1. CSci4211: Multimedia Networking1 A Quick Primer on Multimedia Networking Multimedia vs. (conventional) Data Applications –analog “continuous” media: encoding, decoding & playback –service requirements Classifying multimedia applications –Streaming (stored) multimedia –Live multimedia broadcasting –Interactive multimedia applications Making the best of best effort service: streaming Stored Multimedia over “Best-Effort “Internet –client buffering, rate adaption, etc. Large-scale video delivery over the Internet: YouTube and Netflix case studies (optional) Required Readings: csci4221 Textbook, sections 7.1-7.2

2. 2 Multimedia and Quality of Service Multimedia applications: network audio and video ( “ continuous media ” ) network provides application with level of performance needed for application to function. QoS CSci4211: Multimedia Networking

3. 3 Digital Audio Sampling the analog signal –Sample at some fixed rate –Each sample is an arbitrary real number Quantizing each sample –Round each sample to one of a finite number of values –Represent each sample in a fixed number of bits 4 bit representation (values 0-15) CSci4211: Multimedia Networking

4. 4 Audio Examples Speech –Sampling rate: 8000 samples/second –Sample size: 8 bits per sample –Rate: 64 kbps Compact Disc (CD) –Sampling rate: 44,100 samples/second –Sample size: 16 bits per sample –Rate: 705.6 kbps for mono, 1.411 Mbps for stereo CSci4211: Multimedia Networking

5. 5 Why Audio Compression Audio data requires too much bandwidth –Speech: 64 kbps is too high for a dial-up modem user –Stereo music: 1.411 Mbps exceeds most access rates Compression to reduce the size –Remove redundancy –Remove details that human tend not to perceive Example audio formats –Speech: GSM (13 kbps), G.729 (8 kbps), and G.723.3 (6.4 and 5.3 kbps) –Stereo music: MPEG 1 layer 3 (MP3) at 96 kbps, 128 kbps, and 160 kbps CSci4211: Multimedia Networking

6. 6 A few words about audio compression Analog signal sampled at constant rate –telephone: 8,000 samples/sec –CD music: 44,100 samples/sec Each sample quantized, i.e., rounded –e.g., 2 8 =256 possible quantized values Each quantized value represented by bits –8 bits for 256 values Example: 8,000 samples/sec, 256 quantized values --> 64,000 bps Receiver converts it back to analog signal: –some quality reduction Example rates CD: 1.411 Mbps MP3: 96, 128, 160 kbps Internet telephony: 5.3 - 13 kbps

7. 7 Digital Video Sampling the analog signal –Sample at some fixed rate (e.g., 24 or 30 times per sec) –Each sample is an image Quantizing each sample –Representing an image as an array of picture elements –Each pixel is a mixture of colors (red, green, and blue) –E.g., 24 bits, with 8 bits per color

8. CSci5221: Multimedia8 The 320 x 240 hand The 2272 x 1704 hand

9. 9 A few words about video compression Video is sequence of images displayed at constant rate –e.g. 24 images/sec Digital image is array of pixels Each pixel represented by bits Redundancy –spatial –temporal Examples: MPEG 1 (CD-ROM) 1.5 Mbps MPEG2 (DVD) 3-6 Mbps MPEG4 (often used in Internet, < 1 Mbps) Research: Layered (scalable) video –adapt layers to available bandwidth CSci4211: Multimedia Networking

10. 10 Video Compression: Within an Image Image compression –Exploit spatial redundancy (e.g., regions of same color) –Exploit aspects humans tend not to notice Common image compression formats –Joint Pictures Expert Group (JPEG) –Graphical Interchange Format (GIF) Uncompressed: 167 KBGood quality: 46 KBPoor quality: 9 KB CSci4211: Multimedia Networking

11. 11 Video Compression: Across Images Compression across images –Exploit temporal redundancy across images Common video compression formats –MPEG 1: CD-ROM quality video (1.5 Mbps) –MPEG 2: high-quality DVD video (3-6 Mbps) –Proprietary protocols like QuickTime and RealNetworks CSci4211: Multimedia Networking

12. 12 MM Networking Applications Fundamental characteristics: Typically delay sensitive –end-to-end delay –delay jitter But loss tolerant: infrequent losses cause minor glitches Antithesis of data, which are loss intolerant but delay tolerant. Classes of MM applications: 1) Streaming stored audio and video 2) Streaming live audio and video 3) Real-time interactive audio and video Jitter is the variability of packet delays within the same packet stream CSci4211: Multimedia Networking

13. 13 Application Classes Streaming –Clients request audio/video files from servers and pipeline reception over the network and display –Interactive: user can control operation (similar to VCR: pause, resume, fast forward, rewind, etc.) –Delay: from client request until display start can be 1 to 10 seconds CSci4211: Multimedia Networking

