The State of MPEG-DASH 2016
In the meantime, Microsoft introduced experimental VP9 support in its Edge browser (no WebM support yet) and joined forces with other major industry actors in the Alliance for Open Media to develop the next-generation video format—let’s call it “post-HEVC”—where Google’s VP9 can be one of the starting points with Mozilla’s Daala or Cisco’s Thor. All this movement around free codecs generated a new interest in VP9, as it’s the most advanced among the Alliance for Open Media choices. But VP9 was still stuck in the WebM envelope, until Netflix’s David Ronca and Microsoft’s Kilroy Hughes drafted a proposal for binding Video Partition structured video codecs (such as VP8/9/10) to ISO-BMFF, including the necessary adjustments to be used with CENC and DASH.
This preliminary work, once finished, will make it easy for VP9/VP10 codecs to be introduced alongside a new set of profiles into DASH-IF’s interop guidelines, as these guidelines rely solely on the ISO-BMFF container. The resulting DASH-VPx interop point might then be an interesting alternative to DASH-265 on devices where VP9 decoding is supported in hardware—for instance, the Nvidia Shield Android TV—or software-based browser clients. Here we see another case where the industry is moving toward convergence while preserving space for competition or alternatives in the codec area—and obviously the ISO-BMFF media container standard is one of the strongest forces fostering such a convergence. Let’s examine how DASH’s next evolutions can reinforce this industry move.
Toward the Third Edition of the MPEG-DASH Core Specification
MPEG-DASH’s first edition in 2012 was a major milestone for the streaming industry, as witnessed by the widespread support among encoder/packager solutions on the market today, the second edition was not as widely implemented when it came out in 2014, maybe because it didn’t introduce crucial features on top of the initial version. As a reminder, those new features included media timeline events (to support server-driven interactions and trigger manifest updates), empty periods (to support media blackout signaling), content asset identifiers (to categorize main and advertising contents), and improved ad-insertion support. We can expect suppliers to catch up and implement the upcoming third edition of MPEG-DASH, as it will represent a quantum leap in terms of features and derived user experience. The question is how long it will take to see market solutions implementing the specification, as some components might be difficult to support. In the core scope of features, we’ll find interesting advances, such as an enhanced client-server synchronization logic; improved authentication and authorization mechanisms; the much-awaited Spatial Relationship Description, which will allow the combination of several spatially related videos in a tiled or zoomed immersive combination; a standardized tracking framework for analytics; and a new set of accessibility features to facilitate description and client-side mixing of alternate media tracks for hearing-impaired and blind users.
Manifests aggregation with Playlist Program Description
DASH manifests will also become dynamic through client-side parsing and injection of the query string parameters used in the MPD URL, which might prove useful for adding inline access tokens or custom parameters on media segments. With the companion URI signing specification, DASH will represent a standardized but powerful way to generate and validate access tokens for a given program—something that was difficult in previous technologies, when each CDN required its own access token logic implementation. The Segment Independent SAP Signaling (SISSI) feature will allow fast channel switching and short time to playback through the use of very short segment lengths during channel transition phases, switching as possible to segments of a standard duration.
Additional feature sets, resulting from some of the core experiments initiated by MPEG, will also be able to make their way as extensions to the core specification. The Server and Network Assisted DASH (SAND) will eventually show up as ISO 23009-5, providing a set of QoS parameters to be cross-referenced and shared across the chain by origin servers, network equipment, CDNs, and DASH players in order to bring enhanced ABR decision-making capability to the client. SAND uses the WebSocket protocol to convey the messages through the distribution chain node, which might be challenging for public CDNs to support because WebSocket connections are persistent. The CAPCO core experiment led to a proposal regarding the playback of several chained MPDs inside a dynamic Playlist Program Description (PPD), where a general timeline can be composed from individual MPD timelines or portions of it, mixing live, on-demand, and advertising content, with the content aggregator located anywhere in the chain from the service backend up to the client layer, which sounds like a flexible and efficient way of tackling the problem of dynamic and personalized playlists.
