Chapter 7 Multimedia Networking

Transcription

1 Chapter 7 Multimedia Networking Principles Classify multimedia applications Identify the network services and the requirements the apps need Making the best of best effort service Mechanisms for providing QoS Protocols and Architectures Specific protocols for best-effort Architectures for QoS Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July Sistemi e Servizi Multimediali Multimedia Networking - 1 Current Scenario Explosive growth of audio and video applications IP telephony, radio on Internet, video streaming on-demand, video conference, interactive games, virtual worlds, e-learning Different requirements than traditional applications ( , Web, remote login, file sharing) delay sensitive loss tolerant Antithesis of data, which are loss intolerant but delay tolerant Need for protocols and architectures for QoS Sistemi e Servizi Multimediali Multimedia Networking - 2

2 Multimedia, Quality of Service: What is it? Multimedia applications: network audio and video ( continuous media ) QoS network provides application with level of performance needed for application to function. Sistemi e Servizi Multimediali Multimedia Networking - 3 MM Networking Applications Classes of MM applications: 1) Streaming stored audio and video 2) Streaming live audio and video 3) Real-time interactive audio and video NOT download-and-play apps Fundamental characteristics: Typically delay sensitive end-to-end delay delay jitter But loss tolerant: infrequent losses cause minor glitches Jitter is the variability of packet delays within the same packet stream Sistemi e Servizi Multimediali Multimedia Networking - 4

3 Cumulative data 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 timing constraints not as strong as in the case of real-time interactive apps Sistemi e Servizi Multimediali Multimedia Networking - 5 Streaming Stored Multimedia: What is it? 1. video recorded 2. video sent network delay 3. video received, played out at client time streaming: at this time, client playing out early part of video, while server still sending later part of video Sistemi e Servizi Multimediali Multimedia Networking - 6

4 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 RTSP often used (more later) Commercial products RealPlayer of RealNetworks QuickTime of Apple Windows Media of Microsoft Sistemi e Servizi Multimediali Multimedia Networking - 7 Streaming Live Multimedia Similar to traditional radio and TV, but on the Internet Examples: Internet radio talk show Live sporting event Interactivity fast forward impossible rewind, pause possible! Streaming playback buffer playback can lag tens of seconds after transmission still have timing constraint (10 s.) Sistemi e Servizi Multimediali Multimedia Networking - 8

5 Interactive, Real-Time Multimedia allow people to communicate in real time applications: IP telephony, video conference, distributed interactive worlds end-end delay requirements: audio: < 150 msec good, < 400 msec OK includes application-level (packetization) and network delays higher delays noticeable, impair interactivity Sistemi e Servizi Multimediali Multimedia Networking - 9 Interactive Real-Time Audio Telephony over Internet: for the end-user perspective similar to traditional telephony Born before 1990 (before Web!) PC-telephone applications popular at the end of 90 NetPhone and Dialpad provide low cost PC-telephone services Telephone-Internet-Telephone apps allow to call with traditional phones at very low cost American phone cards Skype, Microsoft Instant Messenger provide PC-PC and PC-telephone Sistemi e Servizi Multimediali Multimedia Networking - 10

6 Multimedia Over Today s Internet TCP/UDP/IP: best-effort service no guarantees on delay, loss?????? But you said multimedia apps requires QoS and level of performance to be?? effective!??? Today s Internet multimedia applications use application-level techniques to mitigate (as best possible) effects of delay, loss Sistemi e Servizi Multimediali Multimedia Networking - 11 How should the Internet evolve to better support multimedia? Integrated services philosophy: Fundamental changes in Internet so that apps can reserve end-to-end bandwidth Requires new, complex software in hosts & routers Laissez-faire no major changes more bandwidth when needed content distribution networks application-layer multicast (!=multicast IP) Differentiated services philosophy: Fewer changes to Internet infrastructure, yet provide 1st and 2nd class service. Protocol to reserve bandwidth Registration policy at routers Description of future traffic for apps and monitoring Bandwidth provisioning Sistemi e Servizi Multimediali Multimedia Networking - 12

7 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 PCM Pulse Code Modulation Example: 8,000 samples/sec, 256 quantized values --> 64,000 bps Receiver converts it back to analog signal: some quality reduction Example rates CD: samples/sec, 16 bits Mbps (stereo) Compression techniques MP3 (stereo): 96, 128, 160 kbps Internet telephony: kbps (GSM) Sistemi e Servizi Multimediali Multimedia Networking - 13 A few words about video compression Video is sequence of images displayed at constant rate e.g. 24 images/sec Digital image (not compressed) is an 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 Sistemi e Servizi Multimediali Multimedia Networking - 14

