Editing Advanced Systems Format (section)

==Overview and features==
ASF is based on [[serialization|serialized]] ''[[Object-oriented programming|objects]]'' which are essentially [[byte]] sequences identified by a [[Globally Unique Identifier|GUID]] marker.

The format does not specify how (i.e. with which [[codec]]) the video or audio should be encoded; it just specifies the structure of the video/audio stream. This is similar to the function performed by the [[QuickTime File Format]], [[Audio Video Interleave|AVI]], or [[Ogg]] formats. One of the objectives of ASF was to support playback from [[Streaming media|digital media servers]], [[HTTP server]]s, and local storage devices such as [[hard disk drive]]s.

The most common media contained within an ASF file are [[Windows Media Audio]] (WMA) and [[Windows Media Video]] (WMV). The most common file extensions for ASF files are extension {{samp|.WMA}} (audio-only files using Windows Media Audio, with MIME-type <code>audio/x-ms-wma</code>) and {{samp|.WMV}} (files containing video, using the Windows Media Audio and Video codecs, with MIME-type <code>video/x-ms-asf</code>). These files are identical to the old {{samp|.ASF}} files but for their extension and MIME-type. The different extensions are used to make it easier to identify the content of a media file.<ref>{{Cite web|url=http://support.microsoft.com/kb/284094|title=The Difference Between ASF and WMV/WMA Files}}</ref>

ASF files can also contain objects representing [[Metadata (computing)|metadata]], such as the artist, title, album and genre for an audio track, or the director of a video track, much like the [[ID3]] tags of [[MP3]] files. It supports scalable media types and stream prioritization; as such, it is a format optimized for streaming.

The ASF container provides the framework for [[digital rights management]] in Windows Media Audio and Windows Media Video. An analysis of an older scheme used in WMA reveals that it is using a combination of [[elliptic curve cryptography]] key exchange, [[Data Encryption Standard|DES]] block cipher, a custom block cipher, [[RC4 cipher|RC4]] stream cipher and the [[SHA-1]] hashing function.

ASF container-based media are sometimes still streamed on the internet either through the [[Microsoft Media Services|MMS]] protocol or the [[Real Time Streaming Protocol|RTSP]] protocol. Mostly, however, they contain material encoded for 'progressive download', which can be distributed by any webserver and then offers the same advantages as streaming: the file starts playing as soon as a minimum number of bytes is received and the rest of the download continues in the background while one is watching or listening.

The [[Library of Congress]] Digital Preservation project considers ASF to be the ''de facto'' successor of [[Resource Interchange File Format|RIFF]].<ref name="LoC" /> In 2010 [[Google]] picked RIFF as the container format for [[WebP]].