The State of MPEG-DASH 2015
”DRM interoperability is a must-have in a video streaming ecosystem which such a broad variety of devices, with some devices only using specific DRM products, leading to a dynamic yet fragmented multi-DRM marketplace,” says Laurent Piron, solution architect at Nagra. “Extending interoperability to the head-end is the main goal of Content Protection Information Exchange Format (CPIXF). It allows DRM license service providers to perform easier pre-integration with other components of the streaming backend such as encoders and CDNs, hence delivering significant cost savings while improving time to market and agility.”
Talking about the DRM ecosystem convergence around this initiative, Piron adds: “In a market with multiple DRM and encoder vendors all aiming to grow the streaming market, the CPIXF workgroup within DASH IF, under NAGRA’s leadership, has now gotten the involvement of all the key players in the internet and TV space, except Apple, which still has not joined DASH IF.
“It shows that the industry is committed to simplify product integration and create a dynamic open streaming ecosystem. It the end, it’s an all-win proposition for all parties, including consumers who get the best experience and content on all devices.”
With CPIXF, the working group has actually crafted a new XML-based data exchange format that all entities involved in the protected content preparation workflow (CMS, encoder/transcoder, packager/encryptor, DRM client, and DRM service) can use to safely and efficiently exchange content keys and DRM Signalization. The specification covers live, on-demand, catch-up, and electronic sell-through use cases, as well as secure push and pull data exchange modes between the DRM systems and the encryptors. This is huge progress in terms of simplifying content preparation workflows, and there is a high probability that implementations will increase in 2015, as the demand for multiple DRMs is skyrocketing. After that, we will only need to see the browser providers address the other part of the same problem, the need to authorize various Content Decryption Modules to install on the various browsers, as the perspective of using one DRM per ecosystem is not very attractive when MPEG-DASH is attempting to simplify workflows.
Several organizations, including Microsoft, Intertrust, ETSI, and Fraunhofer, have made proposals for the CDM Interface API that needs to be standardized, but the discussion still needs to happen in the same industry convergence that has produced DASH.
“CDM and CPIXF are both addressing the same problem but at the exact opposite side of the delivery chain,” Piron says. “Browser vendors need to support multiple DRMs to maximize their market reach and thus will also experience similar business benefits as the backend vendors have found out. So it’s expected that the standardization occurring on the backend side of the chain will provide momentum to accelerate standardization on the client side. That’s where MPEG-DASH, as an open industry standard, brings a powerful vehicle to drive such important interoperability effort for the whole industry.”
The two other main evolutions to monitor at DASH IF are the introduction of the EBU TT-D subtitling format as the reference format for the future. Currently the Timed Text space is crippled with divergent implementation profiles, and it’s virtually impossible to cover all of the workflow, from content production to multiscreen distribution, with a unique format. EBU is currently completing the TT-D specifications set to cover all use cases and to provide mappings from legacy formats to TT-D. But organizations such as DVB have already introduced the new format in their specifications, and it makes sense for DASH IF to align with DVB on this topic. “On Timed-Text, W3C is currently working on profiling of TTML including EBU TT-D and other variations,” Sodagar says. “DASH can deliver any of those TTML and other closed caption formats. But convergence and easy translation from one closed caption format to another would help achieving higher interoperability and greater reach faster.”
This is roughly the same situation as UltraHD. The DASH UltraHD profile(s) will need to align their specifications with the broadcast approach, in order to allow economies of scale throughout the industry and to see UltraHD contents easily shift from one screen type to another. “On UHD, several organizations are working to define their UHD format and DASH-IF is collaborating with them on DASH UHD profiles,” Sodagar says. “The hope is that industry converges on a few UHD formats and that consequently we define DASH UHD Interoperability Points for those formats.”
DASH v3 Core Experiments Shaping Up the Future
In order to bring more intelligence and performance to DASH, MPEG has launched several exploratory tracks named “Core Experiments.” These are designed to validate the proposals and lay out the basis of the DASH v3 specification.
