ASF file is based on serialized objects which are essentially byte sequences identified by a 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, 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.
ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV). Note that the file extension abbreviations are different from the codecs which have the same name. Files containing only WMA audio can be named using a .WMA extension, and files of audio and video content may have the extension .WMV. Both may use the .ASF extension if desired.
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 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.
