W3C WebTV: Adaptive Streaming & Content Protection

Feb 15, 2011


Last week, the W3C held its Second Web & TV Workshop in Berlin. The workshop focused on the convergence of web technology and broadcasting. In other words, how will web and television work together to eventually merge?

Along with sessions on second-screen scenarios and accessibility, the workshops covered adaptive streaming and content protection. Both sessions were very compelling considering that streaming and protection are two important limitations of today’s HTML5 video support.

Adaptive Streaming: DASH

Adaptive streaming is a technology that enables high-quality video streaming from any regular web server. Each adaptive stream is stored in multiple quality levels. Video players continuously request small fragments from these files (e.g. through range-requests) and seamlessly glue them together into one presentation. This technology is especially well suited for mobile (3G, 4G, WiFi) video delivery because video players can quickly adapt to changing bandwidth conditions by loading fragments from another quality level.

In the workshop’s adaptive streaming session, the main focus was on MPEG DASH, a just-released specification from the Motion Pictures Experts Group. DASH aims to standardize streaming of video over HTTP, since today’s solutions from Apple, Adobe and Microsoft are 95% the same but 100% incompatible.

In a nutshell, DASH specifies the format of the XML file that lists the available quality levels (the manifest). The spec provides guidance around encoding video for adaptive streaming (mostly for MP4) and around stitching the fragments together in a video player. See this paper and these slides from MPEG DASH co-author Thomas Stockhammer for more information.

Moving forward, two outstanding issues must be resolved in order to make DASH a widely used standard. First, DASH should be cleared of patent claims (if there are any), so it can be used in free software (e.g. for WebM). Second, specifications should be written for connecting DASH to HTML5, so adaptive streams can load in a video tag. For example, simply through the @src attribute.

Content Protection: PIFF

Content protection is another hot topic for online video. Currently, HTML5 video provides no content protection mechanisms, while closed systems like Flash and Silverlight do. Again, there is no standard for this, forcing companies like Netflix to encode their content multiple times for various DRM systems. And since DRM (by nature) is hard to specify in an open format, standardization seems far away.

There is progress though, in the form of PIFF (Protected Interoperable File Format). PIFF is based upon the MP4 file format and specifies what encryption should be used and how it should be applied. The beauty of this format is that it solely focuses on a common encryption mechanism, while leaving the rights management part untouched. This allows content owners to encrypt and store their videos once, even when using multiple DRM systems. See this paper by John Simmons for a more in-depth explanation.

The separation of encryption and rights management also opens the door for scenarios in which only encryption (no rights management) is used. Such scenarios should appeal to both publishers (for basic protection or privacy reasons) and open browsers like Firefox and Chrome. Next step in this area is the investigation of a common decryption workflow, in either HTML5 or DASH.

More News Soon

In summary, the Second Web & TV Workshop was exciting and productive, but there is still work ahead. Recognizing this, the W3C started a Web+TV Interest Group, which will continue work on such things as investigating the legal state of MPEG DASH. Stay tuned for more updates down the road…