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RTP

Audio and Video for the Internet

Colin Perkins

rVAddison-Wesley

T T

Boston • San Francisco • New York • Toronto • Montreal London • Munich • Paris • Madrid

Capetown • Sydney • 'lokyo • Singapore • Mexico City

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CONTENTS

PREFACE xi ACKNOWLEDGMENTS xvii

Part I Introduction to Networked Multimedia

Chapter 1 AN INTRODUCTION TO RTP 3

A Brief Histoiy of Audio/Video Networking 4 Early Packet Voice and Video Experiments 4

Audio and Video on the Internet 5

ITU Standards 7 Audio/Video Streaming 9

A Snapshot of RTP 9 Related Standards 10 Overview of an RTP Implementation 11

Behavior of an RTP Sender 11 Behavior of an RTP Receiver 13

Summary 14

Chapter 2 VOICE AND VIDEO COMMUNICATION OVER

PACKET NETWORKS 15

TCP/IP and the OSI Reference Model 16 Performance Characteristics of an IP Network 23

Measuring IP Network Performance 25

iii

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iv CONTENTS

Average Packet Loss 26 Packet Loss Patterns 29 Packet Duplication 30 Packet Corruption 31 Network Transit Time 32 Acceptable Packet Sizes 37 Effects of Multicast 37 Effects of Network Technologies 39

Conclusions about Measured Characteristics . . . 40

Effects of Transport Protocols 42

UDP/IP 42 TCP/IP 43 Requirements for Audio/Video Transport

in Packet Networks 45 Benefits of Packet-Based Audio/Video 47

Summary 48

Part II Media Transport Using RTP

Chapter 3 THE REAL-TIME TRANSPORT PROTOCOL 51

Fundamental Design Philosophies of RTP 51

Application-Level Framing 52 The End-to-End Principle 53 Achieving Flexibility 54 Standard Elements of RTP 55 The RTP Specification 55

RTP Profiles 57 RTP Payload Formats 59

Optional Elements 60 Related Standards 61

Call Setup and Control 62 Session Description 63 Quality of Service 63

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CONTENTS V

Future Standards Development 65

Summary 66

Chapter 4 RTP DATA TRANSFER PROTOCOL 67

RTP Sessions 67 The RTP Data Transfer Packet 70

Header Elements 71 Header Extensions 85 Payload Headers 86 Payload Data 88 Packet Validation 89 Translators and Mixers 91

Translators 91 Mixers 92 Summary 94

Chapter 5 RTP CONTROL PROTOCOL 95

Components of RTCP 96 Transport of RTCP Packets 97 RTCP Packet Formats 97

RTCP RR: Receiver Reports 101 RTCP SR: Sender Reports 107 RTCP SDES: Source Description 110 RTCP BYE: Membership Control 116 RTCP APP: Application-Defined RTCP Packets . . 118

Packing Issues 118 Security and Privacy 120 Packet Validation 122 Participant Database 124

Timing Rules 128 Reporting Interval 129

Basic Transmission Rules 132 Forward Reconsideration 134

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v i CONTENTS

Reverse Reconsideration 138 BYE Reconsideration 139 Comments on Reconsideration 141

Common Implementation Problems 141

Summary 142

Chapter 6 MEDIA CAPTURE, PLAYOUT, AND TIMING 145

Behavior of a Sender 146 Media Capture and Compression 147

Audio Capture and Compression 147 Video Capture and Compression 150 Use of Prerecorded Content 152

Generating RTP Packets 152 Timestamps and the RTP Timing Model 154

Fragmentation 155 Payload Format-Specific Headers 157

Behavior of a Receiver 157 Packet Reception 159

Receiving Data Packets l60 Receiving Control Packets 163

The Playout Buffer 166 Basic Operation 167 Playout Time Calculation 170

Adapting the Playout Point 192 Playout Adaptation for Audio with Silence

Suppression 193 Playout Adaptation for Audio without Silence

Suppression 196 Playout Adaptation for Video 198

Decoding, Mixing, and Playout 200

Decoding 200 Audio Mixing 201 Audio Playout 204 Video Playout 206

Summary 207

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C O N T E N T S VÜ

Chapter 7 LIP SYNCHRONIZATION 209

Sender Behavior 211 Receiver Behavior 216 Synchronization Accuracy 222

Summary 223

Part III Robustness

Chapter 8 ERROR CONCEALMENT 227

Techniques for Audio Loss Concealment 227

Measuring Audio Quality 228 Silence Substitution 229 Noise Substitution 231

Repetition 234 Other Techniques for Repairing Speech Signals. . 237

Techniques for Video Loss Concealment 242 Motion-Compensated Repetition 243 Other Techniques for Repairing Video

Packet Loss 245 Dependency Reduction 245

Interleaving 247 Summary 250

Chapter 9 ERROR CORRECTION 251

Forward Error Correction 252

Parity FEC 254 Unequal Error Protection 263

Reed-Solomon Codes 266 Audio Redundancy Coding 266

Channel Coding 273 Partial Checksum 273

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viii C O N T E N T S

Reference Picture Selection 275

Retransmission 276 RTCP as a Framework for Retransmission . . . . 277

Applicability 281 Implementation Considerations 283

At a Receiver 283 At the Sender 285

Summary 285

Chapter 10 CONGESTION CONTROL 287

The Need for Congestion Control 287 Congestion Control on the Internet 290

Implications for Multimedia 296 Congestion Control for Multimedia 298

TCP-Like Rate Control 298 TCP-Friendly Rate Control 300

Layered Coding 306

Summary 310

Part IV Advanced Topics

Chapter 11 HEADER COMPRESSION 315

Introductory Concepts 316 Patterns, Robustness, and Local Implementation . 317

Standards 318 Compressed RTP 319

Operation of CRTP: Initialization and Context . . . 321 Operation of CRTP: Compression

and Decompression 323 Effects of Packet Loss 326 Robust Header Compression 327

Operation of ROHC: States and Modes 329

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CONTENTS ix

Operation of ROHC: Robustness and

Compression Efficiency 331 Considerations for RTP Applications 334

Summary 336

Chapter 12 MULTIPLEXING AND TUNNELING 339

The Motivation for Multiplexing 339 Tunneling Multiplexed Compressed RTP 342

Basic Concepts of TCRTP 343 Implementing TCRTP 346

Performance 347 Other Approaches to Multiplexing 349

GeRM 349 Application-Specific Multiplexing 354

Summary 357

Chapter 13 SECURITY CONSIDERATIONS 359

Privacy 36O Confidentiality 362

Confidentiality Features in the RTP Specification . 36З Confidentiality Using the Secure RTP Profile . . Зб7

Confidentiality Using IP Security 372

Other Considerations 375

Authentication 376 Authentication Using Standard RTP 377

Authentication Using the Secure RTP Profile . . 378

Authentication Using IP Security 378

Replay Protection 380 Denial of Service 381 Mixers and Translators 381

Active Content 382 Other Considerations 382

Summary 383

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X CONTENTS

REFERENCES 385

IETF RFC Standards 386 IETF Internet-Drafts 389 Other Standards 390 Conference and Journal Papers 391

Books 395 WebSites 396 Other References 396

INDEX 397

References

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