List of Figures xix
List of Tables xxvii
Preface xxix
Author xxxi

1 Networked Multimedia Service

1.1 Introduction 1
1.2 Functional Characteristics 1
1.3 Performance Characteristics 1
1.4 Summary 3
References 3

2 Basic Session Initiation Protocol 5

2.1 Introduction 5
2.2 Terminology 5
2.3 Multimedia Session 5
2.4 Session Initiation Protocol 19
2.4.1 Augmented Backus–Naur Form for the SIP 20
2.4.2 SIP Messages 36
2.4.3 SIP Message Structure 38
2.4.4 SIP Network Functional Elements 39
2.5 SIP Request Messages 42
2.6 SIP Response Messages 42
2.7 SIP Call and Media Trapezoid Operation 51
2.8 SIP Header Fields 62
2.8.1 Overview 62
2.8.2 Header-Field Descriptions 72
2.9 SIP Tags 72
2.10 SIP Option Tags 72
2.11 SIP Media Feature Tags 154
2.11.1 Contact Header Field 154
2.11.2 Feature Tag Name, Description, and Usage 154
2.11.3 Conveying Feature Tags with REFER 155
2.12 Summary 164
References 165

3 SIP Message Elements 167

3.1 Introduction 167
3.1.1 SIP UA General Behavior 167
3.1.2 UAC General Behavior 168
3.1.3 UAS General Behavior 172
3.1.4 Redirect Server General Behavior 175
3.2 Canceling a Request 175
3.3 Registration 176
3.3.1 Registration without Managing Client-Initiated Connection 176
3.3.2 Discovering a SIP Registrar 179
3.3.3 Multiple-AOR Registration 179
3.3.4 Registration Call Flows 179
3.3.5 Registration for Multiple Phone Numbers in SIP 181
3.4 Indicating UA Capabilities 192
3.4.1 Contact Header Field 192
3.4.2 Capability Expression Using Media Feature Tag 193
3.4.3 Usage of the Content Negotiation Framework 193
3.4.4 Indicating Feature Sets in Remote Target URIs 196
3.4.5 OPTIONS Processing 196
3.5 Discovering UA and Proxy Capabilities 196
3.5.1 OPTIONS Request 197
3.5.2 Response to OPTIONS Request 197
3.6 Dialogs 197
3.6.1 Creation of a Dialog 198
3.6.2 Requests within a Dialog 199
3.6.3 Termination of a Dialog 201
3.6.4 Example of Dialog State 201
3.6.5 Multiple Dialogs 202
3.6.6 Early Dialog Termination Indication 202
3.7 Initiating a Session 206
3.7.1 Overview of Operation 206
3.7.2 UAC Processing 208
3.7.3 UAS Processing 210
3.8 Modifying an Existing Session 211
3.8.1 UAC Behavior 212
3.8.2 UAS Behavior 212
3.8.3 UPDATE 213
3.8.4 SDP Offer and Answer 215
3.8.5 Re-INVITE and Target Refresh Request Handling in SIP 224
3.9 Handling Message Body 233
3.9.1 Objective 233
3.9.2 Message-Body Encoding 233
3.9.3 Message Bodies: Multipart 234
3.9.4 Message Bodies: Multipart/Mixed 235
3.9.5 Message Bodies: Multipart/Alternative 235
3.9.6 Message Bodies: Multipart/Related 235
3.9.7 Disposition Types 236
3.9.8 Message-Body Processing 238
3.9.9 Future SIP Extensions 238
3.10 Terminating a Session 239
3.10.1 Terminating a Session with a BYE Request 239
3.11 Proxy Behavior 240
3.11.1 Overview 240
3.11.2 Stateful Proxy 240
3.11.3 Request Validation 241
3.11.4 Route Information Preprocessing 242
3.11.5 Determining Request Targets 242
3.11.6 Request Forwarding 243
3.11.7 Response Processing 247
3.11.8 Processing Timer C 250
3.11.9 Handling Transport Errors 250
3.11.10 CANCEL Processing 250
3.11.11 Stateless Proxy 250
3.11.12 Summary of Proxy Route Processing 251
3.12 Transactions 253
3.12.1 Client Transaction 256
3.12.2 Server Transaction 260
3.13 Transport 262
3.13.1 Clients 263
3.13.2 Servers 264
3.13.3 Framing 265
3.13.4 Error Handling 265
3.14 Summary 265
References 266

