  A value of NONE indicates
      that the content does not require output copy protection.

      Encrypted Variant Streams with different HDCP levels SHOULD use
      different media encryption keys.

      The HDCP-LEVEL attribute is OPTIONAL.  It SHOULD be present if any
      content in the Variant Stream will fail to play without HDCP.
      Clients without output copy protection SHOULD NOT load a Variant
      Stream with an HDCP-LEVEL attribute unless its value is NONE.



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      ALLOWED-CPC

      The ALLOWED-CPC attribute allows a server to indicate that the
      playback of a Variant Stream containing encrypted Media Segments
      is to be restricted to devices that guarantee a certain level of
      content protection robustness.  Its value is a quoted-string
      containing a comma-separated list of entries.  Each entry consists
      of a KEYFORMAT attribute value followed by a colon character (:)
      followed by a sequence of Content Protection Configuration (CPC)
      Labels separated by slash (/) characters.  Each CPC Label is a
      string containing characters from the set [A..Z], [0..9], and '-'.

      For example: ALLOWED-CPC="com.example.drm1:SMART-TV/PC,
      com.example.drm2:HW"

      A CPC Label identifies a class of playback device that implements
      the KEYFORMAT with a certain level of content protection
      robustness.  Each KEYFORMAT can define its own set of CPC Labels.
      The "identity" KEYFORMAT does not define any labels.  A KEYFORMAT
      that defines CPC Labels SHOULD also specify its robustness
      requirements in a secure manner in each key response.

      A client MAY play the Variant Stream if it implements one of the
      listed KEYFORMAT schemes with content protection robustness that
      matches one or more of the CPC Labels in the list.  If it does not
      match any of the CPC Labels then it SHOULD NOT attempt to play the
      Variant Stream.

      The ALLOWED-CPC attribute is OPTIONAL.  If it is not present or
      does not contain a particular KEYFORMAT then all clients that
      support that KEYFORMAT MAY play the Variant Stream.

      VIDEO-RANGE

      The value is an enumerated-string; valid strings are SDR, HLG and
      PQ.

      The value MUST be SDR if the video in the Variant Stream is
      encoded using one of the following reference opto-electronic
      transfer characteristic functions specified by the
      TransferCharacteristics code point: [CICP] 1, 6, 13, 14, 15.  Note
      that different TransferCharacteristics code points can use the
      same transfer function.

      The value MUST be HLG if the video in the Variant Stream is
      encoded using a reference opto-electronic transfer characteristic
      function specified by the TransferCharacteristics code point 18,




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      or consists of such video mixed with video qualifying as SDR (see
      above).

      The value MUST be PQ if the video in the Variant Stream is encoded
      using a reference opto-electronic transfer characteristic function
      specified by the TransferCharacteristics code point 16, or
      consists of such video mixed with video qualifying as SDR or HLG
      (see above).

      This attribute is OPTIONAL.  Its absence implies a value of SDR.
      Clients that do not recognize the attribute value SHOULD NOT
      select the Variant Stream.

      AUDIO

      The value is a quoted-string.  It MUST match the value of the
      GROUP-ID attribute of an EXT-X-MEDIA tag elsewhere in the Master
      Playlist whose TYPE attribute is AUDIO.  It indicates the set of
      audio Renditions that SHOULD be used when playing the
      presentation.  See Section 4.4.6.2.1.

      The AUDIO attribute is OPTIONAL.

      VIDEO

      The value is a quoted-string.  It MUST match the value of the
      GROUP-ID attribute of an EXT-X-MEDIA tag elsewhere in the Master
      Playlist whose TYPE attribute is VIDEO.  It indicates the set of
      video Renditions that SHOULD be used when playing the
      presentation.  See Section 4.4.6.2.1.

      The VIDEO attribute is OPTIONAL.

      SUBTITLES

      The value is a quoted-string.  It MUST match the value of the
      GROUP-ID attribute of an EXT-X-MEDIA tag elsewhere in the Master
      Playlist whose TYPE attribute is SUBTITLES.  It indicates the set
      of subtitle Renditions that can be used when playing the
      presentation.  See Section 4.4.6.2.1.

      The SUBTITLES attribute is OPTIONAL.

      CLOSED-CAPTIONS

      The value can be either a quoted-string or an enumerated-string
      with the value NONE.  If the value is a quoted-string, it MUST
      match the value of the GROUP-ID attribute of an EXT-X-MEDIA tag



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      elsewhere in the Playlist whose TYPE attribute is CLOSED-CAPTIONS,
      and it indicates the set of closed-caption Renditions that can be
      used when playing the presentation.  See Section 4.4.6.2.1.

      If the value is the enumerated-string value NONE, all EXT-X-
      STREAM-INF tags MUST have this attribute with a value of NONE,
      indicating that there are no closed captions in any Variant Stream
      in the Master Playlist.  Having closed captions in one Variant
      Stream but not another can trigger playback inconsistencies.

      The CLOSED-CAPTIONS attribute is OPTIONAL.

4.4.6.2.1.  Alternative Renditions

   When an EXT-X-STREAM-INF tag contains an AUDIO, VIDEO, SUBTITLES, or
   CLOSED-CAPTIONS attribute, it indicates that alternative Renditions
   of the content are available for playback of that Variant Stream.

   When defining alternative Renditions, the following constraints MUST
   be met to prevent client playback errors:

   o  All playable combinations of Renditions associated with an EXT-X-
      STREAM-INF tag MUST have an aggregate bandwidth less than or equal
      to the BANDWIDTH attribute of the EXT-X-STREAM-INF tag.

   o  If an EXT-X-STREAM-INF tag contains a RESOLUTION attribute and a
      VIDEO attribute, then every alternative video Rendition MUST have
      an optimal display resolution matching the value of the RESOLUTION
      attribute.

   o  Every alternative Rendition associated with an EXT-X-STREAM-INF
      tag MUST meet the constraints for a Variant Stream described in
      Section 6.2.4.

   The URI attribute of the EXT-X-MEDIA tag is REQUIRED if the media
   type is SUBTITLES, but OPTIONAL if the media type is VIDEO or AUDIO.
   If the media type is VIDEO or AUDIO, a missing URI attribute
   indicates that the media data for this Rendition is included in the
   Media Playlist of any EXT-X-STREAM-INF tag referencing this EXT-
   X-MEDIA tag.  If the media TYPE is AUDIO and the URI attribute is
   missing, clients MUST assume that the audio data for this Rendition
   is present in every video Rendition specified by the EXT-X-STREAM-INF
   tag.

   The URI attribute of the EXT-X-MEDIA tag MUST NOT be included if the
   media type is CLOSED-CAPTIONS.





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4.4.6.3.  EXT-X-I-FRAME-STREAM-INF

   The EXT-X-I-FRAME-STREAM-INF tag identifies a Media Playlist file
   containing the I-frames of a multimedia presentation.  It stands
   alone, in that it does not apply to a particular URI in the Master
   Playlist.  Its format is:

   #EXT-X-I-FRAME-STREAM-INF:<attribute-list>

   All attributes defined for the EXT-X-STREAM-INF tag (Section 4.4.6.2)
   are also defined for the EXT-X-I-FRAME-STREAM-INF tag, except for the
   FRAME-RATE, AUDIO, SUBTITLES, and CLOSED-CAPTIONS attributes.  In
   addition, the following attribute is defined:

      URI

      The value is a quoted-string containing a URI that identifies the
      I-frame Media Playlist file.  That Playlist file MUST contain an
      EXT-X-I-FRAMES-ONLY tag.

   Every EXT-X-I-FRAME-STREAM-INF tag MUST include a BANDWIDTH attribute
   and a URI attribute.

   The provisions in Section 4.4.6.2.1 also apply to EXT-X-I-FRAME-
   STREAM-INF tags with a VIDEO attribute.

   A Master Playlist that specifies alternative VIDEO Renditions and
   I-frame Playlists SHOULD include an alternative I-frame VIDEO
   Rendition for each regular VIDEO Rendition, with the same NAME and
   LANGUAGE attributes.

4.4.6.4.  EXT-X-SESSION-DATA

   The EXT-X-SESSION-DATA tag allows arbitrary session data to be
   carried in a Master Playlist.

   Its format is:

   #EXT-X-SESSION-DATA:<attribute-list>

   The following attributes are defined:

      DATA-ID

      The value of DATA-ID is a quoted-string that identifies a
      particular data value.  The DATA-ID SHOULD conform to a reverse
      DNS naming convention, such as "com.example.movie.title"; however,
      there is no central registration authority, so Playlist authors



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      SHOULD take care to choose a value that is unlikely to collide
      with others.  This attribute is REQUIRED.

      VALUE

      VALUE is a quoted-string.  It contains the data identified by
      DATA-ID.  If the LANGUAGE is specified, VALUE SHOULD contain a
      human-readable string written in the specified language.

      URI

      The value is a quoted-string containing a URI.  The resource
      identified by the URI MUST be formatted as JSON [RFC8259];
      otherwise, clients may fail to interpret the resource.

      LANGUAGE

      The value is a quoted-string containing a language tag [RFC5646]
      that identifies the language of the VALUE.  This attribute is
      OPTIONAL.

   Each EXT-X-SESSION-DATA tag MUST contain either a VALUE or URI
   attribute, but not both.

   A Playlist MAY contain multiple EXT-X-SESSION-DATA tags with the same
   DATA-ID attribute.  A Playlist MUST NOT contain more than one EXT-X-
   SESSION-DATA tag with the same DATA-ID attribute and the same
   LANGUAGE attribute.

