Background

Advanced ( formerly Active ) Streaming Format was developed by Microsoft in 1995-1998. Its main purpose is to serve as an universal format for storing and streaming media. There are two versions of ASF. Version that is known as 2.0 is well-documented and its specifications are publicly available. Unfortunately, they are not very helpful for developers because this format is not widely used ( if used at all ).
On the other hand, there's another version of ASF format ( 1.0 ). It is extremely popular. All files with extensions .asf, .asx, .wmv and .wma that you can find in the 'Net are stored in ASF 1.0. Microsoft never released any documentation covering this format. There's a rumour that this format is even patented! This situation similar to the one with MPEG-4 specifications: Microsoft appears to take active part in development of specifications for MPEG-4 but does not use these formats in its products, instead, it promotes their closed-source variations ( DivX ;-) and Windows Media Video ).
As long as Microsoft does not provide implementations of ASF reader or writer for any platforms except Windows and Macintosh, it is necessary to have at least minimal specification of the format to implement tools for working with ASF 1.0 on all other platforms. This document tries to organize all available information covering the format, received from different sources.
Readers are encouraged to get acquainted with ASF 2.0 specifications to better understand the ideas beyond the format and other features that it offers.

Disclaimer

This specification was created by analyzing data contained in freely-available media files. No reverse-engineering or other illegal activity took place during collection of this information. Neither author nor any contributors guarantee that any bit of this information is correct.