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As described in the first post in this series: What is Dynamic Streaming?, HTTP Dynamic Streaming (HDS) allows for the delivery of streaming content over the HTTP protocol. Streaming video over HTTP requires some knowledge and preparation. The following is a rundown and description for each of the components involved with HTTP streaming.
The files: The F4F and F4M file formats
The F4F file format specification (http://www.adobe.com/devnet/f4v.html) describes how to divide media content into segments and fragments. These segments and fragments are what make ‘streaming’ content over HTTP possible. Basically, the file is broken up into multiple pieces and parts when it is prepared for for HTTP streaming.
A sample segment and its fragments
These way the segments and each segments fragments are created is based on multiple factors such as the length of the video content, the the number of keyframes and the length specified for each segment. The result of packaging your video content for HDS delivery is a set of files that look similar to the following for a simple, single bit-rate file:
The set of files created by the f4fpackager for a short video clip.
From the image above:
- sample.f4m: This is an XML file created by the f4fpackager that contains the information necessary for a client player to playback the video file. There will be only one of these for each source video file.
- sampleSeg1.f4f: This segment file contains the fragments that the client player will request for playback. There can be multiple of these files for each source video file.
- sampleSeg1.f4x: This is an index file that contains specific information about the fragments inside the segment files. There will be one of these types of files for each segment file. The HTTP Origin Module uses the data in this file to determine the fragment to send back to the client after a request.
Each F4F file is a single segment and the segment can contain multiple fragments. So, if you inspect the HTTP requests as a media item is being played back you will see files being requested that map to a fragment within a segment. For example:
Segments and fragments being requested for HDS video
This request specifies to the HTTP Origin Module what segment’s fragment to pull out of the F4F file and deliver to the client for playback. These requests are based on where the time code of where the media item is during playback. So if you scrub to later in the video, the Requests might look something like the following:
Segments and fragment requests while scrubbing HDS video content
This allows the client to request any piece of the video content and start playing back almost immediately. You can control how the files are segmented and fragmented when preparing content with the f4fpackager. The basic concept is to balance the size of the fragment being delivered and the number of HTTP requests that a client needs to make for playback. Please note, the fragments may not be sequential, so you cannot rely on this when requesting content. The the fragments are based on the settings passed into the packager and can skip fragment numbers. So , you can expect to see fragment sequences like the following (no scrubbing involved):
This doesn’t mean that the file is incomplete, it is just how the fragments were created by the packager.
The F4M File Format
The F4M or Flash Media Manifest file format contains the information about the package of files created when video content is packaged for HDS. The information included in the manifest file can include some or all of the following:
- Media location
- Media type
- Media bootstrap
- Multi-bitrate (MBR) availability
- Digital Rights Management (DRM) authentication
- DVR information
- moov atom, metadata block and XMP metadata block
When playing back HDS video content, the F4M file is expected to be loaded as the ‘media file’. The client is responsible for inspecting the data included in the F4M file to authenticate (if DRM authentication is required), determine if MBR content is available and select the best MBR version of the content and then request the media from the server.
Sample of a simple F4M file with a single media item:
Installing and configuring the Apache HTTP Origin Module: http://help.adobe.com/en_US/HTTPStreaming/1.0/Using/WS7b362c044b7dd076-735e76121260080a90e-8000.html
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