4 Addressing in SIP 269

4.1 Introduction 269
4.2 SIP Public Address 269
4.2.1 SIP and SIPS Uniform Resource Indicators 270
4.2.2 Telephone URI 274
4.2.3 Use of SIPS URI Scheme in SIP 276
4.3 Globally Routable UA URI 284
4.3.1 Overview 284
4.3.2 GRUU Grammar 284
4.3.3 Operation 284
4.3.4 Obtaining a GRUU 287
4.3.5 Using a GRUU 287
4.3.6 Dereferencing a GRUU 287
4.3.7 UA Behavior 287
4.3.8 Registrar Behavior 290
4.3.9 Proxy Behavior 292
4.3.10 GRUU Example 293
4.4 Services URI 295
4.4.1 Messaging Services 296
4.4.2 Media Services 298
4.5 Summary 302

5 SIP Event Framework and Packages 303

5.1 Introduction 303
5.2 Event Framework 303
5.2.1 Overview 303
5.2.2 Subscription, Notification, and Publication Event Model 304
5.3 Event Package 307
5.4 Summary 307

6 Presence and Instant Messaging in SIP 309

6.1 Introduction 309
6.2 SIP Presence 309
6.2.1 Overview 309
6.2.2 SIP Extensions for Presence 310
6.2.3 Presence Data Formats and Processing 311
6.2.4 Presence Operations 311
6.3 SIP Instant Messaging 312
6.3.1 Pager-Mode Single Recipient 312
6.3.2 Pager-Mode Multiple Recipients 313
6.3.3 Two-Party Session Mode 313
6.3.4 Multiparty Session Mode 315
6.4 Summary 316

7 Media Transport Protocol and Media Negotiation 317

7.1 Introduction 317
7.2 Real-Time Transmission and Control Protocol 318
7.2.1 Overview 318
7.2.2 RTP Specification 318
7.2.3 RTCP Specification 321
7.3 Secure RTP (SRTP 323
7.4 ZRTP 324
7.5 Real-Time Streaming Protocol (RTSP 324
7.6 Media Resource Control Protocol (MRCP 326
7.7 Session Description Protocol (SDP 327
7.7.1 Overview 327
7.7.2 SDP Specification 327
7.7.3 SDP Field Description 328
7.7.4 SDP Media 333
7.7.5 SDP Content-Agnostic Attributes 334
7.7.6 SDP Transport-Independent Bandwidth Modifier 339
7.7.7 SDP Format for BFCP Streams 345
7.7.8 SDP Content Attribute 347
7.8 Summary 349
References 350

8 DNS and ENUM in SIP 351

8.1 Introduction 351
8.2 Domain Name System 352
8.2.1 Namespace 352
8.2.2 Resource Records 353
8.2.3 Name Servers 353
8.2.4 Locating/Discovering SIP Entities 356
8.3 ENUM 361
8.3.1 DDDS Algorithm 366
8.3.2 DDDS Algorithm Application to ENUM 366
8.3.3 ENUM with Compound NAPTRs 370
8.3.4 ENUM Operations 370
8.3.5 ENUM Service Registration for SIP Addresses of Record (AORs 371
8.3.6 ENUM Services Registration in XML Chunk 372
8.3.7 Using E.164 Numbers with SIP 373
8.3.8 ENUM for SIP Services 377
8.3.9 ENUM Implementation Issues 379
8.4 DSN and ENUM Security 380
8.4.1 Cache Poisoning 380
8.4.2 Client Flooding 381
8.4.3 Dynamic Updates Vulnerability 381
8.4.4 Information Leakage 381
8.4.5 Compromising Authoritative Data 381
8.5 Summary 381