4.4.6.5.  EXT-X-SESSION-KEY

   The EXT-X-SESSION-KEY tag allows encryption keys from Media Playlists
   to be specified in a Master Playlist.  This allows the client to
   preload these keys without having to read the Media Playlist(s)
   first.

   Its format is:

   #EXT-X-SESSION-KEY:<attribute-list>

   All attributes defined for the EXT-X-KEY tag (Section 4.4.4.4) are
   also defined for the EXT-X-SESSION-KEY, except that the value of the
   METHOD attribute MUST NOT be NONE.  If an EXT-X-SESSION-KEY is used,
   the values of the METHOD, KEYFORMAT, and KEYFORMATVERSIONS attributes
   MUST match any EXT-X-KEY with the same URI value.






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   EXT-X-SESSION-KEY tags SHOULD be added if multiple Variant Streams or
   Renditions use the same encryption keys and formats.  An EXT-X-
   SESSION-KEY tag is not associated with any particular Media Playlist.

   A Master Playlist MUST NOT contain more than one EXT-X-SESSION-KEY
   tag with the same METHOD, URI, IV, KEYFORMAT, and KEYFORMATVERSIONS
   attribute values.

   The EXT-X-SESSION-KEY tag is optional.

5.  Key Files

5.1.  Structure of Key Files

   An EXT-X-KEY tag with a URI attribute identifies a Key file.  A Key
   file contains a cipher key that can decrypt Media Segments in the
   Playlist.

   [AES_128] encryption uses 16-octet keys.  If the KEYFORMAT of an EXT-
   X-KEY tag is "identity", the Key file is a single packed array of 16
   octets in binary format.

5.2.  IV for AES-128

   [AES_128] REQUIRES the same 16-octet IV to be supplied when
   encrypting and decrypting.  Varying this IV increases the strength of
   the cipher.

   An IV attribute on an EXT-X-KEY tag with a KEYFORMAT of "identity"
   specifies an IV that can be used when decrypting Media Segments
   encrypted with that Key file.  IV values for AES-128 are 128-bit
   numbers.

   An EXT-X-KEY tag with a KEYFORMAT of "identity" that does not have an
   IV attribute indicates that the Media Sequence Number is to be used
   as the IV when decrypting a Media Segment, by putting its big-endian
   binary representation into a 16-octet (128-bit) buffer and padding
   (on the left) with zeros.

6.  Client/Server Responsibilities

6.1.  Introduction

   This section describes how the server generates the Playlist and
   Media Segments and how the client should download them for playback.






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6.2.  Server Responsibilities

6.2.1.  General Server Responsibilities

   The production of the source media is outside the scope of this
   document, which simply presumes a source of continuous encoded media
   containing the presentation.

   The server MUST divide the source media into individual Media
   Segments whose duration (when rounded to a whole second) is less than
   or equal to the Target Duration.  Segments longer than that can
   trigger playback stalls and other errors.

   The server SHOULD attempt to divide the source media at points that
   support effective decode of individual Media Segments, such as on
   packet and key frame boundaries.

   The server MUST create a URI for every Media Segment that enables its
   clients to obtain the segment data.  If a server supports partial
   loading of resources (e.g., via HTTP Range requests), it MAY specify
   segments as sub-ranges of larger resources using the EXT-X-BYTERANGE
   tag.

   The absence of media data (due to, for example, the temporary
   unavailability of an encoder) SHOULD be signaled by adding one or
   more Media Segments to the Playlist whose Segment durations add up to
   the duration of absent media; these Media Segments MUST have EXT-
   X-GAP tags applied to them.  Similarly, such Partial Segments MUST
   have a GAP=YES attribute.  Attempting to download these segments MAY
   produce an error, such as HTTP 404 or 410.

   A Media Segment MUST be available for immediate download at the full
   speed of the link to the Client when it is added to a Playlist unless
   it has been marked with an EXT-X-GAP tag; otherwise playback errors
   can occur.  Once download starts, its transfer rate SHOULD NOT be
   constrained by the segment production process.

   A Partial Segment MUST be similarly available at the time it is added
   to a Playlist.

   HTTP servers SHOULD transfer text files -- such as Playlists and
   WebVTT segments -- using the "gzip" Content-Encoding if the client
   indicates that it is prepared to accept it.

   The server must create a Media Playlist file (Section 4) that
   contains a URI for each Media Segment that the server wishes to make
   available, in the order in which they are to be played.




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   The value of the EXT-X-VERSION tag (Section 4.4.1.2) SHOULD NOT be
   greater than what is required for the tags and attributes in the
   Playlist (see Section 7).

   Changes to the Playlist file MUST be made atomically from the point
   of view of the clients, or playback errors MAY occur.

   The server MUST NOT change the Media Playlist file, except to:

      Append lines to it (Section 6.2.1).

      Remove Media Segment URIs from the Playlist in the order that they
      appear, along with any tags that apply only to those segments
      (Section 6.2.2).

      Remove Media Metadata tags that no longer apply to the
      presentation (Section 6.2.1).

      Remove EXT-X-PART tags no longer at the live edge (Section 6.2.2).

      Increment the value of the EXT-X-MEDIA-SEQUENCE or EXT-X-
      DISCONTINUITY-SEQUENCE tags (Section 6.2.2).

      Add an EXT-X-ENDLIST tag to the Playlist (Section 6.2.1).

   A Media Playlist has further constraints on its updates if it
   contains an EXT-X-PLAYLIST-TYPE tag.  An EXT-X-PLAYLIST-TYPE tag with
   a value of VOD indicates that the Playlist file MUST NOT change.  An
   EXT-X-PLAYLIST-TYPE tag with a value of EVENT indicates that the
   Server MUST NOT change or remove any part of the Playlist file, with
   the exception of EXT-X-PART tags and Media Metadata tags as described
   above; the Server MAY append lines to the Playlist.

   The value of the EXT-X-TARGETDURATION tag in the Media Playlist MUST
   NOT change.  A typical Target Duration is 6 seconds.

   Playlist changes other than those allowed here can trigger playback
   errors and inconsistent client behavior.

   Each Media Segment in a Media Playlist has an integer Discontinuity
   Sequence Number.  The Discontinuity Sequence Number can be used in
   addition to the timestamps within the media to synchronize Media
   Segments across different Renditions.

   A segment's Discontinuity Sequence Number is the value of the EXT-X-
   DISCONTINUITY-SEQUENCE tag (or zero if none) plus the number of EXT-
   X-DISCONTINUITY tags in the Playlist preceding the URI line of the
   segment.



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   The server MAY associate an absolute date and time with a Media
   Segment by applying an EXT-X-PROGRAM-DATE-TIME tag to it.  This
   defines an informative mapping of the (wall-clock) date and time
   specified by the tag to the first media timestamp in the segment,
   which may be used as a basis for seeking, for display, or for other
   purposes.  If a server provides this mapping, it SHOULD apply an EXT-
   X-PROGRAM-DATE-TIME tag to every segment that has an EXT-
   X-DISCONTINUITY tag applied to it.

   The Server MUST NOT add any EXT-X-PROGRAM-DATE-TIME tag to a Playlist
   that would cause the mapping between program date and Media Segment
   to become ambiguous.

   The server MUST NOT remove an EXT-X-DATERANGE tag from a Playlist if
   any date in the range maps to a Media Segment in the Playlist.

   The server MUST NOT reuse the ID attribute value of an EXT-
   X-DATERANGE tag for any new Date Range in the same Playlist.

   Once the Following Range of a Date Range with an END-ON-NEXT=YES
   attribute is added to a Playlist, the Server MUST NOT subsequently
   add a Date Range with the same CLASS attribute whose START-DATE is
   between that of the END-ON-NEXT=YES range and its Following Range.

   For Date Ranges with a PLANNED-DURATION attribute, the Server SHOULD
   signal the actual end of the range once it has been established.  It
   can do so by adding another EXT-X-DATERANGE tag with the same ID
   attribute value and either a DURATION or an END-DATE attribute or, if
   the Date Range has an END-ON-NEXT=YES attribute, by adding a
   Following Range.

   If the Media Playlist contains the final Media Segment of the
   presentation, then the Playlist file MUST contain the EXT-X-ENDLIST
   tag; this allows clients to minimize unproductive Playlist reloads.

   If a Media Playlist does not contain the EXT-X-ENDLIST tag, the
   server MUST make a new version of the Playlist file available that
   contains at least one new Media Segment.  It MUST be made available
   no later than 1.5 times the Target Duration after the previous time
   the Playlist was updated with a Media Segment.  This allows clients
   to utilize the network efficiently.

   If a Media Playlist without an EXT-X-ENDLIST tag contains Partial
   Segments, the Server MUST add a new Partial Segment to the Playlist
   within one Part Target Duration after it added the previous Partial
   Segment.





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   If the server wishes to remove an entire presentation, it SHOULD
   provide a clear indication to clients that the Playlist file is no
   longer available (e.g., with an HTTP 404 or 410 response).  It MUST
   ensure that all Media Segments in the Playlist file remain available
   to clients for at least the duration of the Playlist file at the time
   of removal to prevent interruption of in-progress playback.

6.2.2.  Live Playlists

   The server MAY limit the availability of Media Segments by removing
   Media Segments from the Playlist file (Section 6.2.1).  If Media
   Segments are to be removed, the Playlist file MUST contain an EXT-X-
   MEDIA-SEQUENCE tag.  Its value MUST be incremented by 1 for every
   Media Segment that is removed from the Playlist file; it MUST NOT
   decrease or wrap.  Clients can malfunction if each Media Segment does
   not have a consistent, unique Media Sequence Number.

   EXT-X-PART tags SHOULD be removed from the Playlist after they are
   greater than three Target Durations from the end of the Playlist.