8 Streaming Stored Multimedia Major consumer available bandwith Increasing trend for many reasons Low costs for storage (terabyte) Improvement of Internet infrastructure (connection, delivery techniques, QoS protocols) Potential growth of requests for high quality video streaming on demand: TV and Web on demand Sistemi e Servizi Multimediali Multimedia Networking - 15 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 Current Web browser rely on helper applications: Media Player (e. g., RealPlayer, Media Player) Media Player decompression jitter removal error concealment (loss) graphical user interface w/ controls for interactivity Sistemi e Servizi Multimediali Multimedia Networking - 16

9 Internet multimedia: simplest approach HTTP (TCP) audio or video stored in file video often divided in two files two HTTP requests files transferred as HTTP object received in entirety at client then passed to player (header HTTP) audio, video not streamed: no, pipelining, long delays until playout! Sistemi e Servizi Multimediali Multimedia Networking - 17 Internet multimedia: streaming approach But TCP and HTTP browser GETs metafile with info (URL, ) browser launches player, passing metafile player contacts server server streams audio/video to player Sistemi e Servizi Multimediali Multimedia Networking - 18

10 Streaming from a streaming server This architecture allows for non-http protocol between server and media player Can also use UDP instead of TCP. Different options for communication between streaming server and media player (equal rate or client buffer) Sistemi e Servizi Multimediali Multimedia Networking - 19 Streaming Multimedia: Client Buffering Cumulative data constant bit rate video transmission variable network delay client video reception buffered video constant bit rate video playout at client client playout delay time Client-side buffering, playout delay compensate for network-added delay, delay jitter Sistemi e Servizi Multimediali Multimedia Networking - 20

11 Streaming Multimedia: Client Buffering variable fill rate, x(t) constant drain rate, d buffered video Client-side buffering, playout delay compensate for network-added delay, delay jitter Sistemi e Servizi Multimediali Multimedia Networking - 21 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 superior quality of audio/video HTTP/TCP passes more easily through firewalls Sistemi e Servizi Multimediali Multimedia Networking - 22

12 Streaming Multimedia: client rate(s) 1.5 Mbps encoding 28.8 Kbps encoding Q: how to handle different client receive rate capabilities? 28.8 Kbps dialup 100Mbps Ethernet A: server stores, transmits multiple copies of video, encoded at different rates Sistemi e Servizi Multimediali Multimedia Networking - 23 User Control of Streaming Media: RTSP (Real-Time Streaming Protocol) HTTP Does not target multimedia content No commands for fast forward, etc. RTSP: RFC 2326 Client-server application layer protocol. For user to control display: rewind, fast forward, pause, resume, repositioning, etc What it doesn t do: does not define how audio/video is compressed does not define how audio/video is encapsulated for streaming over network does not restrict how streamed media is transported; it can be transported over UDP or TCP does not specify how the media player buffers audio/video Sistemi e Servizi Multimediali Multimedia Networking - 24

13 RTSP: out of band protocol FTP uses an out-of-band control channel: A file is transferred over one TCP connection. Control information (directory changes, file deletion, file renaming, etc.) is sent over a separate TCP connection. The out-of-band and inband channels use different port numbers. RTSP messages are also sent out-of-band: RTSP control messages use different port numbers than the media stream: out-of-band. Port 554 The media stream is considered in-band. Sistemi e Servizi Multimediali Multimedia Networking - 25 RTSP Example Scenario: metafile communicated to web browser browser launches player player sets up an RTSP control connection, data connection to streaming server Sistemi e Servizi Multimediali Multimedia Networking - 26

14 RTSP Operation Sistemi e Servizi Multimediali Multimedia Networking - 27 Metafile Example <title>twister</title> <session> <group language=en lipsync> <switch> <track type=audio e="pcmu/8000/1" Synchronization Choice between two audio streams src = "rtsp://audio.example.com/twister/audio.en/lofi"> <track type=audio e="dvi4/16000/2" pt="90 DVI4/8000/1" src="rtsp://audio.example.com/twister/audio.en/hifi"> </switch> <track type="video/jpeg" src="rtsp://video.example.com/twister/video"> </group> </session> Sistemi e Servizi Multimediali Multimedia Networking - 28

15 RTSP Exchange Example C: SETUP rtsp://audio.example.com/twister/audio RTSP/1.0 Cseq: 1 Transport: rtp/udp; compression; port=3056; mode=play S: RTSP/ OK Cseq: 1 Session: 4231 C: PLAY rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Cseq: 2 Range: npt=0- C: PAUSE rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Cseq: 3 Range: npt=37 C: TEARDOWN rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231; Cseq: 4 S: RTSP/ OK Session: 4231; Cseq: 4 Sistemi e Servizi Multimediali Multimedia Networking - 29