Finally, the upcoming Full Duplex DASH (future ISO 23009-6) will bring standard mechanisms to transport DASH over HTTP/2 and WebSockets. The HTTP/2 flavor will certainly have the most impact on the delivery aspects of DASH, first because it will be natively supported by CDNs on the short term, and also because of its intrinsic capabilities, such as data multiplexing— which allows several segments to be sent in a row over a single connection—and its push model—which can fetch content to the client without having him request it—in contrast to the current model, where only the client makes decisions on bitrates requested. Applied to DASH, this would suppose that the CDN edge understands the segment request flow generated by a DASH playback session and calculates the next segment name to push it as soon as it is available. This is relatively easy to accomplish with SegmentList or Number-based SegmentTemplate fragmentation types, but becomes more complex when Time-based SegmentTemplate fragmentation is used instead. Nevertheless, HTTP/2 without push mode will still be interesting out of the box for header compression and data multiplexing, as well as the ability to combine it into UDP-based transport protocols. The Full Duplex DASH is not an incremental change from the performance perspective; it’s a game changer that will significantly boost the QoS of playback sessions from 720p over ADSL up to 4K over fiber.
DASH-IF Interoperability Points Moving Toward UHD
The DASH Industry Forum acts as a relay to MPEG, providing complementary Interoperability Points (IOP) and best practices for deploying DASH, and specifying DASH profiles that combine precise container/codec/ security options in order to narrow down the scope of what’s possible in the DASH core specification (which is a lot). In June 2013, version two of the Interoperability Guidelines brought to DASH-264 support of HD video up to 1080p and Multichannel Audio. That was a transition to version three, which was a major jump forward in April 2015, with a specific chapter for live services; a developed ad insertion model; and support for H.265/HEVC, CEA6608/708, and stream events. In June 2015, DASH-IF released a companion implementation guidelines document for the Content Protection Information Exchange Format (CPIX), which aims to unify the exchanges between entities in charge of content protection (content provider, encoder, packager, player, DRM license delivery service, etc.). The proposed data model allows secure and standardized key exchanges between those entities, which was not possible before as all options relied on proprietary interfaces. Further 2015 revisions of the Interoperability Guidelines v3 introduced support for ISMSC1 Timed Text (of which EBU TT is a subset) and Dolby AC-4, as well as Adaptation Set switching (which can be used to mix H.264 and H.265 representations in the same MPD), live Key rotation, and MPEG-H 3D Audio.
The next big topic for the DASH-IF is the design of new UHD Interoperability Points. This is a challenge, as the overall video ecosystem is still discussing exactly how UHD technologies will combine. Hopefully things will settle down a bit more now that the UHD Alliance announced the Ultra HD Premium certification requirements at CES 2016, summarized as 4K HEVC 10bits/WCG/BT.2020/HDR SMPTE ST2084 EOTF. DASH-IF is working closely with other standardization bodies concerned with DASH and/or UHD (such as DVB, ATSC, SCTE, DECE, or MPEG) in order to ensure that its new UHD IOPs are aligned with other DASH UHD profiles throughout the broadcast industry. Given the complexity and the scope, a few additional iterations will most likely be required before the version four Guidelines, including the new UHD IOPs, are released for community review. Could the expectation for UHD slow the adoption of DASH-IF Guidelines version three? Possibly, but these guidelines condense what is required today for 1080p AVC/HEVC services and the UHD IOPs will not revolutionize with version three, but rather incrementally modify it with specific UHD requirements and recommendations.
There’s no doubt that DASH will be a cornerstone of all major media initiatives, such as the Global Internet Video Ecosystem (GIVE) Project, which wants to push HTML5 video forward as quickly as possible. But starting today, you can deploy DASH in production and take advantage of its technical strengths and vivid ecosystem.
This article appears in the 2016 Streaming Media Industry Sourcebook.
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