   Media Segments and EXT-X-PART tags MUST be removed from the Playlist
   in the order that they appear in the Playlist; otherwise, client
   playback can malfunction.

   The server MUST NOT remove a Media Segment from a Playlist file
   without an EXT-X-ENDLIST tag if that would produce a Playlist whose
   duration is less than three times the Target Duration.  Doing so can
   trigger playback stalls.

   The Availability Duration of a Media Segment is the duration of the
   segment plus the duration of the longest-duration Playlist
   distributed by the server containing that segment.  If the server
   removes a Media Segment URI from a Playlist that contains an EXT-
   X-ENDLIST tag, clients MUST be able to download the corresponding
   Media Segment until the time of removal plus the segment's
   Availability Duration.  If the server removes a Media Segment URI
   from a Playlist that does not contain an EXT-X-ENDLIST tag, clients
   MUST be able to download the segment until the time at which it first
   appeared in the Playlist plus the segment's Availability Duration.

   If the server wishes to remove segments from a Media Playlist
   containing an EXT-X-DISCONTINUITY tag, the Media Playlist MUST
   contain an EXT-X-DISCONTINUITY-SEQUENCE tag.  Without the EXT-X-
   DISCONTINUITY-SEQUENCE tag, it can be impossible for a client to
   locate corresponding segments between Renditions.

   If the server removes an EXT-X-DISCONTINUITY tag from the Media
   Playlist, it MUST increment the value of the EXT-X-DISCONTINUITY-



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   SEQUENCE tag so that the Discontinuity Sequence Numbers of the
   segments still in the Media Playlist remain unchanged.  The value of
   the EXT-X-DISCONTINUITY-SEQUENCE tag MUST NOT decrease or wrap.
   Clients can malfunction if each Media Segment does not have a
   consistent Discontinuity Sequence Number.

   If a server plans to remove a Media Segment after it is delivered to
   clients over HTTP, it SHOULD ensure that the HTTP response contains
   an Expires header that reflects the planned time-to-live.

   A Live Playlist MUST NOT contain the EXT-X-PLAYLIST-TYPE tag, as no
   value of that tag allows Media Segments to be removed.

6.2.3.  Encrypting Media Segments

   Media Segments MAY be encrypted.  Every encrypted Media Segment MUST
   have an EXT-X-KEY tag (Section 4.4.4.4) applied to it with a URI that
   the client can use to obtain a Key file (Section 5) containing the
   decryption key.

   A Media Segment can only be encrypted with one encryption METHOD,
   using one encryption key and IV.  However, a server MAY offer
   multiple ways to retrieve that key by providing multiple EXT-X-KEY
   tags, each with a different KEYFORMAT attribute value.

   The server MAY set the HTTP Expires header in the key response to
   indicate the duration for which the key can be cached.

   Any unencrypted Media Segment in a Playlist that is preceded by an
   encrypted Media Segment MUST have an EXT-X-KEY tag applied to it with
   a METHOD attribute of NONE.  Otherwise, the client will misinterpret
   those segments as encrypted.

   If the encryption METHOD is AES-128 and the Playlist does not contain
   the EXT-X-I-FRAMES-ONLY tag, AES encryption as described in
   Section 4.4.4.4 SHALL be applied to individual Media Segments.

   If the encryption METHOD is AES-128 and the Playlist contains an EXT-
   X-I-FRAMES-ONLY tag, the entire resource MUST be encrypted using
   AES-128 CBC with PKCS7 padding [RFC5652].  Encryption MAY be
   restarted on 16-byte block boundaries, unless the first block
   contains an I-frame.  The IV used for encryption MUST be either the
   Media Sequence Number of the Media Segment or the value of the IV
   attribute of the EXT-X-KEY tag, as described in Section 5.2.  These
   constraints allow a client to load and decrypt individual I-frames
   specified as sub-ranges of regular encrypted Media Segments, and
   their Media Initialization Sections.




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   If the encryption METHOD indicates Sample Encryption, media samples
   MAY be encrypted prior to encapsulation in a Media Segment.

   The server MUST NOT remove an EXT-X-KEY tag from the Playlist file if
   it applies to any Media Segment in the Playlist file, or clients who
   subsequently load that Playlist will be unable to decrypt those Media
   Segments.

6.2.4.  Providing Variant Streams

   A server MAY offer multiple Media Playlist files to provide different
   encodings of the same presentation.  If it does so, it SHOULD provide
   a Master Playlist file that lists each Variant Stream to allow
   clients to switch between encodings dynamically.

   Master Playlists describe regular Variant Streams with EXT-X-STREAM-
   INF tags and I-frame Variant Streams with EXT-X-I-FRAME-STREAM-INF
   tags.

   If an EXT-X-STREAM-INF tag or EXT-X-I-FRAME-STREAM-INF tag contains
   the CODECS attribute, the attribute value MUST include every media
   format [RFC6381] present in any Media Segment in any of the
   Renditions specified by the Variant Stream.

   The server MUST meet the following constraints when producing Variant
   Streams in order to allow clients to switch between them seamlessly:

      Each Variant Stream MUST present the same content.

      Matching content in Variant Streams MUST have matching timestamps.
      This allows clients to synchronize the media.

      Matching content in Variant Streams MUST have matching
      Discontinuity Sequence Numbers (see Section 4.4.3.3).

      Each Media Playlist in each Variant Stream MUST have the same
      Target Duration.  The only exceptions are SUBTITLES Renditions and
      Media Playlists containing an EXT-X-I-FRAMES-ONLY tag, which MAY
      have different Target Durations if they have an EXT-X-PLAYLIST-
      TYPE of VOD.

      Content that appears in a Media Playlist of one Variant Stream but
      not in another MUST appear either at the beginning or at the end
      of the Media Playlist file and MUST NOT be longer than the Target
      Duration.






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      If any Media Playlists have an EXT-X-PLAYLIST-TYPE tag, all Media
      Playlists MUST have an EXT-X-PLAYLIST-TYPE tag with the same
      value.

      If the Playlist contains an EXT-X-PLAYLIST-TYPE tag with the value
      of VOD, the first segment of every Media Playlist in every Variant
      Stream MUST start at the same media timestamp.

      If any Media Playlist in a Master Playlist contains an EXT-X-
      PROGRAM-DATE-TIME tag, then all Media Playlists in that Master
      Playlist MUST contain EXT-X-PROGRAM-DATE-TIME tags with consistent
      mappings of date and time to media timestamps.

      Each Variant Stream MUST contain the same set of Date Ranges.  The
      EXT-X-DATERANGE tags of corresponding Date Ranges MUST have the
      same ID attribute value and contain the same set of attribute/
      value pairs.

      If any Media Playlist in a Master Playlist contains an EXT-X-
      SERVER-CONTROL tag, then all Media Playlists in that Master
      Playlist MUST contain that tag, with the same attributes and
      values.

   In addition, for broadest compatibility, Variant Streams SHOULD
   contain the same encoded audio bitstream.  This allows clients to
   switch between Variant Streams without audible glitching.

   The rules for Variant Streams also apply to alternative Renditions
   (see Section 4.4.6.2.1).

6.2.5.  Delivery Directives Interface

   A server MAY provide a set of services to its clients by implementing
   support for Delivery Directives.  Delivery Directives are transmitted
   by the Client to the Server as Query Parameters in Playlist request
   URIs.

   Servers advertise the availability of Delivery Directives using the
   EXT-X-SERVER-CONTROL tag (Section 4.4.3.8).

   Currently-defined Delivery Directives are _HLS_skip, _HLS_msn and
   _HLS_part.

6.2.5.1.  Playlist Delta Updates

   Live presentations involve frequent Playlist downloads.  When
   Playlists are large and a Client already has the previous version,




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   the transfer cost can be reduced considerably by sending only the
   newest information in response to a Playlist update request.

   A Server advertises support for Playlist Delta Updates that skip
   older Media Segments by adding the CAN-SKIP-UNTIL attribute to the
   EXT-X-SERVER-CONTROL tag.  A Server can also offer support for
   Playlist Delta Updates that skip older EXT-X-DATERANGE tags by adding
   the CAN-SKIP-DATERANGES attribute to the EXT-X-SERVER-CONTROL tag.

   When a Server receives a request for a Playlist containing the CAN-
   SKIP-UNTIL attribute but no EXT-X-ENDLIST tag, and the requested URL
   contains an _HLS_skip directive whose value is YES or v2, it MUST
   respond with a Playlist Delta Update.

   The Playlist Delta Update is a version of the Playlist in which Media
   Segments that are further from the end of the Playlist than the Skip
   Boundary (Section 4.4.3.8), as well as their associated tags, are
   replaced by an EXT-X-SKIP tag (Section 4.4.5.1.2).

   When the _HLS_skip directive has a value of v2, the Playlist Delta
   Update additionally MUST NOT contain EXT-X-DATERANGE tags that were
   added to the Playlist more than CAN-SKIP-UNTIL seconds before the
   Playlist request.  The RECENTLY-REMOVED-DATERANGES attribute of the
   EXT-X-SKIP tag MUST list the date ranges that were removed from the
   Playlist within CAN-SKIP-UNTIL seconds of the Playlist request.

   All tags that were not skipped MUST remain in the Playlist Delta
   Update.

   A Server MUST ignore the _HLS_skip directive if the Playlist does not
   contain the CAN-SKIP-UNTIL attribute, or if it contains an EXT-
   X-ENDLIST tag.

6.2.5.2.  Blocking Playlist Reload

   A Server MAY offer Blocking Playlist Reloads, which enable immediate
   client discovery of Playlist updates as an alternative to polling.