9 Routing in SIP 383

9.1 Introduction 383
9.2 SIP Registrar 383
9.3 SIP Proxy 385
9.4 Traversing a Strict-Routing Proxy 386
9.5 Rewriting Record-Route Header Field Values 387
9.5.1 Problems and Recommendation 387
9.6 Record-Routing with Globally Routable UA URI 387
9.7 Double Route-Record 388
9.8 Transport Parameter Usage Problems and Remedies 390
9.8.1 UA Implementation 390
9.8.2 Proxy Implementation 392
9.8.3 Symmetric Response Routing 392
9.9 Caller Preferences-Based Routing 394
9.9.1 Overview 394
9.9.2 Operation 395
9.9.3 UAC Behavior 395
9.9.4 UAS Behavior 396
9.9.5 Proxy Behavior 397
9.9.6 Mapping Feature Parameters to a Predicate 400
9.9.7 Header Field Definitions 401
9.9.8 Augmented BNF 402
9.10 Location-Based Routing 402
9.10.1 Overview 402
9.10.2 Basic SIP Location Conveyance Operations 403
9.10.3 Geolocation Examples 405
9.11 Loop Detection 407
9.11.1 Enhancements in Loop- Detection Algorithm 408
9.11.2 Max-Breadth Header Field 409
9.12 Summary 412

10 User and Netw]ork- Asserted Identity in SIP 415

10.1 Introduction 415
10.2 Multiple User Identities 415
10.3 Public User Identity 416
10.4 Private User Identity 416
10.4.1 P-Asserted-Identity 417
10.4.2 P-Preferred-Identity 417
10.4.3 Identity 417
10.4.4 Recommended Use of Asserted Identity with SIP Messages 418
10.5 Network-Asserted Identity 420
10.5.1 Overview 420
10.5.2 Trust Domain Identities, NAI, and Trust Domain Specification 421
10.5.3 Generation of NAI 422
10.5.4 Transport of NAI 422
10.5.5 Parties with NAIs 422
10.5.6 Types of NAI 422
10.6 Summary 423

11 Early Media in SIP 425

11.1 Introduction 425
11.2 Early Media and Session Establishment in SIP 425
11.3 Early-Media Solution Models 426
11.4 Early-Media Solution Model with Disposition-Type: Early-Session 426
11.4.1 Overview 426
11.4.2 Issues Related to Early-Media Session Establishment 427
11.4.3 Early-Session Disposition Type 427
11.4.4 Preconditions 428
11.4.5 Option Tag 428
11.4.6 Example 428
11.4.7 Early-Media Solution with Application Server and Gateway Model 428
11.5 Early-Media Solution Model with P-Early-Media Header 432
11.5.1 Early-Media Policy 432
11.5.2 Early-Media Application Environments 432
11.5.3 Early-Media Authorization 432
11.5.4 Applicability of Content-Disposition and Application/Gateway Model 433
11.5.5 Operation 434
11.5.6 Limitations of the P-Early-Media Header Field 434
11.5.7 P-Early-Media Header Field 435
11.6 Summary 436

12 Service and Served-User Identity in SIP 437

12.1 Introduction 437
12.2 Communications Service ID 438
12.2.1 Derived Service ID 438
12.2.2 SIP’s Expressiveness for Negotiation 438
12.2.3 Presence 438
12.2.4 Intradomain 438
12.2.5 Device Dispatch 439
12.3 Asserted- and Preferred-Service ID 439
12.3.1 Overview 439
12.3.2 Applicability Statement 440
12.3.3 Header Fields 440
12.3.4 Usage of Header Fields in Requests 442
12.3.5 Usage of Header Fields in Responses 442
12.3.6 Examples of Usage 442
12.4 Served-User ID for Handling Services 444
12.4.1 P-Served-User Header 444
12.4.2 Application Service Invocation 445
12.4.3 P-Served-User Header Field Usage, Definition, and Syntax 447
12.4.4 Proxy Behavior: Generating the P-Served-User Header 447
12.4.5 Proxy Behavior: Consuming the P-Served-User Header 447
12.4.6 Applicability and Limitations 448
12.5 Summary 448

