What is MPEG DASH?
MPEG DASH is the latest hot topic in the online video space. Here we break down what it is, and what its implications might be for video delivery in the future.
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Posted on November 22, 2011Page 1
MPEG DASH (Dynamic Adaptive Streaming over HTTP) is a developing ISO Standard (ISO/IEC 23009-1) that should be finalized by early 2012. As the name suggests, DASH is a standard for adaptive streaming over HTTP that has the potential to replace existing proprietary technologies like Microsoft Smooth Streaming, Adobe Dynamic Streaming, and Apple HTTP Live Streaming (HLS). A unified standard would be a boon to content publishers, who could produce one set of files that play on all DASH-compatible devices.
The DASH working group has industry support from a range of companies, with contributors including critical stakeholders like Apple, Adobe, Microsoft, Netflix, Qualcomm, and many others. However, while Microsoft has indicated that it will likely support the standard as soon as it’s finalized, Adobe and Apple have not given the same guidance, and until DASH is supported by these two major players, it will gain little traction in the market.
A more serious problem is that MPEG DASH doesn’t resolve the HTML5 codec issue. That is, DASH is codec agnostic, which means that it can be implemented in either H.264 or WebM. Since neither codec is universally supported by all HTML5 browsers, this may mean that DASH users will have to create multiple streams using multiple codecs, jacking up encoding, storage, and administrative costs.
Finally, at this point, it remains unclear whether DASH usage will be royalty-free. This may impact adaption by many potential users, including Mozilla, who has already commented that it’s “unlikely to implement” DASH unless and until it’s completely royalty-free. With Firefox currently sitting at around 22% of market share, this certainly dims DASH’s impact in the HTML5 market.
Introduction to MPEG DASH
Adaptive streaming involves producing several instances of a live or on-demand source file and making them available to various clients depending upon their delivery bandwidth and CPU processing power. By monitoring CPU utilization and/or buffer status, adaptive streaming technologies can change streams when necessary to ensure continuous playback or to improve the experience.
One key difference between adaptive streaming technologies is the streaming protocol utilized. For example, Adobe’s RTMP-based Dynamic Streaming uses Adobe’s proprietary Real Time Messaging Protocol (RTMP), which requires a streaming server and a near-continuous connection between the server and player. Requiring a streaming server can increase implementation cost, while RTMP-based packets can be blocked by firewalls[.
A near-continuous connection means that RTMP can’t take advantage of caching on plain-vanilla servers like those used for Hypertext Transfer Protocol (HTTP) delivery, the delivery protocol used by Apple’s HTTP Live Streaming (HLS), Microsoft’s Smooth Streaming, and Adobe’s HTTP-based Dynamic Streaming (HDS). All three of these delivery solutions use standard HTTP web servers to deliver streaming content, obviating the need for a streaming server. In addition, HTTP packets are firewall friendly and can utilize HTTP caching mechanisms on the web. This latter capability should both decrease total bandwidth costs associated with delivering the video, since more data can be served from web-based caches rather than the origin server, and improve quality of service, since cached data is generally closer to the viewer and more easily retrievable.
While most of the video streamed over the web today is still delivered via RTMP, an increasing number of companies will convert to HTTP delivery over time.
All HTTP-based adaptive streaming technologies use a combination of encoded media files and manifest files that identify alternative streams and their respective URLs. The respective players monitor buffer status (HLS) and CPU utilization (Smooth Streaming and HTTP Dynamic Streaming) and change streams as necessary, locating the alternate stream from the URLs specified in the manifest file. HLS uses MPEG-2 Transport Stream (M2TS) segments, stored as thousands of tiny M2TS files, while Smooth Streaming and HDS use time-code to find the necessary fragment of the appropriate MP4 elementary streams.
Existing HTTP-based adaptive streaming technologies are so similar that at Streaming Media West in November, 2011, Will Law, Akamai’s principal architect for media engineering, commented, “We’ve spent the past five years delivering a variety of adaptive video formats—SmoothHD, HDNI, HLS and HDS—all of which are 80 percent the same but 100 percent incompatible.”
DASH is an attempt to combine the best features of all HTTP-based adaptive streaming technologies into a standard that can be utilized from mobile to OTT devices.
MPEG DASH Technology Overview
As mentioned, all HTTP-based adaptive streaming technologies have two components: the encoded A/V streams themselves and manifest files that identify the streams for the player and contain their URL addresses. For DASH, the actual A/V streams are called the Media Presentation, while the manifest file is called the Media Presentation Description.
As you can see in Figure 1, the Media Presentation is a collection of structured audio/video content that incorporates periods, adaptation sets, representations, and segments.
Figure 1. The Media Presentation Data Model. Taken from MPEG-DASH presentation at Streaming Media West, 2011.
The media presentation defines the video sequence with one or more consecutive periods that break up the video from start to finish. Each period contains multiple adaptation sets that contain the content that comprises the audio/video experience. This content can be muxed, in which case there might be one adaptation set, or represented in elementary streams, as shown in Figure 1, enabling features like multiple language support for audio.
Each adaptation set contains multiple representations, each a single stream in the adaptive streaming experience. In the figure, Representation 1 is 640×480@500Kbps, while Representation 2 is 640×480@250Kbps.
Each representation is divided into media segments, essentially the chunks of data that all HTTP-based adaptive streaming technologies use. Data chunks can be presented in discrete files, as in HLS, or as byte ranges in a single media file. Presentation in a single file helps improve file administration and caching efficiency as compared to chunked technologies that can create hundreds of thousands of files for a single audio/video event.