   A Server advertises support for Blocking Playlist Reload by adding
   the CAN-BLOCK-RELOAD=YES attribute to the EXT-X-SERVER-CONTROL tag.

   A Client requests a Blocking Playlist Reload using an _HLS_msn
   directive with a decimal-integer value M.  When the Playlist URI
   contains an _HLS_msn directive and no _HLS_part directive, the Server
   MUST defer responding to the request until the Playlist contains a
   Media Segment with a Media Sequence Number of M or later or it
   responds with an error.




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   The Playlist URI MAY also contain an _HLS_part directive with a
   decimal-integer value N.  When the Playlist URI contains both an
   _HLS_msn directive and an _HLS_part directive, the Server MUST defer
   responding to the request until the Playlist contains the Partial
   Segment with Part Index N and with a Media Sequence Number of M or
   later or it responds with an error.

   If the Client requests a Part Index greater than that of the final
   Partial Segment of the Parent Segment, the Server MUST treat the
   request as one for Part Index 0 of the following Parent Segment.

   The Server MUST deliver the entire Playlist, even if the requested
   Media Segment is not the last one in the Playlist, and even if it is
   no longer in the Playlist.

   A Server MUST ignore _HLS_msn and _HLS_part if the Playlist contains
   an EXT-X-ENDLIST tag.

   If the _HLS_msn is greater than the Media Sequence Number of the last
   Media Segment in the current Playlist plus two, or if the _HLS_part
   exceeds the last Partial Segment in the current Playlist by the
   Advance Part Limit, then the server SHOULD immediately return Bad
   Request, such as HTTP 400.  The Advance Part Limit is three divided
   by the Part Target Duration if the Part Target Duration is less than
   one second, or three otherwise.

   If the Playlist URI contains an _HLS_part directive but no _HLS_msn
   directive, the Server MUST return Bad Request, such as HTTP 400.

   A Server that cannot provide the requested Playlist after blocking
   for more than three Target Durations SHOULD return Service
   Unavailable, such as HTTP 503.

6.2.6.  Providing Preload Hints

   The Server MAY add EXT-X-PRELOAD-HINT tags (Section 4.4.5.1.3) to the
   Playlist to allow Clients playing the stream to request upcoming
   resources in advance.

   A hinted resource MUST be available for request when its EXT-X-
   PRELOAD-HINT tag is added to the Playlist.

   When processing requests for a URL or a byte range of a URL that
   includes one or more Partial Segments that are not yet completely
   available to be sent - such as requests made in response to an EXT-X-
   PRELOAD-HINT tag - the server MUST refrain from transmitting any
   bytes belonging to a Partial Segment until all bytes of that Partial
   Segment can be transmitted at the full speed of the link to the



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   client.  If the requested range includes more than one Partial
   Segment then the server MUST enforce this delivery guarantee for each
   Partial Segment in turn.  This enables the client to perform accurate
   Adaptive Bit Rate (ABR) measurements.

   The Server SHOULD NOT hint a byte range that it does not expect its
   clients to require in the near term.

   The server SHOULD respond with "Not Found" (such as HTTP 404) to a
   request for a resource that it cannot find and that is not specified
   by an EXT-X-PRELOAD-HINT tag in an active Media Playlist.

   A server MAY choose not to publish previously-hinted resources if the
   planned segmentation changes, such as the case of early return from
   an ad.  The server SHOULD respond to client requests for those
   resources with "Not Found" (such as HTTP 404).

   If a Partial Segment is created as a subrange of a larger resource
   and its length is not known at the time that its hint is added to the
   Playlist, the BYTERANGE-LENGTH attribute SHOULD be omitted.  The
   BYTERANGE-OFFSET SHOULD indicate the Partial Segment's starting
   offset into the larger resource.

   The Server SHOULD NOT add more than one EXT-X-PRELOAD-HINT tag with
   the same TYPE to a Playlist.

6.3.  Client Responsibilities

6.3.1.  General Client Responsibilities

   How the client obtains the URI to the Playlist file is outside the
   scope of this document; it is presumed to have done so.

   The client obtains the Playlist file from the URI.  If the Playlist
   file so obtained is a Master Playlist, the client can select a
   Variant Stream to load from the Master Playlist.

   Clients MUST ensure that loaded Playlists comply with Section 4 and
   that the EXT-X-VERSION tag, if present, specifies a protocol version
   supported by the client; if either check fails, the client MUST NOT
   attempt to use the Playlist, or unintended behavior could occur.

   When parsing Playlist elements that are subject to variable
   substitution, a Variable Reference whose Variable Name has been
   provided by an EXT-X-DEFINE tag that precedes the Variable Reference
   MUST be replaced by the corresponding Variable Value.  Such
   replacements themselves are NOT subject to variable substitution.




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   When parsing Playlist elements that are subject to variable
   substitution, a Variable Reference whose Variable Name has NOT been
   provided by an EXT-X-DEFINE tag preceding the Variable Reference MUST
   trigger a parsing error.

   If any URI element in a Playlist contains an URI scheme that the
   client cannot handle, the client MUST stop playback.  All clients
   MUST support HTTP schemes.

   To support forward compatibility, when parsing Playlists, clients
   MUST:

   o  ignore any unrecognized tags.

   o  ignore any attribute/value pair with an unrecognized
      AttributeName.

   o  ignore any tag containing an attribute/value pair of type
      enumerated-string whose AttributeName is recognized but whose
      AttributeValue is not recognized, unless the definition of the
      attribute says otherwise.

   When identifying playable Renditions, Clients SHOULD consider an
   audio Rendition having unrecognized CHANNELS parameters to be
   playable if its associated CODECS attribute is supported.  However,
   an equivalent Rendition with the same audio codec and recognized
   CHANNELS parameters SHOULD be preferred if it is present in the
   Master Playlist.

   Algorithms used by the client to switch between Variant Streams are
   beyond the scope of this document.

6.3.2.  Loading the Media Playlist File

   Every time a Media Playlist is loaded or reloaded from a Playlist
   URI, the client MUST determine the next Media Segment to load, as
   described in Section 6.3.5, if it intends to play the presentation
   normally (i.e., in Playlist order at the nominal playback rate).

   If the Media Playlist contains the EXT-X-MEDIA-SEQUENCE tag, the
   client SHOULD assume that each Media Segment in it will become
   unavailable at the time that the Playlist file was loaded plus the
   duration of the Playlist file.

   A client MAY use the segment Media Sequence Number to track the
   location of a Media Segment within a Playlist when the Playlist is
   reloaded.




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   A client MUST NOT assume that segments with the same Media Sequence
   Number in different Variant Streams or Renditions have the same
   position in the presentation; Playlists MAY have independent Media
   Sequence Numbers.  Instead, a client MUST use the relative position
   of each segment on the Playlist timeline and its Discontinuity
   Sequence Number to locate corresponding segments.

   Clients using Delivery Directives (Section 6.2.5) MUST ensure that
   all query parameters appear in UTF-8 order within the URI.  This
   improves Server cache utilization.

   A client MUST load the Media Playlist file of every Rendition
   selected for playback in order to locate the media specific to that
   Rendition.  But, to prevent unnecessary load on the server, it SHOULD
   NOT load the Playlist file of any other Rendition.

   For some Variant Streams, it is possible to select Renditions that do
   not include the Rendition specified by the EXT-X-STREAM-INF tag.  As
   noted above, the client SHOULD NOT load that Rendition in those
   cases.

6.3.3.  Playing the Media Playlist File

   The client SHALL choose which Media Segment to play first from the
   Media Playlist when playback starts.  If the EXT-X-ENDLIST tag is not
   present and the client intends to play the media normally, the client
   SHOULD NOT choose a segment closer to the end of the Playlist than
   described by the HOLD-BACK and PART-HOLD-BACK attributes.  Doing so
   can trigger playback stalls.

   Normal playback can be achieved by playing Media Segments or Partial
   Segments in the order that they appear in the Playlist.  The client
   MAY present the available media in any way it wishes, including
   normal playback, random access, and trick modes.

   The client SHOULD NOT attempt to load Media Segments that have been
   marked with an EXT-X-GAP tag, or to load Partial Segments with a
   GAP=YES attribute.  Instead, clients are encouraged to look for
   another Variant Stream of the same Rendition which does not have the
   same gap, and play that instead.

   The encoding parameters for samples in a Media Segment and across
   multiple Media Segments in a Media Playlist SHOULD remain consistent.
   However, clients SHOULD deal with encoding changes as they are
   encountered, for example, by scaling video content to accommodate a
   resolution change.  If the Variant Stream includes a RESOLUTION
   attribute, clients SHOULD display all video within a rectangle with
   the same proportions as that resolution.



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   Clients SHOULD be prepared to handle multiple tracks of a particular
   type (e.g., audio or video).  A client with no other preference
   SHOULD choose the track with the lowest numerical track identifier
   that it can play.

   Clients SHOULD ignore private streams inside Transport Streams that
   they do not recognize.  Private streams can be used to support
   different devices with the same stream, although stream authors
   SHOULD be sensitive to the additional network load that this imposes.

   The client MUST be prepared to reset its parser(s) and decoder(s)
   before playing a Media Segment that has an EXT-X-DISCONTINUITY tag
   applied to it; otherwise, playback errors can occur.

   The client SHOULD attempt to load Media Segments in advance of when
   they will be required for uninterrupted playback to compensate for
   temporary variations in latency and throughput.

   The client MAY use the value of the EXT-X-PROGRAM-DATE-TIME tag to
   display the program origination time to the user.  If the value
   includes time zone information, the client SHALL take it into
   account; if it does not, the client MAY assume the time to be local.

   Note that dates in Playlists can refer to when the content was
   produced (or to other times), which have no relation to the time of
   playback.