13 Connections Management and Overload Control in SIP 449

13.1 Introduction 449
13.2 Connections Management in SIP Network 449
13.2.1 Overview 449
13.2.2 Flow-Based Connections Setup 450
13.2.3 Keep-Alive Mechanisms 450
13.2.4 Grammar 451
13.2.5 Connections Management Procedures for SIP Entities 451
13.2.6 Keep-Alive Mechanisms in SIP Network 460
13.2.7 Connection Management Example 463
13.2.8 Connection Reuse in SIP 467
13.3 Loss-Based Overload Control in SIP Network 472
13.3.1 Overview 472
13.3.2 Operations 472
13.3.3 Via Header Parameters for Overload Control 473
13.3.4 General Behavior 474
13.3.5 Loss-Based Overload Control Scheme 478
13.3.6 Relationship with Other SIP Load Control Schemes 480
13.3.7 Syntax 480
13.3.8 Design Considerations for Overload Control 480
13.3.9 Salient Features of Overload Control 482
13.4 Rate-Based Overload Control in SIP Network 483
13.4.1 Overview 483
13.4.2 Rate-Based Algorithm Scheme 483
13.4.3 Example 487
13.4.4 Syntax 487
13.5 Summary 487
References 488

14 Interworking Services in SIP 489

14.1 Introduction 489
14.2 SIP Session Border Controller 489
14.2.1 Objective 489
14.2.2 Background on SBCs 490
14.2.3 Functions of SBCs 492
14.2.4 Derived Requirements for Future SIP Standardization Work 499
14.3 NAT Crossing by SIP 499
14.3.1 Overview 499
14.3.2 NAT-Crossing Protocols 499
14.4 SIP–PSTN/ISDN Protocols Interworking 501
14.4.1 Overview 501
14.4.2 SIP-PSTN/ISDN Protocols Interworking Framework 501
14.5 Summary 503
Reference 503

15 Resource Priority and Quality of Service in SIP 505

15.1 Introduction 505
15.2 Communications Resource Priority in SIP 506
15.2.1 Overview 506
15.2.2 Resource-Priority SIP Header Field 507
15.2.3 Behavior of SIP Elements That Receive Prioritized Requests 508
15.2.4 UAC Behavior 510
15.2.5 UAS Behavior 511
15.2.6 Proxy Behavior 511
15.2.7 Third-Party Authentication 511
15.2.8 Backwards Compatibility 511
15.2.9 Examples 512
15.2.10 Handling Multiple Concurrent Namespaces 513
15.2.11 Registering Namespaces 514
15.2.12 Namespace Definitions 515
15.3 Preemption Events in SIP 516
15.3.1 Overview 516
15.3.2 Access Preemption Events 517
15.3.3 Network Preemption Events 518
15.3.4 Hybrid Infrastructure Preemption Events 520
15.3.5 Preemption Reason Header Cause Codes and Semantics 520
15.4 QOS in SIP 524
15.4.1 Overview 524
15.4.2 SDP Parameters 525
15.4.3 Usage of Preconditions with Offer–Answer 525
15.4.4 Suspending and Resuming Session Establishment 527
15.4.5 Status Confirmation 528
15.4.6 Refusing an Offer 528
15.4.7 Unknown Precondition Type 529
15.4.8 Multiple Preconditions per Media Stream 529
15.4.9 Option Tag for Preconditions 529
15.4.10 Indicating Capabilities 529
15.4.11 Examples 529
15.4.12 Enhancements in Precondition Procedures and Use in Session Mobility 532
15.4.13 SIP Performance Metrics 535
15.5 SDP Media Streams Mapping to QOS Flows 538
15.5.1 Overview 538
15.5.2 SRF Semantics 538
15.5.3 Applicability Statement 538
15.5.4 Examples 538
15.5.5 IANA Registration: SDP Attribute for Group 538
15.6 QOS Mechanism Selection in SDP 539
15.6.1 Overview 539
15.6.2 SDP Attribute Definitions 539
15.6.3 Offer–Answer Behavior 539
15.6.4 Example 540
15.6.5 IANA Registration: SDP Attribute and Token for QOS 540
15.7 SIP Signaling Compression 540
15.8 Summary 541
References 542