14. 14 Application Classes (more) Unidirectional Real-Time: –E.g., real-time video broadcasting of a sport event –similar to existing TV and radio stations, but delivery on the network –Non-interactive, just listen/view Interactive Real-Time : –Phone conversation or video conference –E.g., skype, Google handout, VoIP & SIP, … –More stringent delay requirement than Streaming and Unidirectional because of real-time nature –Video: < 150 msec acceptable –Audio: < 150 msec good, <400 msec acceptable CSci4211: Multimedia Networking

15. 15 Multimedia Over Today ’ s Internet TCP/UDP/IP: “ best-effort service ” no guarantees on delay, loss Today ’ s Internet multimedia applications use application-level techniques to mitigate (as best possible) effects of delay, loss But you said multimedia apps requires QoS and level of performance to be effective! ? ? ?? ? ? ? ? ? ? ? CSci4211: Multimedia Networking

16. 16 Challenges TCP/UDP/IP suite provides best-effort, no guarantees on expectation or variance of packet delay Streaming applications delay of 5 to 10 seconds is typical and has been acceptable, but performance deteriorate if links are congested (transoceanic) Real-Time Interactive requirements on delay and its jitter have been satisfied by over-provisioning (providing plenty of bandwidth), what will happen when the load increases?... CSci4211: Multimedia Networking

17. 17 Internet (Stored) Multimedia: Simplest Approach audio, video not streamed: no, “ pipelining, ” long delays until playout! audio or video stored in file files transferred as HTTP object –received in entirety at client –then passed to player CSci4211: Multimedia Networking

18. 18 Streaming Stored Multimedia Streaming: media stored at source transmitted to client streaming: client playout begins before all data has arrived timing constraint for still-to-be transmitted data: in time for playout CSci4211: Multimedia Networking

19. 19 Streaming Stored Multimedia: What is it? 1.Video pre-recorded 2. video sent 3. video received, played out at client Cumulative data streaming: at this time, client playing out early part of video, while server still sending later part of video network delay time CSci4211: Multimedia Networking

20. 20 Internet multimedia: Streaming Approach browser GETs metafile browser launches player, passing metafile player contacts server server streams audio/video to player CSci4211: Multimedia Networking

21. 21 Streaming from a streaming server This architecture allows for non-HTTP protocol between server and media player Can also use UDP instead of TCP. CSci4211: Multimedia Networking

22. 22 Streaming Stored Multimedia Application-level streaming techniques for making the best out of best effort service: – client side buffering – use of UDP versus TCP – multiple encodings of multimedia jitter removal decompression error concealment graphical user interface w/ controls for interactivity Media Player

23. 23 Streaming Multimedia: UDP or TCP? UDP server sends at rate appropriate for client (oblivious to network congestion !) –often send rate = encoding rate = constant rate –then, fill rate = constant rate - packet loss short playout delay (2-5 seconds) to compensate for network delay jitter error recover: time permitting TCP send at maximum possible rate under TCP fill rate fluctuates due to TCP congestion control larger playout delay: smooth TCP delivery rate HTTP/TCP passes more easily through firewalls

24. 24 video sent at Certain bit rates Cumulative data time variable network delay client video reception constant bit rate video playout at client client playout delay buffered video Streaming Multimedia: Client Buffering Client-side buffering, playout delay compensate for network-added delay, delay jitter

25. 25 Streaming Multimedia: Client Buffering Client-side buffering, playout delay compensate for network-added delay, delay jitter buffered video variable fill rate, x(t) “ constant ” drain rate, d CSci4211: Multimedia Networking

26. 26 Streaming Stored Multimedia: Interactivity VCR-like functionality: client can pause, rewind, FF, push slider bar –10 sec initial delay OK –1-2 sec until command effect OK timing constraint for still-to-be transmitted data: in time for playout CSci4211: Multimedia Networking

27. 27 Streaming Live Multimedia Examples: Internet radio talk show Live sporting event Streaming playback buffer playback can lag tens of seconds after transmission still have timing constraint Interactivity fast forward impossible rewind, pause possible! CSci4211: Multimedia Networking

28. 28 Interactive, Real-Time Multimedia end-end delay requirements: –audio: < 150 msec good, < 400 msec OK includes application-level (packetization) and network delays higher delays noticeable, impair interactivity session initialization –how does callee advertise its IP address, port number, encoding algorithms? applications: IP telephony, video conference, distributed interactive worlds CSci4211: Multimedia Networking