   If the first EXT-X-PROGRAM-DATE-TIME tag in a Playlist appears after
   one or more Media Segment URIs, the client SHOULD extrapolate
   backward from that tag (using EXTINF durations and/or media
   timestamps) to associate dates with those segments.  To associate a
   date with any other Media Segment that does not have an EXT-X-
   PROGRAM-DATE-TIME tag applied to it directly, the client SHOULD
   extrapolate forward from the last EXT-X-PROGRAM-DATE-TIME tag
   appearing before that segment in the Playlist.

6.3.4.  Reloading the Media Playlist File

   The client MUST periodically reload a Media Playlist file to learn
   what media is currently available, unless it contains an EXT-X-
   PLAYLIST-TYPE tag with a value of VOD, or a value of EVENT and the
   EXT-X-ENDLIST tag is also present.

   However, the client MUST NOT attempt to reload the Playlist file more
   frequently than specified by this section, in order to limit the
   collective load on the server.





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   When a client loads a Playlist file for the first time or reloads a
   Playlist file and finds that it has changed since the last time it
   was loaded, the client MUST wait for at least the duration of the
   last segment in the Playlist before attempting to reload the Playlist
   file again, measured from the last time the client began loading the
   Playlist file.

   If the client reloads a Playlist file and finds that it has not
   changed, then it MUST wait for a period of one-half the Target
   Duration before retrying.

   After reloading a Media Playlist, the client SHOULD verify that each
   Media Segment in it has the same URI (and byte range, if specified)
   as the Media Segment with the same Media Sequence Number in the
   previous Media Playlist.  It SHOULD halt playback if it does not, as
   this normally indicates a server error.

   In order to reduce server load, the client SHOULD NOT reload the
   Playlist files of Variant Streams or alternate Renditions that are
   not currently being played.  If it decides to switch playback to a
   different Variant Stream, it SHOULD stop reloading the Playlist of
   the old Variant Stream and begin loading the Playlist of the new
   Variant Stream.  It can use the EXTINF durations and the constraints
   in Section 6.2.4 to determine the approximate location of
   corresponding media.  Once media from the new Variant Stream has been
   loaded, the timestamps in the Media Segments can be used to
   synchronize the old and new timelines precisely.

   A client MUST NOT attempt to use the Media Sequence Number to
   synchronize between streams (see Section 6.3.2).

6.3.5.  Determining the Next Segment to Load

   The client MUST examine the Media Playlist file every time it is
   loaded or reloaded to determine the next Media Segment to load, as
   the set of available media MAY have changed.

   The first segment to load is generally the segment that the client
   has chosen to play first (see Section 6.3.3).

   In order to play the presentation normally, the next Media Segment to
   load is the one with the lowest Media Sequence Number that is greater
   than the Media Sequence Number of the last Media Segment loaded.








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6.3.6.  Decrypting Encrypted Media Segments

   If a Media Playlist file contains an EXT-X-KEY tag that specifies a
   Key file URI, the client can obtain that Key file and use the key
   inside it to decrypt all Media Segments to which that EXT-X-KEY tag
   applies.

   A client MUST ignore any EXT-X-KEY tag with an unsupported or
   unrecognized KEYFORMAT attribute, to allow for cross-device
   addressability.  If the Playlist contains a Media Segment to which
   only EXT-X-KEY tags with unrecognized or unsupported KEYFORMAT
   attributes are applied, playback SHOULD fail.

   A client MUST NOT attempt to decrypt any segments whose EXT-X-KEY tag
   has a METHOD attribute that it does not recognize.

   If the encryption METHOD is AES-128, AES-128 CBC decryption SHALL be
   applied to individual Media Segments, whose encryption format is
   described in Section 4.4.4.4.

   If the encryption METHOD is AES-128 and the Media Segment is part of
   an I-frame Playlist (Section 4.4.3.6) and it has an EXT-X-BYTERANGE
   tag applied to it, special care needs to be taken in loading and
   decrypting the segment, because the resource identified by the URI is
   encrypted in 16-byte blocks from the start of the resource.

   The decrypted I-frame can be recovered by first widening its byte
   range, as specified by the EXT-X-BYTERANGE tag, so that it starts and
   ends on 16-byte boundaries from the start of the resource.

   Next, the byte range is widened further to include a 16-byte block at
   the beginning of the range.  This 16-byte block allows the correct IV
   for the following block to be calculated.

   The widened byte range can then be loaded and decrypted with AES-128
   CBC using an arbitrary IV.  The number of bytes added to the
   beginning and the end of the original byte range are discarded from
   the decrypted bytes; what remains is the decrypted I-frame.

   If the encryption METHOD indicates Sample Encryption, decryption
   SHALL be applied to encrypted media samples within the Media Segment.

   An EXT-X-KEY tag with a METHOD of NONE indicates that the Media
   Segments it applies to are not encrypted.







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6.3.7.  Requesting Playlist Delta Updates

   If a Media Playlist file contains an EXT-X-SERVER-CONTROL tag with a
   CAN-SKIP-UNTIL attribute and no EXT-X-ENDLIST tag, a Client MAY use
   the _HLS_skip Delivery Directive to request Playlist Delta Updates.

   A Client SHOULD NOT request a Playlist Delta Update unless it already
   has a version of the Playlist that is no older than one-half of the
   Skip Boundary.

   The client can request a Playlist Delta Update that skips older Media
   Segments by adding an "_HLS_skip=YES" directive to the Media Playlist
   URL when it requests the Playlist.

   Alternately, if the EXT-X-SERVER-CONTROL tag contains a CAN-SKIP-
   DATERANGES=YES attribute, the client can request a Playlist Delta
   Update that skips both older Segments and older EXT-X-DATERANGE tags
   by adding an "_HLS_skip=v2" directive to the Media Playlist URL when
   it requests the Playlist.

   A Client MUST merge the contents of a Playlist Delta Update with its
   previous version of the Playlist to form an up-to-date version of the
   Playlist.  If a Client receives a Playlist containing an EXT-X-SKIP
   tag and finds that it does not already have all of the information
   that was skipped, it MUST obtain a complete copy of the Playlist by
   reissuing its Playlist request without the _HLS_skip directive.

6.3.8.  Issuing Blocking Requests

   Clients MUST NOT request Blocking Playlist Reloads unless the
   Playlist contains an EXT-X-SERVER-CONTROL tag with a CAN-BLOCK-
   RELOAD=YES attribute.

   If Blocking Playlist Reloads are supported, Clients SHOULD use the
   _HLS_msn Delivery Directive (and _HLS_part, if the Playlist contains
   Partial Segments) to obtain Playlist updates in preference to the
   polling regime described in Section 6.3.4.

   If up-to-date information on the next expected Media Sequence Number
   of a Rendition is not available, a Client SHOULD use a tune-in
   algorithm such as the one described in Appendix C to obtain a recent
   version of the Playlist.

   Clients MUST ignore EXT-X-PRELOAD-HINT tags with unrecognized TYPE
   attributes.  Clients SHOULD ignore all but the first EXT-X-PRELOAD-
   HINT tag in a Playlist with a particular TYPE attribute.





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   When processing a Playlist containing an EXT-X-PRELOAD-HINT tag with
   TYPE=PART, a Client with sufficient space in its download pipeline
   that is not already loading the hinted resource SHOULD request it.
   This will typically happen at the same time as its blocking request
   for the next Playlist update.

   When processing a Playlist containing an EXT-X-PRELOAD-HINT tag with
   TYPE=MAP, a Client with sufficient space in its download pipeline
   that has not already cached the hinted Media Initialization
   Section SHOULD request it.

   A Client SHOULD cancel a request for a hinted resource if it is not
   present in a subsequent Playlist update, such as in an EXT-X-PRELOAD-
   HINT tag or as part of another tag such as EXT-X-PART.  The client
   SHOULD ignore the results of such requests.

   A Client SHOULD recognize when a Partial Segment indicated by an EXT-
   X-PART tag is a subrange of a hint download and obtain the Partial
   Segment from the hint download.  Clients SHOULD recognize contiguous
   ranges between existing Partial Segments and Partial Segment hints
   and avoid duplicate downloads.

7.  Protocol Version Compatibility

   Protocol compatibility is specified by the EXT-X-VERSION tag.  A
   Playlist that contains tags or attributes that are not compatible
   with protocol version 1 MUST include an EXT-X-VERSION tag.

   A client MUST NOT attempt playback if it does not support the
   protocol version specified by the EXT-X-VERSION tag, or unintended
   behavior could occur.

   A Media Playlist MUST indicate an EXT-X-VERSION of 2 or higher if it
   contains:

   o  The IV attribute of the EXT-X-KEY tag.

   A Media Playlist MUST indicate an EXT-X-VERSION of 3 or higher if it
   contains:

   o  Floating-point EXTINF duration values.

   A Media Playlist MUST indicate an EXT-X-VERSION of 4 or higher if it
   contains:

   o  The EXT-X-BYTERANGE tag.

   o  The EXT-X-I-FRAMES-ONLY tag.



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   A Media Playlist MUST indicate an EXT-X-VERSION of 5 or higher if it
   contains:

   o  An EXT-X-KEY tag with a METHOD of SAMPLE-AES.

   o  The KEYFORMAT and KEYFORMATVERSIONS attributes of the EXT-X-KEY
      tag.

   o  The EXT-X-MAP tag.

   A Media Playlist MUST indicate an EXT-X-VERSION of 6 or higher if it
   contains:

   o  The EXT-X-MAP tag in a Media Playlist that does not contain EXT-
      X-I-FRAMES-ONLY.