16 Call Services in SIP 543

16.1 Introduction 543
16.2 Call Transfer and Related Call Services 544
16.2.1 Overview 544
16.2.2 Actors and Roles 544
16.2.3 Requirements 544
16.2.4 Using REFER to Achieve Call Transfer 544
16.2.5 Basic Transfer 545
16.2.6 Transfer with Consultation Hold 549
16.2.7 Transfer with Referred-By 562
16.2.8 Transfer as an Ad Hoc Conference 564
16.2.9 Transfer with Multiple Parties 564
16.2.10 Gateway Transfer Issues 565
16.2.11 Call Services with Shared Appearances of a SIP AOR 567
16.2.12 Completion of Call Services in SIP 570
16.3 Call Diversion Indication 575
16.3.1 Overview 575
16.3.2 Diversion and History-Info Header Interworking in SIP 576
16.4 Call Services Using Session Border Controller 582
16.4.1 Overview 582
16.4.2 Distributed SBC Architecture 583
16.4.3 Conclusion 587
16.5 Referring Call to Multiple Resources 588
16.5.1 Overview 588
16.5.2 Operation 588
16.5.3 Multiple-Refer SIP Option Tag 588
16.5.4 Suppressing REFER’s Implicit Subscription 588
16.5.5 URI-List Format 589
16.5.6 Behavior of SIP REFER-Issuers 590
16.5.7 Behavior of REFER-Recipients 590
16.5.8 Example 590
16.6 Call Services with Content Indirection 591
16.6.1 Overview 591
16.6.2 Use-Case Examples 592
16.6.3 Requirements 593
16.6.4 Application of MIME-URI Standard to Content Indirection 593
16.6.5 Examples 596
16.7 Transcoding Call Services 596
16.7.1 Transcoding Services Framework 596
16.7.2 Third-Party Transcoding Services 597
16.7.3 Conference Bridging Transcoding Call Control Flows 604
16.8 INFO Method—Mid-Call Information Transfer 607
16.8.1 Overview 607
16.8.2 Motivation 607
16.8.3 UAs Are Allowed to Enable Both Legacy INFO Usages and Info 608
16.8.4 INFO Method 608
16.8.5 INFO Packages 609
16.8.6 Formal INFO Method Definition and Header Fields 611
16.8.7 INFO Package Considerations 611
16.8.8 Alternative Mechanisms 611
16.8.9 INFO Package Requirements 612
16.8.10 Examples 614
16.9 SIP Call Control UUI Transfer Services 616
16.9.1 Overview 616
16.9.2 Requirements for UUI Transport 616
16.9.3 Possible Approaches for UUI Transport in SIP 617
16.9.4 SIP Extensions for UUI Transport 619
16.9.5 Normative Definition 619
16.9.6 Guidelines for UUI Packages 621
16.9.7 Use Cases 622
16.10 Call Services Using DTMF 623
16.11 Emergency Call Services in SIP 624
16.11.1 Overview 624
16.11.2 Emergency Services Uniform Resource Name 625
16.11.3 Multilevel Precedence and Preemption 625
16.12 Summary 626
References 627

