Presentation on theme: "A Technical Analysis of the Adaptive Bit Rate Technology in DASH – Dynamic Adaptive Streaming over HTTP Brian Bresnahan,"— Presentation transcript:
A Technical Analysis of the Adaptive Bit Rate Technology in DASH – Dynamic Adaptive Streaming over HTTP Brian Bresnahan, Harvard University Extension School CSCI-E139 Spring 2013, Len Evenchik
Basic Network Diagram and Data Flow
Fundamental Problem How can you maintain seemless video and audio playback over a network that can not guarantee quality of service ? Solution: A streaming algorithm that adjusts to the bandwidth available on the network.
Video Streaming Technologies Adobe Flash progressive download Apple HTTP Live Streaming Microsoft Smooth Streaming (Netflix) Adobe HTTP Dynamic Streaming NEW! DASH – Dynamic Adaptive Streaming over HTTP
Empirical Network Analysis of Video Streaming Providers Video Company Proto- col CDNPlayer TechnologyNotes HuluHTTPAkamaiAdobe Flash YoutubeHTTPnoAdobe FlashHTTP GET of a file of mime-type video/x-flv. For 360p, every 6-10 seconds a 1.7 MB file is downloaded. At 720p, player continuously downloads using HTTP a large file with mime-type video/mp4. e.g. 53 minute video is 1.3 GB NetflixHTTPLimelight or Netflix Microsoft Smooth Streaming and Silverlight Silverlight is the player software technology. Packet capture analysis done with Wireshark and Google Chrome browser Developer tools on my home network.
Video Codecs and Bitrates CodecBitrate Resolution Width (pixels) Resolution Height (pixels) Notes AVC1256 Kbps AVC1512 Kbps AVC11 Mbps AVC Mbps AVC11.5 Mbps AVC12.0 Mbps MPEG-27.0 Mbps DVD resolution. (playback bitrate from PS3) AVC122 Mbps Blu-Ray DVD resolution. (Playback bitrate from PS3. May burst up to 40 Mbps) ?? “2K” UHDTV. Also “4K”.
Adaptive Bitrate Algorithm A video streaming system should dynamically adapt to the network conditions and playback device CPU load then automatically select the highest possible resolution the network can support. The general concept is called “adaptive bitrate streaming” (ABR). It was conceived by the DVD Forum in 2002 (Wikipedia, Adaptive Bit Rate). ABR follows this simple algorithm: 1.Encode the media at various resolutions, color depth, frame rate. The result is multiple data sets with different sizes which in turn will require different network bandwidths for streaming for playback. 2.As the media is played, the client monitors the playback. If the playback stalls, use a lower bitrate encoding. 3.If the playback is not stalling and there is bandwidth available such that the player can use a higher bitrate encoding, switch to the higher bitrate encoding
DASH Specification The specification was promoted by the Motion Pictures Expert Group (MPEG), “a working group of ISO/IEC with the mission to develop standards for coded representation of digital audio and video and related data.” (http://mpeg.chiariglione.org) DASH under development since Dynamic Adaptive Streaming over HTTP (DASH) ISO Specification Number
DASH Architecture Elements HTTP/Web Servers, not proprietary servers HTTP – Hypertext Transfer Protocol – Based on TCP, a reliable protocol that guarantees byte order – Already has encryption standards – Is stateless, making the server simple. Forces the complexity to the client where ABR algorithm needs to run. – Allows for “partial GET” XML based Manifest.MPD file Goal: support Live Event streaming as well as static encoded media
DASH Network Model
Operational Steps 1.A back-end process is used to prepare and encode the media. This is the box on the left. This needs to be done before any clients can view the media. 2.The DASH client requests from the DASH server a “manifest” XML file called the MPD (Media Presentation Description) file. This file contains information regarding all available bitrates i.e. “Representations” for a particular video. 3.The media playing software application uses the DASH client to select a video and audio bitrate from the manifest and begins downloading both separately with HTTP. 4.The DASH client is codec agnostic so it passes the received video frames and audio packets to the media application to render them with the codecs on the operating system the application is running on. 5.The DASH client runs the ABR algorithm previously described. If the video or audio segments are downloading well, the next higher bit rate may be switched to. Similarly, if the segment download rate is insufficient for the chosen bitrate, a lower bitrate stream will be selected. 6.The DASH client repeatedly does HTTP GETs for segments of video and audio data until the video and audio playback is complete.
DASH Data Model Elements
Media Stream Access Points Required for the DASH client to be able to seamlessly switch bitrates as network bandwidth increases or decreases.