   Note that in protocol version 6, the semantics of the EXT-
   X-TARGETDURATION tag changed slightly.  In protocol version 5 and
   earlier it indicated the maximum segment duration; in protocol
   version 6 and later it indicates the the maximum segment duration
   rounded to the nearest integer number of seconds.

   A Master Playlist MUST indicate an EXT-X-VERSION of 7 or higher if it
   contains:

   o  "SERVICE" values for the INSTREAM-ID attribute of the EXT-X-MEDIA
      tag.

   A Playlist MUST indicate an EXT-X-VERSION of 8 or higher if it
   contains:

   o  Variable substitution.

   A Playlist MUST indicate an EXT-X-VERSION of 9 or higher if it
   contains:

   o  The EXT-X-SKIP tag.

   A Playlist MUST indicate an EXT-X-VERSION of 10 or higher if it
   contains:

   o  An EXT-X-SKIP tag that replaces EXT-X-DATERANGE tags in a Playlist
      Delta Update.

   The EXT-X-MEDIA tag and the AUDIO, VIDEO, and SUBTITLES attributes of
   the EXT-X-STREAM-INF tag are backward compatible to protocol version
   1, but playback on older clients may not be desirable.  A server MAY




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   consider indicating an EXT-X-VERSION of 4 or higher in the Master
   Playlist but is not required to do so.

   The PROGRAM-ID attribute of the EXT-X-STREAM-INF and the EXT-X-I-
   FRAME-STREAM-INF tags was removed in protocol version 6.

   The EXT-X-ALLOW-CACHE tag was removed in protocol version 7.

8.  Playlist Examples

8.1.  Simple Media Playlist

   #EXTM3U
   #EXT-X-TARGETDURATION:10
   #EXT-X-VERSION:3
   #EXTINF:9.009,
   http://media.example.com/first.ts
   #EXTINF:9.009,
   http://media.example.com/second.ts
   #EXTINF:3.003,
   http://media.example.com/third.ts
   #EXT-X-ENDLIST

8.2.  Live Media Playlist Using HTTPS

   #EXTM3U
   #EXT-X-VERSION:3
   #EXT-X-TARGETDURATION:8
   #EXT-X-MEDIA-SEQUENCE:2680

   #EXTINF:7.975,
   https://priv.example.com/fileSequence2680.ts
   #EXTINF:7.941,
   https://priv.example.com/fileSequence2681.ts
   #EXTINF:7.975,
   https://priv.example.com/fileSequence2682.ts

8.3.  Playlist with Encrypted Media Segments













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   #EXTM3U
   #EXT-X-VERSION:3
   #EXT-X-MEDIA-SEQUENCE:7794
   #EXT-X-TARGETDURATION:15

   #EXT-X-KEY:METHOD=AES-128,URI="https://priv.example.com/key.php?r=52"

   #EXTINF:2.833,
   http://media.example.com/fileSequence52-A.ts
   #EXTINF:15.0,
   http://media.example.com/fileSequence52-B.ts
   #EXTINF:13.333,
   http://media.example.com/fileSequence52-C.ts

   #EXT-X-KEY:METHOD=AES-128,URI="https://priv.example.com/key.php?r=53"

   #EXTINF:15.0,
   http://media.example.com/fileSequence53-A.ts

8.4.  Master Playlist

   #EXTM3U
   #EXT-X-STREAM-INF:BANDWIDTH=1280000,AVERAGE-BANDWIDTH=1000000
   http://example.com/low.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=2560000,AVERAGE-BANDWIDTH=2000000
   http://example.com/mid.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=7680000,AVERAGE-BANDWIDTH=6000000
   http://example.com/hi.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=65000,CODECS="mp4a.40.5"
   http://example.com/audio-only.m3u8

8.5.  Master Playlist with I-Frames

   #EXTM3U
   #EXT-X-STREAM-INF:BANDWIDTH=1280000
   low/audio-video.m3u8
   #EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=86000,URI="low/iframe.m3u8"
   #EXT-X-STREAM-INF:BANDWIDTH=2560000
   mid/audio-video.m3u8
   #EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=150000,URI="mid/iframe.m3u8"
   #EXT-X-STREAM-INF:BANDWIDTH=7680000
   hi/audio-video.m3u8
   #EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=550000,URI="hi/iframe.m3u8"
   #EXT-X-STREAM-INF:BANDWIDTH=65000,CODECS="mp4a.40.5"
   audio-only.m3u8






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8.6.  Master Playlist with Alternative Audio

   In this example, the CODECS attributes have been condensed for space.
   A '\' is used to indicate that the tag continues on the following
   line with whitespace removed:

   #EXTM3U
   #EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aac",NAME="English", \
      DEFAULT=YES,AUTOSELECT=YES,LANGUAGE="en", \
      URI="main/english-audio.m3u8"
   #EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aac",NAME="Deutsch", \
      DEFAULT=NO,AUTOSELECT=YES,LANGUAGE="de", \
      URI="main/german-audio.m3u8"
   #EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aac",NAME="Commentary", \
      DEFAULT=NO,AUTOSELECT=NO,LANGUAGE="en", \
      URI="commentary/audio-only.m3u8"
   #EXT-X-STREAM-INF:BANDWIDTH=1280000,CODECS="...",AUDIO="aac"
   low/video-only.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=2560000,CODECS="...",AUDIO="aac"
   mid/video-only.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=7680000,CODECS="...",AUDIO="aac"
   hi/video-only.m3u8
   #EXT-X-STREAM-INF:BANDWIDTH=65000,CODECS="mp4a.40.5",AUDIO="aac"
   main/english-audio.m3u8

8.7.  Master Playlist with Alternative Video

   This example shows three different video Renditions (Main,
   Centerfield, and Dugout) and three different Variant Streams (low,
   mid, and high).  In this example, clients that did not support the
   EXT-X-MEDIA tag and the VIDEO attribute of the EXT-X-STREAM-INF tag
   would only be able to play the video Rendition "Main".

   Since the EXT-X-STREAM-INF tag has no AUDIO attribute, all video
   Renditions would be required to contain the audio.

   In this example, the CODECS attributes have been condensed for space.
   A '\' is used to indicate that the tag continues on the following
   line with whitespace removed:












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   #EXTM3U
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="low",NAME="Main", \
      DEFAULT=YES,URI="low/main/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="low",NAME="Centerfield", \
      DEFAULT=NO,URI="low/centerfield/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="low",NAME="Dugout", \
      DEFAULT=NO,URI="low/dugout/audio-video.m3u8"

   #EXT-X-STREAM-INF:BANDWIDTH=1280000,CODECS="...",VIDEO="low"
   low/main/audio-video.m3u8

   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="mid",NAME="Main", \
      DEFAULT=YES,URI="mid/main/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="mid",NAME="Centerfield", \
      DEFAULT=NO,URI="mid/centerfield/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="mid",NAME="Dugout", \
      DEFAULT=NO,URI="mid/dugout/audio-video.m3u8"

   #EXT-X-STREAM-INF:BANDWIDTH=2560000,CODECS="...",VIDEO="mid"
   mid/main/audio-video.m3u8

   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="hi",NAME="Main", \
      DEFAULT=YES,URI="hi/main/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="hi",NAME="Centerfield", \
      DEFAULT=NO,URI="hi/centerfield/audio-video.m3u8"
   #EXT-X-MEDIA:TYPE=VIDEO,GROUP-ID="hi",NAME="Dugout", \
      DEFAULT=NO,URI="hi/dugout/audio-video.m3u8"

   #EXT-X-STREAM-INF:BANDWIDTH=7680000,CODECS="...",VIDEO="hi"
   hi/main/audio-video.m3u8


8.8.  Session Data in a Master Playlist

   In this example, only the EXT-X-SESSION-DATA is shown:

   #EXT-X-SESSION-DATA:DATA-ID="com.example.lyrics",URI="lyrics.json"

   #EXT-X-SESSION-DATA:DATA-ID="com.example.title",LANGUAGE="en", \
           VALUE="This is an example"
   #EXT-X-SESSION-DATA:DATA-ID="com.example.title",LANGUAGE="es", \
           VALUE="Este es un ejemplo"

8.9.  CHARACTERISTICS Attribute Containing Multiple Characteristics

   Certain characteristics are valid in combination, as in:





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   CHARACTERISTICS=
   "public.accessibility.transcribes-spoken-dialog,public.easy-to-read"

8.10.  EXT-X-DATERANGE Carrying SCTE-35 Tags

   This example shows two EXT-X-DATERANGE tags that describe a single
   Date Range, with an SCTE-35 "out" splice_insert() command that is
   subsequently updated with an SCTE-35 "in" splice_insert() command.

   #EXTM3U
   ...
   #EXT-X-DATERANGE:ID="splice-6FFFFFF0",START-DATE="2014-03-05T11:
   15:00Z",PLANNED-DURATION=59.993,SCTE35-OUT=0xFC002F0000000000FF0
   00014056FFFFFF000E011622DCAFF000052636200000000000A0008029896F50
   000008700000000

   ... Media Segment declarations for 60s worth of media

   #EXT-X-DATERANGE:ID="splice-6FFFFFF0",DURATION=59.993,SCTE35-IN=
   0xFC002A0000000000FF00000F056FFFFFF000401162802E6100000000000A00
   08029896F50000008700000000
   ...

8.11.  Low-Latency Playlist

   This example shows the end of a Playlist that contains Partial
   Segments.  Note that EXT-X-PART tags have already been removed from
   earlier Parent Segments.  The Playlist also includes a preload hint,
   a Rendition Report, and a mid-roll advertisement.






