29. Large-scale Internet Video Delivery: YouTube & Netflix Case Studies Based on two active measurement studies we have conducted Reverse-engineering YouTube Delivery Cloud –Google’s New YouTube Architectural Design Unreeling Netflix Video Streaming Service –Cloud-sourcing: Amazon Cloud Services & CDNs CSci4211: Multimedia Networking29

30. YouTube Video Delivery Basics User Front end web-servers Video-servers (front end) 1. HTTP GET request for video URL 2. HTTP reply containing html to construct the web page and a link to stream the FLV file 3. HTTP GET request for FLV stream 4. HTTP reply FLV stream Internet CSci4211: Multimedia Networking30

31. www.youtube.com CSci4211: Multimedia Networking31

32. Embedded Flash Video CSci4211: Multimedia Networking32

33. Google’s New YouTube Video Delivery Architecture Three components Videos and video id space Physical cache hierarchy  three tiers: primary, secondary, & tertiary  primary caches: “Google locations” vs. “ISP locations” Layered organization of namespaces  representing “logical” video servers  five “anycast” namespaces  two “unicast” namespaces Implications: a)YouTube videos are not replicated at all locations! b)only replicated at (5) tertiary cache locations c) Google likely utilizes some form of location-aware load-balancing (among primary cache locations) CSci4211: Multimedia Networking33

34. YouTube Video Id Space Each YouTube video is assigned a unique id e.g., http://www.youtube.com/watch?v=tObjCw_WgKs Each video id is 11 char string first 10 chars can be any alpha-numeric values [0-9, a-z, A-Z] plus “-” and “_” last char can be one of the 16 chars {0, 4, 8,..., A, E,...} Video id space size: 64 11 Video id’s are randomly distributed in the id space CSci4211: Multimedia Networking34

35. Physical Cache Hierarchy & Locations ~ 50 cache locations ~40 primary locations including ~10 non- Google ISP locations 8 secondary locations 5 tertiary locations P: primary S: secondary T: Tertiary Geo-locations using city codes in unicast hostnames, e.g., r1.sjc01g01.c.youtube.com low latency from PLnodes (< 3ms) clustering of IP addresses using latency matrix CSci4211: Multimedia Networking35

36. Layered Namespace Organization Two types of namespaces –Five “anycast” namespaces lscache: “visible” primary ns each ns representing fixed # of “logical” servers logical servers mapped to physical servers via DNS – 2 “unicast” namespaces rhost: google locations rhostisp: ISP locations mapped to a single server CSci4211: Multimedia Networking36

37. YouTube Video Delivery Dynamics: Summary Locality-aware DNS resolution Handling load balancing & hotspots –DNS change –Dynamic HTTP redirection –local vs. higher cache tier Handling cache misses –Background fetch –Dynamic HTTP redirection CSci4211: Multimedia Networking37

38. What Makes Netflix Interesting? Commercial, feature-length movies and TV shows  and not free; subscription-based Nonetheless, Netflix is huge!  25 million subscribers and ~20,000 titles (and growing)  consumes 30% of peak-time downstream bandwidth in North America A prime example of cloud-sourced architecture  Maintains only a small “in-house” facility for key functions  e.g., subscriber management (account creation, payment, …)  Majority of functions are sourced to Amazon cloud (EC2/S3)  user authentication, video search, video storage, …  DNS service is sourced to UltraDNS  Leverage multiple CDNs (content-distribution networks) for video delivery  Akamai, Level 3 and Limelight CSci4211: Multimedia Networking

39. Netflix Architecture Netflix has its own “data center” for certain crucial operations (e.g., user registration, billing, …) Most web-based user-video interaction, computation/storage operations are cloud-sourced to Amazon AWS Video delivery is out/cloud-sourced to 3 CDNs Users need to use MS Silverlight player for video streaming CSci4211: Multimedia Networking

40. Netflix Videos and Video Chunks Netflix uses a numeric ID to identify each movie –IDs are variable length (6-8 digits): 213530, 1001192, 70221086 –video IDs do not seem to be evenly distributed in the ID space –these video IDs are not used in playback operations Each movie is encoded in multiple quality levels, each is identified by a numeric ID (9 digits) –various numeric IDs associated with the same movie appear to have no obvious relations CSci4211: Multimedia Networking40

41. Netflix Videos and Video Chunks Videos are divided in “chunks” (of roughly 4 secs), specified using (byte) “range/xxx-xxx?” in the URL path: Limelight: http://netflix-094.vo.llnwd.net/s/stor3/384/534975384.ismv/range/0- 57689?p=58&e=1311456547&h=2caca6fb4cc2c522e657006cf69d4ace Akamai: http://netflix094.as.nflximg.com.edgesuite.net/sa53/384/534975384.ismv/range/0 -57689?token=1311456547_411862e41a33dc93ee71e2e3b3fd8534 Level3: http://nflx.i.ad483241.x.lcdn.nflximg.com/384/534975384.ismv/range/0- 57689?etime=20110723212907&movieHash=094&encoded=06847414df0656e6 97cbd Netflix uses a version of (MPEG-)DASH for video streaming CSci4211: Multimedia Networking41

