Advanced Systems Format (formerly Advanced Streaming Format, Active Streaming Format) is Microsoft’s proprietary digital audio/digital video container format. ASF is part of the Windows Media framework which is based on serialized objects which are essentially byte sequences identified by a GUID marker. The format, which means to move forward 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, AVI, or Ogg container formats. One of the objectives of ASF was to support playback from digital media servers, HTTP servers, and local storage devices such as hard disk drives.
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 .WMA (audio-only files using Windows Media Audio, with MIME-type ‘audio/x-ms-wma’) and .WMV (files containing video, using the Windows Media Audio and Video codecs, with MIME-type ‘video/x-ms-asf’). These files are identical to the old .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.
ASF files can also contain objects representing 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, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function.
ASF container-based media are sometimes still streamed on the internet either through the MMS protocol or the RTSP protocol. Mostly, however, they contain material encoded for ‘progressive download’, which can be distributed by any web server 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.