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  #EXTM3U
  #EXT-X-TARGETDURATION:4
  ...
  #EXTINF:4.00008,
  fileSequence268.mp4
  #EXTINF:4.00008,
  fileSequence269.mp4
  #EXTINF:4.00008,
  fileSequence270.mp4
  #EXT-X-PART:DURATION=2.00004,INDEPENDENT=YES,URI="filePart271.0.mp4"
  #EXT-X-PART:DURATION=2.00004,URI="filePart271.1.mp4"
  #EXTINF:4.00008,
  fileSequence271.mp4
  #EXT-X-PART:DURATION=2.00004,INDEPENDENT=YES,URI="filePart272.0.mp4"
  #EXT-X-PART:DURATION=0.50001,URI="filePart272.1.mp4"
  #EXTINF:2.50005,
  fileSequence272.mp4
  #EXT-X-DISCONTINUITY
  #EXT-X-PART:DURATION=2.00004,INDEPENDENT=YES,URI="midRoll273.0.mp4"
  #EXT-X-PART:DURATION=2.00004,URI="midRoll273.1.mp4"
  #EXTINF:4.00008,
  midRoll273.mp4
  #EXT-X-PART:DURATION=2.00004,INDEPENDENT=YES,URI="midRoll274.0.mp4"
  #EXT-X-PRELOAD-HINT:TYPE=PART,URI="midRoll274.1.mp4"
  #EXT-X-RENDITION-REPORT:URI="/1M/LL-HLS.m3u8",LAST-MSN=274,LAST-PART=1

9.  Contributors

   Significant contributions to the design of this protocol were made by
   Jim Batson, David Biderman, Bill May, Roger Pantos, Alan Tseng, and
   Eryk Vershen.  Stuart Cheshire helped edit the specification.

   Significant contributions to the update of this protocol were made by
   Bill May, Eryk Vershen, and Peng Zhou.

   In particular, Bill May co-authored the first edition of HTTP Live
   Streaming, [RFC8216], and continues to provide valuable guidance and
   input.

10.  IANA Considerations

   IANA has registered the following media type [RFC2046]:

   Type name: application

   Subtype name: vnd.apple.mpegurl

   Required parameters: none



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   Optional parameters: none

   Encoding considerations: encoded as UTF-8, which is 8-bit text.  This
   media type may require encoding on transports not capable of handling
   8-bit text.  See Section 4 for more information.

   Security considerations: See Section 11.

   Compression: this media type does not employ compression.

   Interoperability considerations: There are no byte-ordering issues,
   since files are 8-bit text.  Applications could encounter
   unrecognized tags, which SHOULD be ignored.

   Published specification: see Section 4.

   Applications that use this media type: Multimedia applications such
   as the iPhone media player in iOS 3.0 and later and QuickTime Player
   in Mac OS X version 10.6 and later.

   Fragment identifier considerations: no Fragment Identifiers are
   defined for this media type.

   Query parameter considerations: the definition of all query
   parameters for resources of this media type which begin with the
   string "_HLS_" are reserved by this specification.  Currently-defined
   query parameters are specified in Section 6.2.5.

   Additional information:

      Deprecated alias names for this type: none
      Magic number(s): #EXTM3U
      File extension(s): .m3u8, .m3u (see Section 4)
      Macintosh file type code(s): none

   Person & email address to contact for further information: David
   Singer, singer AT apple.com.

   Intended usage: LIMITED USE

   Restrictions on usage: none

   Author: Roger Pantos

   Change Controller: David Singer






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11.  Security Considerations

   Since the protocol generally uses HTTP to transfer data, most of the
   same security considerations apply.  See Section 15 of HTTP
   [RFC7230].

   Media file parsers are typically subject to "fuzzing" attacks.
   Implementors SHOULD pay particular attention to code that will parse
   data received from a server and ensure that all possible inputs are
   handled correctly.

   Playlist files contain URIs, which clients will use to make network
   requests of arbitrary entities.  Clients SHOULD range-check responses
   to prevent buffer overflows.  See also the Security Considerations
   section of "Uniform Resource Identifier (URI): Generic Syntax"
   [RFC3986].

   Apart from URL resolution, this format does not employ any form of
   active content.

   Clients SHOULD limit each playback session to a reasonable number of
   concurrent downloads (for example, four) to avoid contributing to
   denial-of-service attacks.

   HTTP requests often include session state ("cookies"), which may
   contain private user data.  Implementations MUST follow cookie
   restriction and expiry rules specified by "HTTP State Management
   Mechanism" [RFC6265] to protect themselves from attack.  See also the
   Security Considerations section of that document, and "Use of HTTP
   State Management" [RFC2964].

   Encryption keys are specified by URI.  The delivery of these keys
   SHOULD be secured by a mechanism such as HTTP Over TLS [RFC2818]
   (formerly SSL) in conjunction with a secure realm or a session token.

12.  References

12.1.  Normative References

   [AC_3]     Advanced Television Systems Committee, "Digital Audio
              Compression (AC-3) (E-AC-3)", ATSC Standard A/52:2010,
              November 2010, <http://atsc.org/wp-
              content/uploads/2015/03/A52-201212-17.pdf>.

   [AES_128]  National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", FIPS PUB 197, DOI
              10.6028/NIST.FIPS.197, November 2001,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197.pdf>.



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   [CEA608]   Consumer Technology Association, "Line 21 Data Services",
              ANSI/CTA Standard 608-E, April 2008,
              <https://standards.cta.tech/kwspub/published_docs/ANSI-
              CTA-608-E-R-2014-Preview.pdf>.

   [CEA708]   Consumer Technology Association, "Digital Television (DTV)
              Closed Captioning", ANSI/CTA Standard CEA-708-E, August
              2013, <https://standards.cta.tech/kwspub/published_docs/
              ANSI-CTA-708-E-Preview.pdf>.

   [CICP]     International Organization for Standardization,
              "Information technology - MPEG systems technologies - Part
              8: Coding-independent code points", ISO/IEC International
              Standard 23001-8:2016, 2016, <https://www.iso.org/obp/
              ui/#iso:std:iso-iec:23001:-8:ed-2:v1:en>.

   [CMAF]     International Organization for Standardization,
              "Information technology -- Multimedia application format
              (MPEG-A) -- Part 19: Common media application format
              (CMAF) for segmented media", ISO/IEC International
              Standard 23000-19:2017, December 2017,
              <https://www.iso.org/standard/71975.html>.

   [COMMON_ENC]
              International Organization for Standardization,
              "Information technology -- MPEG systems technologies --
              Part 7: Common encryption in ISO base media file format
              files", ISO/IEC International Standard 23001-7:2016,
              February 2016, <http://www.iso.org/iso/
              catalogue_detail.htm?csnumber=68042>.

   [H_264]    International Telecommunications Union, "Advanced video
              coding for generic audiovisual services", January 2012,
              <http://www.itu.int/rec/T-REC-H.264>.

   [HDCP]     Digital Content Protection LLC, "High-bandwidth Digital
              Content Protection System - Mapping HDCP to HDMI",
              February 2013, <http://www.digital-
              cp.com/sites/default/files/specifications/
              HDCP%20on%20HDMI%20Specification%20Rev2_2_Final1.pdf>.

   [IMSC1]    W3C, "TTML Profiles for Internet Media Subtitles and
              Captions 1.0 (IMSC1)", April 2016,
              <https://www.w3.org/TR/ttml-imsc1/>.







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   [ISO_13818]
              International Organization for Standardization, "Generic
              coding of moving pictures and associated audio
              information", ISO/IEC International Standard 13818:2007,
              October 2007,
              <http://www.iso.org/iso/catalogue_detail?csnumber=44169>.

   [ISO_13818_3]
              International Organization for Standardization, "Generic
              coding of moving pictures and associated audio information
              -- Part 3: Audio", ISO/IEC International
              Standard 13818-3:1998, April 1998,
              <http://www.iso.org/iso/home/store/catalogue_tc/
              catalogue_detail.htm?csnumber=26797>.

   [ISO_13818_7]
              International Organization for Standardization, "Generic
              coding of moving pictures and associated audio information
              -- Part 7: Advanced Audio Coding (AAC)", ISO/IEC
              International Standard 13818-7:2006, January 2006,
              <http://www.iso.org/iso/home/store/catalogue_tc/
              catalogue_detail.htm?csnumber=43345>.

   [ISO_14496]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 3: Audio", ISO/IEC International
              Standard 14496-3:2009, 2009,
              <http://www.iso.org/iso/catalogue_detail?csnumber=53943>.

   [ISO_8601]
              International Organization for Standardization, "Data
              elements and interchange formats -- Information
              interchange -- Representation of dates and times", ISO/IEC
              International Standard 8601:2004, December 2004,
              <http://www.iso.org/iso/catalogue_detail?csnumber=40874>.

   [ISOBMFF]  International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 12: ISO base media file format", ISO/IEC
              International Standard 14496-12:2015, December 2015,
              <http://www.iso.org/iso/
              catalogue_detail.htm?csnumber=68960>.








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   [MP4_TIMED_TEXT]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 30: Timed text and other visual overlays in ISO
              base media file format", ISO/IEC International
              Standard 14496-30:2014, March 2014,
              <https://www.iso.org/standard/63107.html>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC2964]  Moore, K. and N. Freed, "Use of HTTP State Management",
              BCP 44, RFC 2964, DOI 10.17487/RFC2964, October 2000,
              <https://www.rfc-editor.org/info/rfc2964>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.