The DASH manifest file, called the Media Presentation Description, is an XML file that identifies the various content components and the location of all alternative streams. This enables the DASH player to identify and start playback of the initial segments, switch between representations as necessary to adapt to changing CPU and buffer status, and change adaptation sets to respond to user input, like enabling/disabling subtitles or changing languages.
Other attributes of DASH include:
- DASH is codec-independent, and will work with H.264, WebM and other codecs
- DASH supports both the ISO Base Media File Format (essentially the MP4 format) and MPEG-2 transport streams
- DASH does not specify a DRM method but supports all DRM techniques specified in ISO/IEC 23001-7: Common Encryption
- DASH supports trick modes for seeking, fast forwards and rewind
- DASH supports advertising insertion
In terms of availability, DASH should be completed and ready for deployment in the first half of 2012.
MPEG DASH Intellectual Property Issues
At this point, it’s unclear whether DASH will be encumbered by royalties, and where they might be applied. For example, the MPEG-2 video codec comes with royalty obligations for encoders, decoders, and users of the codec. Many of the participants who are contributing intellectual property to the effort—including Microsoft, Cisco, and Qualcomm—have indicated that they want a royalty-free solution. While these three companies comprise the significant bulk of the IP contributed to the specification, not all contributors agree, so the royalty issue is unclear at this time.
Other issues include whether browser-vendor Mozilla can integrate DASH into their Firefox browser if the underlying media that a DASH MPD reference uses royalty-bearing components to play back. This is one of the key reasons that the company didn’t integrate H.264 playback into the Firefox browser in the past, along with the potential $5 million dollar per year royalty obligation.
We asked Mozilla about their intentions regarding DASH, and they sent this statement from Chris Blizzard, Director of Web Platform.
Mozilla has always been committed to implementing widely adopted royalty-free standards. If the underlying MPEG standards were royalty free we would implement DASH. However, MPEG DASH is currently built on top of MPEG Transport Streams, which are not royalty free. Therefore, we are unlikely to implement at this time.
According to website www.netmarketshare.com, as of November 18, 2011, Firefox enjoyed a 22.5% market share in the desktop browser market. Without support from Firefox, DASH obviously doesn’t represent a standard that will unify the approach to adaptive streaming in the HTML5 market. In addition, as a codec-agnostic technology, DASH also does nothing to resolve the HTML5 codec issue, so even if it was fully adopted by all HTML5-compatible browsers, content producers would still have to encode in both H.264 and WebM for universal playback.
Obviously, this doesn’t preclude DASH from being integrated into plug-ins like Flash or Silverlight or being implemented in mobile or OTT devices, and playing a significant role in these markets. However, as things exist today, it’s hard to see DASH as the cure-all solution for the current lack of live, adaptive streaming, and DRM support in desktop HTML5 browsers. And, in the absence of affirmative statements from Apple or Adobe that they will adopt the standard once finalized, it’s unclear how much immediate traction DASH will gain in the mobile and plug-in markets. Let’s see why.
MPEG DASH Competitive Issues
To a great degree, DASH levels the playing field among competitive players in the adaptive streaming space. For example, Apple’s HLS provides a distinct competitive advantage over other mobile platforms as it’s a widely adapted specification that allows all connected iDevices to play adaptive streams. That’s why Google decided to implement HLS in Android 3.0. Distributing video to Apple iOS devices has been relatively straightforward because of HLS, while the lack of a technology standard and the diversity of devices has made distributing video to Android, Blackberry, and other mobile markets very challenging.
If Apple adopts DASH and implements it on all existing connected iDevices, this competitive advantage disappears, and all DASH-enabled mobile devices are on a level playing field respecting video playback. To be clear, Apple representatives have been active in creating the specification and there is no indication that they won’t support it when it’s released. However, none of the Apple representatives that we contacted were able to comment on Apple’s intent, which is not unusual given that Apple seldom comments on unreleased products. Still, Apple is not known for its competitive graciousness, and adapting DASH would clearly make their products less competitive vis a vis other mobile platforms, at least in the short term.
On the flip side, content publishers want a distribution mechanism with flexible and complete DRM, which iDevices don’t currently provide. If enough content producers support DASH-enabled platforms, but not iDevices, that will obviously motivate Apple to support the spec. However, unless and until Apple supports DASH, it’s unlikely that producers without DRM concerns will stop producing HLS streams, which may lesson the attractiveness of supporting DASH.
To a lesser degree, the same principle holds true for Adobe since the Flash Player’s ubiquity on the desktop is a key competitive advantage over Microsoft’s Silverlight and even HTML5. Though Adobe participated in the standards work, they haven’t committed to supporting DASH in future versions of the Flash Player. Again, Adobe seldom comments on future products, so you can’t draw any conclusion from their silence.
Conclusion
DASH is an extraordinarily attractive technology for web producers, a single standard that should allow them to encode once, and then securely distribute to a universe of players, from mobile to OTT, and to the desktop via plug-ins or HTML5. In addition to not resolving the HTM5 codec issue, it’s also unclear whether publishers will be charged for the privilege of producing files using the DASH spec, which could be a significant negative.
Mozilla has already indicated that they probably won’t support the specification as currently written, and Apple and Adobe have not affirmed if or when they will support the technology. An optimist would assume that the value of DASH to the streaming media marketplace would compel all stakeholders to make their contributes royalty free, and convince Apple, Adobe and Mozilla to support the specification soon after its release. Until all this plays out, though, DASH may play a significant role in some markets, but won’t reach its full potential.
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What is MPEG DASH?
Bron: Streaming Media