Media Presentation Description File mp mp4 streamer.harvard.edu/e139_vid_2_1mbps.mp4 streamer.harvard.edu/e139_vid_2_2mbps.mp4
URL Method 2 Example This example is of method 2 described above. The MPD has 2 separate URLs for retrieving 1 Mbps encoded or 2 Mbps encoded media from the ficticious server streamer.harvard.edu: streamer.harvard.edu/e139_vid_2_1mbps.mp4 streamer.harvard.edu/e139_vid_2_2mbps.mp4 Note that since the Representation does not describe a variable name for the URL, then the expectation is HTTP partial GETs using begin and end bytes will be used to retrieve segment data.
URL Method 3 Example CSCIE_139_Lecture5_ From the above BaseURL tag and Segment Template, the DASH client can synthesize the following URLs over time: CSCIE_139_Lecture5_1130kbps_00000.ts CSCIE_139_Lecture5_1130kbps_00001.ts …
Other DASH Features Metric Reporting – the DASH spec defines 3 “observation points” that can be used to monitor the DASH client retrieving segments with HTTP and feeding the data to the client applicaton. (ISO, DASH Specification, p. 107) Switchable and selectable streams – useful to select different language audio streams or different camera angles if available. Ad insertion – DASH defines the ability to insert ads between Periods or Segments. Multiple Base URLS – the Representation can specify multiple URLs to allow the client to use different servers or CDNs if needed Clock drift control – for live sessions, the UTC time can be included with each segment to allow the client to adjust playback to match the server’s encode rate.
Current DASH Research and Demo Sites France GPAC A team in France is focused on creating multimedia software they collectively call GPAC. GPAC is an Open Source multimedia framework for research and academic purposes. DASH Industry Forum DASH is promoted by this organization which is in turn funded by large corporations interested in DASH mentioned in the overview of this paper. They have developed a reference client media player that implements DASH. Note that you must run Chrome to use their player. Opensource libdash library. This is a C++ library that implements the MPEG-DASH standard specified by ISO It uses the libcurl library which contains HTTP. DASH test method This research team wrote a paper on how to correctly test DASH with reference media. Google/Youtube Google has a team that implemented an experimental DASH client. This is interesting because Youtube streaming is currently based on Adobe Flash.
Youtube/Google DASH Demo Page
Bitrate Transition in Youtube DASH Log 31: DASH MSE/EME demo version gfee : Initializing : 1, XHR for manifest sent 230: 2, Manifest received 393: 3, onSourceOpen() 398: 4, updateRepresentationForm() (AUTHOR’S NOTE: the “87” file is initial video file and “8c” is the audio file.) 896: 2, Sent XHR: url=http://yt-dash-mse-test.commondatastorage.googleapis.com/car mp4, range=bytes= : 2, Sent XHR: url=http://yt-dash-mse-test.commondatastorage.googleapis.com/car c.mp4, range=bytes=0-851 (AUTHOR’S NOTE: REMOVED PART OF LOG. Note the next line shows the player selecting a video file with a different bit rate.) 1899: Selected new rate for bandwidth (from , ) 1900: 3, onRepChange 1904: 1, resetSourceBuffer 2396: 2, Sent XHR: url=http://yt-dash-mse-test.commondatastorage.googleapis.com/car mp4, range=bytes= : 2, Sent XHR: url=http://yt-dash-mse-test.commondatastorage.googleapis.com/car c.mp4, range=bytes=
Conclusion DASH exemplifies the engineering of the internet. That is, it is built on previously designed and proven building blocks. DASH is built largely on HTTP and also on XML. Beneath HTTP is the TCP protocol delivering an error- free, ordered byte stream required for quality video display and audio playback. HTTP and TCP are available in a wide variety of devices today from handheld computers such as iPhones to HD TVs, creating the possibility of DASH being deployed on this wide variety of devices. The design of DASH appears to be solid.
Project References ISO, DASH Specification. Dynamic Adaptive Streaming over HTTP (DASH) ISO Geneva, Switzerland, 1 April DASH Industry Forum. n.d. Iraj Sodagar. The MPEG-DASH Standard for Multimedia Streaming over the Internet. October L. Peterson, B. Davie. Computer Networks, 5th Edition. March w3c. HTML5 Specification Wikipedia. Adaptive Bitrate Streaming Video Demystified, Keith Jack For MPEG-2 and MPEG-4 definitions. DASH Industry Forum, Google / Youtube DASH team, test.appspot.com/decoder-test.htmlhttp://dash-mse- test.appspot.com/decoder-test.html