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   [RFC6381]  Gellens, R., Singer, D., and P. Frojdh, "The 'Codecs' and
              'Profiles' Parameters for "Bucket" Media Types", RFC 6381,
              DOI 10.17487/RFC6381, August 2011,
              <https://www.rfc-editor.org/info/rfc6381>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8216]  Pantos, R., Ed. and W. May, "HTTP Live Streaming",
              RFC 8216, DOI 10.17487/RFC8216, August 2017,
              <https://www.rfc-editor.org/info/rfc8216>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [SCTE35]   Society of Cable Telecommunications Engineers, "Digital
              Program Insertion Cueing Message for Cable", ANSI/SCTE 35,
              August 2014, <http://www.scte.org/documents/pdf/Standards/
              ANSI_SCTE%2035%202014.pdf>.

   [US_ASCII]
              American National Standards Institute, "Coded Character
              Sets - 7-Bit American National Standard Code for
              Information Interchange (7-Bit ASCII)", ANSI X3.4,
              December 1986.

   [WebVTT]   World Wide Web Consortium (W3C), ""WebVTT: The Web Video
              Text Tracks Format", Draft Community Group Report", July
              2013, <http://dev.w3.org/html5/webvtt/>.

12.2.  Informative References

   [ID3]      ID3.org, "The ID3 audio file data tagging format",
              <http://www.id3.org/Developer_Information>.

   [M3U]      Nullsoft, Inc., "The M3U Playlist format, originally
              invented for the Winamp media player",
              <http://wikipedia.org/wiki/M3U>.





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   [SampleEnc]
              Apple Inc., "MPEG-2 Stream Encryption Format for HTTP Live
              Streaming",
              <https://developer.apple.com/library/ios/documentation/
              AudioVideo/Conceptual/HLS_Sample_Encryption/>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <https://www.unicode.org/versions/latest/>.

   [UTI]      Apple Inc., "Uniform Type Identifier",
              <http://developer.apple.com/library/ios/#documentation/
              general/conceptual/DevPedia-CocoaCore/
              UniformTypeIdentifier.html>.

Appendix A.  Changes from RFC 8216

   Several changes have been made since the publication of RFC 8216
   [RFC8216].

   The following tags have been added: EXT-X-GAP, EXT-X-BITRATE, EXT-X-
   SERVER-CONTROL, EXT-X-SKIP, EXT-X-PART-INF, EXT-X-PART, EXT-X-
   PRELOAD-HINT, and EXT-X-RENDITION-REPORT.

   EXT-X-DEFINE was introduced to support variable substitution.

   IMSC has been added to the set of recognized subtitle formats.

   The VIDEO-RANGE attribute has been added to the EXT-X-STREAM-INF and
   EXT-X-I-FRAME-STREAM-INF tags.

   The ALLOWED-CPC attribute has been added to the EXT-X-STREAM-INF and
   EXT-X-I-FRAME-STREAM-INF tags.

   TYPE-1 has been added as a defined value for the HDCP-LEVEL
   attribute.

   The minimum new segment publication latency has been removed from
   server timing model.

   The Availability Duration of a Media Segment now depends on the
   presence of an EXT-X-ENDLIST tag.

   The recommended playlist offset to join a live stream has changed.

   The minimum delay before reloading a Playlist file has changed.

   The definition of peak segment bit rate was changed to ensure every
   segment is included in at least one contiguous set.



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   Media Metadata tags such as EXT-X-DATERANGE may be removed from
   playlists.

   Partial Segments were defined as a means to reduce publishing
   latency.

   Delivery Directives were introduced, including support for Playlist
   Delta Updates and Blocking Playlist Reload.

   A Low-Latency Server Configuration Profile was added to Appendix B.

   Appendix C was added.

   There have been a number of minor editorial changes.

Appendix B.  Server Configuration Profiles

   Server Configuration Profiles specify additional requirements that
   optimize delivery of HTTP Live Streaming for certain use cases.

B.1.  Low-Latency Server Configuration Profile

   Playing at a reduced delay from live requires certain stream and
   transport features to support the timely delivery of media.  Clients
   SHOULD verify that the server meets these requirements before playing
   at a delay-from-live of less than two Target Durations.  Because the
   Low-Latency extensions are additions rather than replacements,
   clients can and SHOULD fall back to regular-latency playback if they
   discover that the server does not meet the requirements of this
   configuration profile.

   This profile places the following requirements on stream production:

      All Media Playlists have EXT-PROGRAM-DATE-TIME tags.  This allows
      more-precise mapping between Segments across Renditions.  Note
      that real-time clocks are NOT required to be synchronized between
      client and server.

      Each (non-Partial) Media Segment in a Media Playlist will contain
      at least one independent frame.

      A Playlist that contains an EXT-X-PART tag but no EXT-X-ENDLIST
      tag will also contain an EXT-X-PRELOAD-HINT tag that specifies the
      next Partial Segment that is expected to be added to the Playlist.

      If the Partial Segment specified by an EXT-X-PRELOAD-HINT tag has
      a different Media Initialization Section than the last Partial
      Segment in the Playlist, the Playlist will also contain an EXT-X-



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      PRELOAD-HINT tag with TYPE=MAP that hints the Media Initialization
      Section of the hinted Partial Segment.

      Each Media Playlist contains one EXT-RENDITION-REPORT tag for each
      Media Playlist (Rendition) in the Master Playlist, except for the
      Media Playlist to which the EXT-X-RENDITION-REPORT tag is being
      added, and Playlists that contain the EXT-X-I-FRAMES-ONLY tag.

   This profile places the following requirements on stream delivery:

      HTTP-delivered Playlists and Segments are served via HTTP/2.
      Efficient delivery requires HTTP/2 priority control (dependencies
      and weights) and support for Ping frames.

      Each server offers the entire set of Variant Streams in the master
      Playlist.  This allows rapid bit rate switching without connection
      reestablishment.

      Servers support HTTP Range requests if Media Playlists contain the
      BYTERANGE, BYTERANGE-START, or BYTERANGE-LENGTH attributes.

      TCP connections support Selective Acknowledgment (SACK) across the
      entire route from client to server.  This improves the performance
      of TCP loss recovery

      Playlist requests are idempotent.

      Playlists are delivered in GZIP format.  This speeds up Media
      Playlist reload and Rendition switching.

      All Renditions in a Master Playlist are updated in sync, within an
      accuracy of one Part Target Duration.

      CDNs and other proxy caches recognize blocking requests for
      Playlists and Media Segments whose cache fill is already pending,
      and hold the duplicate requests until they can be delivered from
      that cache fill.  This minimizes the load on the active origin.

      HTTP caches used to deliver Playlists or Segments will set the Age
      HTTP Response header.

   In addition, the following configurations are recommended:

      TCP connections should set Explicit Congestion Notification (ECN)
      during congestion.  They should also use TCP timestamps, TAIL LOSS
      probe, and TCP RACK.  These configurations improve the performance
      of TCP loss recovery.  See RFC 2018, RFC 3168, RFC 7323, and IETF
      draft-ietf-tcpm-rack for more information about these TCP options.



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      Servers should support TLS 1.3 or higher.  This reduces time to
      connect.  Servers should also support TLS 1.3 0-RTT connections
      for Media Playlists and Media Segments.

      Blocking Playlist Reload allows longer caching of Playlists
      without detriment to Clients.  Successful responses to blocking
      Playlist requests should be cached for six Target Durations.
      Unsuccessful responses (such as 404s) should be cached for four
      Target Durations.  Successful responses to non-blocking Playlist
      requests should be cached for half the Target Duration.
      Unsuccessful responses to non-blocking Playlist requests should be
      cached for for one Target Duration.

      Successful responses to blocking Media segment requests should be
      cached for six Target Durations.  Unsuccessful responses should be
      cached for one Target Duration.

      Origin servers should use Cache-Control headers to communicate the
      desired cache lifetime.

      The recommended Target Duration is six seconds.

      The recommended GOP size is between one and two seconds.  Smaller
      GOPs allow faster switching between Renditions.

Appendix C.  Low-Latency CDN Tune-in

   Clients SHOULD support delivery of low-latency streams through CDNs
   and other HTTP caches.  Correctly implementing PART-HOLD-BACK, the
   server-recommended playback delay from live, requires that the client
   first obtain a reasonably up-to-date version of the Media Playlist.

   There are various approaches that a client may take to obtain a
   recent version of a Media Playlist.  The following algorithm
   typically requires two Playlist requests to obtain a Playlist that is
   within one Part Target Duration of the current Playlist:

   1.  Send a request for the Media Playlist that does not include an
       _HLS_msn or _HLS_part directive.

   2.  Record the first Playlist response, including its received time
       and Age header.  If there's no Age header in the first Playlist
       response, consider the Playlist to be up to date.  (No Age header
       means that the response came directly from the origin, rather
       than being held for a period of time in an intervening HTTP
       cache.)





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   3.  If there is an Age header in the first Playlist response, set the
       goalDuration to match the Age value.  Increase the goalDuration
       by one second if the Part Target Duration is less than 1.0.

   4.  While the Age value is greater than or equal to the floor of the
       Part Target Duration:

       A.  Set currentGoal to be the goalDuration plus the amount of
           time since the first Playlist response.

       B.  If the current version of the Playlist has at least
           currentGoal more media in it than the first Playlist,
           consider the current Playlist to be up to date.

       C.  Use the Target Duration and the Part Target Duration to
           estimate how many more segments and parts the server will add
           to the Playlist to contribute at least currentGoal more media
           to it.

       D.  Request the Media Playlist again, using the _HLS_msn and
           _HLS_part directives to obtain the Playlist that has the
           estimated additional duration of media since the first
           Playlist.

       E.  Update the current Playlist and the Age value from the
           Playlist response.

Author's Address

   Roger Pantos (editor)
   Apple Inc.
   Cupertino, California
   United States

   Email: http-live-streaming-review@group.apple.com
















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