42. CSci4211: Multimedia Networking DASH: dynamic adaptive streaming over HTTP Not really a protocol; it provides formats to enable efficient and high-quality delivery of streaming services over the Internet –Enable HTTP-CDNs; reuse of existing technology (codec, DRM,…) –Move “intelligence” to client: device capability, bandwidth adaptation, … In particular, it specifies Media Presentation Description (MPD) Ack & ©: Thomas Stockhammer 42

43. CSci4211: Multimedia Networking DASH Data Model and Manifest Files DASH MPD: Segment Info Initialization Segment http://www.e.com/ahs-5.3gp Initialization Segment http://www.e.com/ahs-5.3gp Media Presentation Period, start=0s … Period, start=100s … Period, start=295s … … Period, start=100 baseURL=http://www.e.com/ Representation 1 500kbit/s … Representation 2 100kbit/s … Representation 1 bandwidth=500kbit/s width 640, height 480 Segment Info duration=10s Template:./ahs-5-$Index$.3gs … Media Segment 1 start=0s http://www.e.com/ahs-5-1.3gs Media Segment 1 start=0s http://www.e.com/ahs-5-1.3gs Media Segment 2 start=10s http://www.e.com/ahs-5-2.3gs Media Segment 2 start=10s http://www.e.com/ahs-5-2.3gs Media Segment 3 start=20s http://www.e.com/ahs-5-3.3gh Media Segment 3 start=20s http://www.e.com/ahs-5-3.3gh Media Segment 20 start=190s http://www.e.com/ahs-5-20.3gs Media Segment 20 start=190s http://www.e.com/ahs-5-20.3gs Segment Indexing: MPD only; MPD+segment; segment only Segment Index in MPD only...... seg1.mp4 seg2.mp4......... seg.mp4 Ack & ©: Thomas Stockhammer 43

44. Netflix Manifest Files A manifest file contains metadata Netflix manifest files contain a lot of information o Available bitrates for audio, video and trickplay o MPD and URLs pointing to CDNs o CDNs and their "rankings" level3 6 1 140 limelight 4 2 120 akamai 9 3 100 CSci4211: Multimedia Networking44

45. Netflix Manifest Files … A section of the manifest containing the base URLs, pointing to CDNs 1311456547 9 http://netflix094.as.nflximg.com.edgesuite.net/sa73/531/943233531.ismv?token=131145 6547_e329d4271a7ff72019a550dec8ce3840 1311456547 4 http://netflix- 094.vo.llnwd.net/s/stor3/531/943233531.ismv?p=58&e=1311456547&h=8adaa52cd06db9 219790bbdb323fc6b8 1311456547 6 http://nflx.i.ad483241.x.lcdn.nflximg.com/531/943233531.ismv?etime=20110723212907 &movieHash=094&encoded=0473c433ff6dc2f7f2f4a CSci4211: Multimedia Networking45

46. Netflix: Adapting to Bandwidth Changes Two possible approaches  Increase/decrease quality level using DASH  Switch CDNs Experiments  Play a movie and systematically throttle available bandwidth  Observe server addresses and video quality Bandwidth throttling using the “dummynet” tool  Throttling done on the client side by limiting how fast it can download from any given CDN server  First throttle the most preferred CDN server, keep throttling other servers as they get selected 46CSci4211: Multimedia Networking

47. Adapting to Bandwidth Changes Lower quality levels in response to lower bandwidth Switch CDN only when minimum quality level cannot be supported Netflix seems to use multiple CDNs only for failover purposes! 47CSci4211: Multimedia Networking

48. CDN Bandwidth Measurement Use both local residential hosts and PlanetLab nodes  13 residential hosts and 100s PlanetLab nodes are used  Each host downloads small chunks of Netflix videos from all three CDN servers by replaying URLs from the manifest files Experiments are done for several hours every day for about 3 weeks  total experiment duration was divided into 16 second intervals  the clients downloaded chunks from CDN 1, 2 and 3 at the beginning of seconds 0, 4 and 8.  at the beginning of the 12th second, the clients tried to download the chunks from all three CDNs simultaneously Measure bandwidth to 3 CDNs separately as well as the combined bandwidth Perform analysis at three time-scales  average over the entire period  daily averages  instantaneous bandwidth 48CSci4211: Multimedia Networking

49. There is no Single Best CDN CSci4211: Multimedia Networking49