User manual APPLE HTTP LIVE STREAMING OVERVIEW

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[. . . ] Currently HTTP Live Streaming supports AES-128 encryption using 16-octet keys. The format of the key file is a packed array of these 16 octets in binary format. The media stream segmenter available from Apple provides encryption and supports three modes for configuring encryption. The first mode allows you to specify a path to an existing key file on disk. In this mode the segmenter inserts the URL of the existing key file in the index file. The second mode instructs the segmenter to generate a random key file, save it in a specified location, and reference it in the index file. All media files are encrypted using this randomly generated key. Session Types 2009-09-09 | © 2009 Apple Inc. All Rights Reserved. 13 CHAPTER 2 Using HTTP Live Streaming The third mode instructs the segmenter to generate a random key file, save it in a specified location, reference it in the index file, and then regenerate and reference a new key file every n files. Note: All media files may be encrypted using the same key, or new keys may be required at intervals. The theoretical limit is one key per media file, but because each media key adds a file request and transfer to the overhead for presenting the following media segments, changing to a new key periodically is less likely to impact system performance than changing keys for each segment. You may also choose to protect the delivery of the key files using your own session-based authentication scheme. Caching and Delivery Protocols HTTPS is commonly used to deliver key files. It may also be used to deliver the content files and index files, but this is not recommended when scalability is important, since HTTPS requests often bypass web server caches, causing all content requests to be routed through your server and defeating the purpose of edge network distribution systems. For this very reason, however, it is important to make sure that any content delivery network you use understands that the . M3U8 index files are not to be cached for longer than one media segment duration. Stream Alternates Index files may reference alternate streams of content. References can be used to support delivery of multiple streams of the same content with varying quality levels for different bandwidths or devices. The client software uses heuristics to determine appropriate times to switch between the alternates. Currently, these heuristics are based on recent trends in measured network throughput. The index file points to alternate streams of media by including a specially tagged list of other index files, as illustrated in xName-No-Link 14 Caching and Delivery Protocols 2009-09-09 | © 2009 Apple Inc. All Rights Reserved. CHAPTER 2 Using HTTP Live Streaming Figure 2-1 Stream alternates Alternate-A Index file . ts Index file Alternate-B Index file . ts Alternate-C Index file . ts Note that the client may choose to change to an alternate stream at any time, such as when a mobile device enters or leaves a WiFi hotspot. Failover Protection If your playlist contains alternate streams, they can not only operate as bandwidth or device alternates, but as failure fallbacks. Starting with iPhone OS 3. 1, if the client is unable to reload the index file for a stream (due to a 404 error, for example), the client attempts to switch to an alternate stream. In the event of an index load failure on one stream, the client chooses the highest bandwidth alternate stream that the network connection supports. If there are multiple alternates at the same bandwidth, the client chooses among them in the order listed in the playlist. You can use this feature to provide redundant streams that will allow media to reach clients even in the event of severe local failures, such as a server crashing or a content distributor node going down. [. . . ] The index file can contain an instruction to the client that content should not be cached. Otherwise, the client may cache data for performance optimization when seeking within the media. 19 2009-09-09 | © 2009 Apple Inc. All Rights Reserved. CHAPTER 3 Frequently Asked Questions 16. It has inherent latency corresponding to the size and duration of the media files containing stream segments. [. . . ]