Systems Format (formerly Advanced Streaming
Format) is Microsoft's proprietary digital audio/digital
video container format, especially meant for streaming
media. ASF is part of the Windows Media framework.
format does not specify how the video or audio should
be encoded, but instead just specifies the structure of
the video/audio stream. What this means is that ASF files
can be encoded with basically any audio/video codec and
still would be in ASF format. This is similar to the function
performed by the QuickTime, AVI, or Ogg formats. One of
the objectives of the ASF format was to support playback
from digital media servers, HTTP servers, and local storage
ASF format is based on serialized objects which are essentially
byte-sequences identified by a GUID marker.
most common filetypes contained within an ASF file are
Windows Media Audio (WMA) and Windows Media Video (WMV).
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.
containing only WMA audio can be named using a .wma extension,
and files of only audio and video content may have the
extension .wmv. Both may use the .asf extension if desired.
error-correcting techniques related to ASF are patented
in the United States (United States Patent 6,041,345 Levi,
et al. March 21, 2000) by Microsoft. Although the format
is publicly documented by Microsoft, its license limits
implementations to closed-source development projects
only. Interestingly, Apple's iTunes software now has the
capability to convert WMA files to any iTunes supported
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.
files have MIME type application/vnd.ms-asf or video/x-ms-asf.
(Advanced Stream Redirector (ASX) files also have MIME
ASF container is usually streamed on the net through Microsoft's
proprietary MMS protocol.