Acknowledging that there are dozens of use cases where it doesn’t make sense to let the player alone make the decisions on the quality variant it is requesting, MPEG has launched the Server and Network Assisted DASH (SAND) initiative. SAND introduced the concept of DASH Aware Network Element (DANE), which is able to understand that DASH content is being delivered, as well as a new range of information exchanges between the DASH Client, the Media Origin, the DANE, and third-party DASH distribution monitoring servers. These include SAND parameters passed by the delivery network elements to assist the DASH client operations, DASH metrics collected by the client on the DASH service and Parameters for Enhancing delivery by DANE provided by the Media Origin. The bidirectional communication channel of WebSocket is tipped as an efficient real-time transport vector for this information. This instrumented approach is supposed to prevent clients from cannibalizing the bandwidth and triggering an infinite cycle in bitrate switches, as well as to allow network operators to better control video that is delivered over their networks.
But the mission of MPEG does not inevitably cover the definition of all standards needed to get SAND implemented across all the chain. “The goal of SAND is to define parameters (basically and most importantly the semantics) that help improving DASH-based applications and services along the delivery/distribution path/ network,” says Dr. Christian Timmerer, CIO at Bitmovin. “Similar to DASH metrics (which are implemented in bitdash), it’s important to define the semantics of the parameter which shall be interpreted as requirements from a DASH perspective that can help standardizing whatever is needed within the delivery/distribution path/network whoever is working on standards in that domain (e.g. IETF, W3C, 3GPP...). Thus, I don’t think we should provide too much in terms of specifications as the main purpose is to inform others what we think is needed within their domain to help DASH to evolve.”
Another study track has been launched to see how DASH would have to be adapted to push-based transport protocols HTTP/2 and WebSocket. It’s called DASH over Full Duplex HTTP-based Protocols (FDH). It aims to provide new options for large scale and low-latency streaming, based on the ability of the DASH delivery node to push the new segments to DASH players as soon as they propagate through the network, thus avoiding the waiting time generated by client-driven requests. The new mechanisms will be interoperable with HTTP/1.1, work with existing DASH contents and support caching by CDNs. If all these goals are reached, the FDH initiative certainly conveys the hope for OTT video delivery to get closer to the scalability and the low latency of the broadcast distribution, which is still a challenge.
There are other ongoing experiments. The goal of the URI Signing for DASH (CE-USD) is to create a solution to prevent a client from skipping pre-roll and mid-roll advertisements, grant temporary access to the contents based on tokens, or prevent deep linking. IETF’s mechanism for URI signing for CDN Interconnection is under review for these purposes. The SAP-Independent Segment Signaling (SISSI) experiment tackles problems including low-latency live streaming and the Fast Tune In use case, where a client accesses a representation with high SAP frequency, then switches to a representation with less IDR frames when the playback session is initiated (in both unicast and hybrid cases).
While recognizing the interest of SISSI, Timmerer also places the problem in context: “Low latency is an issue for live scenarios, but it’s about solving a problem which starts at wrong assumptions based on OTT delivery chain being attached to the content production chain too late,” he says. “Often the input to the OTT delivery chain is the same as what consumers see on their TV, which already introduces significant latency, much more than OTT itself has.”
The Spatial Relationship Description (SRD) experiment (go2sm.com/dashspatial) is designed to explore how we can combine several spatially related videos in a tiled or zoomed combination, while providing backward compatibility with ordinary consumption of adaptation sets. This feature is unique to DASH (until it’s copied by HLS) and provides an immersive approach to tiled streaming in the upcoming perspective of the facilitated UltraHD video production.
With these new features waiting on the corner, MPEG-DASH has positioned itself at the crossroads of innovation. The question for many industry actors is no longer to determine if the DASH revolution has happened, but rather to embrace its vivid development pace and tame its complexity which is also its key success factor. The final word about DASH comes from Dr. Leonardo Chiariglione, co-founder and chair of the Moving Picture Experts Group (MPEG): “I do believe that the multimedia industry has now the full protocol stack in place for offering services and MPEG is the capability to embed innovation at all layers of the stack.”
[This article appears in the 2015 Streaming Media Industry Sourcebook as The State of MPEG-DASH.]
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