# HLS and LL-HLS in AWS Elemental MediaPackage
<a name="hls-overview"></a>

MediaPackage supports HTTP Live Streaming (HLS) and Low-Latency HLS (LL-HLS) for delivering content to a wide range of devices, particularly Apple devices and browsers. HLS is one of the most widely supported adaptive bitrate streaming protocols, while LL-HLS extends the standard HLS protocol to provide reduced latency for near real-time streaming experiences.

## HLS and LL-HLS features and capabilities
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When using HLS or LL-HLS in MediaPackage, you can take advantage of the following features:
+ **Container types**: HLS and LL-HLS support both TS and CMAF container types, providing flexibility for different use cases.
+ **Program date/time**: MediaPackage can insert `EXT-X-PROGRAM-DATE-TIME` tags at configurable intervals, enabling features like viewer seek in the playback timeline and time display on the player.
+ **Start tag**: MediaPackage supports adding `EXT-X-START` tags to control where playback begins in the manifest.
+ **I-frame only streams**: MediaPackage can include additional I-frame only streams to enable player functionality like fast forward and rewind.
+ **Audio rendition groups**: For TS containers, MediaPackage can group all audio tracks into a single rendition group, allowing all other tracks to be used with any audio rendition.
+ **SCTE-35 ad markers**: MediaPackage supports signaling ad markers in HLS manifests using Daterange format or SCTE-35 enhanced CUE tags.
+ **Encryption**: HLS supports AES-128 and Sample AES encryption methods with FairPlay (Sample AES only) and Clear Key AES-128 (AES-128 only) DRM systems.

## HLS manifest structure
<a name="hls-manifest-structure"></a>

An HLS manifest is a text-based M3U8 playlist that describes the available media segments and their characteristics. HLS uses a hierarchical structure with two types of playlists:
+ **Multivariant playlist**: Contains information about the available variants (different bitrates and resolutions) and references to the media playlists.
+ **Media playlist**: Contains references to the actual media segments for a specific variant.

Key tags in HLS manifests include:
+ **EXT-X-STREAM-INF**: Describes a variant stream in the multivariant playlist.
+ **EXT-X-MEDIA**: Describes alternative renditions such as audio or subtitles.
+ **EXTINF**: Specifies the duration of a media segment.
+ **EXT-X-PROGRAM-DATE-TIME**: Associates a segment with an absolute date and time.
+ **EXT-X-KEY**: Specifies how to decrypt encrypted segments.
+ **EXT-X-SESSION-KEY**: Provides encryption key information in the multivariant playlist to enable client key pre-fetching for DRM-protected content.
+ **EXT-X-DATERANGE**: Signals ad markers and program transition events.

LL-HLS extends the standard HLS protocol with additional tags and features to reduce latency:
+ **EXT-X-PART**: Identifies partial segments that can be delivered before a complete segment is available.
+ **EXT-X-PRELOAD-HINT**: Provides hints to the player about upcoming content.
+ **EXT-X-SERVER-CONTROL**: Specifies server parameters for low-latency streaming.

## Differences between HLS and LL-HLS
<a name="hls-ll-hls-differences"></a>

While HLS and LL-HLS share many features, there are important differences to consider:
+ **Latency**: Standard HLS typically has latency of 18-30 seconds, while LL-HLS can achieve latency as low as 3-5 seconds.
+ **Program date/time**: Program date/time (PDT) is optional for standard HLS but required for LL-HLS.
+ **Segment delivery**: LL-HLS delivers partial segments to reduce latency, while standard HLS delivers complete segments.
+ **Player compatibility**: LL-HLS requires players that specifically support the low-latency extensions.

When choosing between HLS and LL-HLS, consider your latency requirements, player compatibility, and the importance of features like trick play (fast forward/rewind).

## HLS and LL-HLS use cases
<a name="hls-use-cases"></a>

Consider using HLS or LL-HLS endpoints in the following scenarios:
+ Delivering content to Apple devices (iPhone, iPad, Apple TV) and browsers
+ Implementing low-latency streaming for live events or interactive experiences (LL-HLS)
+ Supporting DRM-protected content with FairPlay or AES-128 encryption
+ Creating multi-protocol streaming workflows that need to include HLS alongside other formats
+ Enabling advanced playback features like fast forward and rewind with I-frame only streams

For information about creating HLS or LL-HLS endpoints, see [Create an HLS or LL-HLS manifest](endpoints-create.md#hls-llhls-manifest).