17 Media Server Interfaces in SIP 629

17.1 Introduction 629
17.2 SIP Interface to VoiceXML Media Server 630
17.2.1 Overview 630
17.2.2 Use Cases 630
17.2.3 VoiceXML Session Establishment and Termination 632
17.2.4 Media Support 637
17.2.5 Returning Data to the Application Server 639
17.2.6 Outbound Calling 640
17.2.7 Call Transfer 640
17.3 Summary 642
References 643

18 Multiparty Conferencing in SIP 645

18.1 Introduction 645
18.2 Multiparty Multimedia Conferencing 645
18.3 Third-Party Multiparty Conferencing 646
18.3.1 3PCC Call Establishment 646
18.3.2 Recommendations for 3PCC Call Setups 649
18.3.3 Multiparty Call Establishment Error Handling 649
18.3.4 Continued Call Processing in 3PCC 650
18.3.5 3PCC and Early Media 650
18.3.6 3PCC and SDP Preconditions 651
18.3.7 3PCC Service Examples 652
18.3.8 3PCC Implementation Recommendations 654
18.3.9 Concluding Remarks 654
18.4 Summary 654

19 Security Mechanisms in SIP 657

19.1 Introduction 657
19.2 Multilevel Security Characteristics in SIP 658
19.2.1 Overview 658
19.2.2 Session-Level Security 658
19.2.3 Media-Level Security 672
19.3 Security Mechanisms Negotiation 679
19.3.1 Security Mechanisms Negotiation 680
19.3.2 Backwards Compatibility 683
19.3.3 Security Algorithms Negotiation Example 683
19.3.4 Security Considerations 684
19.3.5 Syntax of IPsec–3GPP Security Headers 685
19.4 Authentication in SIP 686
19.4.1 Background 686
19.4.2 Framework 687
19.4.3 User-to-User Authentication 688
19.4.4 Proxy-to-User Authentication 689
19.4.5 Digest Authentication Scheme 690
19.4.6 Domain Certificates over TLS for Authentication in SIP 690
19.4.7 Authenticated Identity Body Format in SIP 697
19.4.8 Cryptographic Authentication Scheme 701
19.4.9 HTTP Digest Authentication Using AKA in SIP 716
19.4.10 Key-Derivation Authentication Scheme in SIP 721
19.4.11 DNS-Based Authentication for TLS Sessions in SIP 723
19.5 Authorization in SIP 725
19.5.1 Trait-Based Authorization in SIP 725
19.5.2 Authorization through Dialog Identification in SIP 732
19.5.3 Media Authorization in SIP 737
19.5.4 Early-Media Authorization in SIP 742
19.5.5 Framework for Session Setup with Media Authorization 742
19.6 Integrity and Confidentiality in SIP 743
19.6.1 S/MIME Certificates 743
19.6.2 S/MIME Key Exchange 743
19.6.3 Securing MIME Bodies 744
19.6.4 SIP Header Confidentiality and Integrity Using S/MIME: Tunneling SIP 745
19.7 Security for SIP URI-List Services 748
19.7.1 Objective 748
19.7.2 Requirements 748
19.7.3 Framework 748
19.7.4 Security Considerations 749
19.8 Consent-Based Communications for Enhancing Security in SIP 750
19.8.1 Objective 750
19.8.2 Definitions and Terminology 751
19.8.3 Relays and Translations 751
19.8.4 Architecture 752
19.8.5 Framework Operations 754
19.8.6 Security Considerations 761
19.9 SIP Forking Proxy Security 762
19.9.1 Overview 762
19.9.2 Vulnerability: Leveraging Forking to Flood a Network 762
19.9.3 Security Considerations 764
19.10 Nonrepudiation Services in SIP 764
19.11 Call Flows Explaining SIP Security Features 765
19.11.1 Registration 765
19.11.2 Session Setup 765
19.12 Threat Model and Security Usage Recommendations in SIP 767
19.12.1 Attacks and Threat Models 768
19.12.2 Security Mechanisms 769
19.12.3 Implementing Security Mechanisms 771
19.12.4 Limitations 774
19.13 Summary 775
References 777

20 Privacy and Anonymity in SIP 779

20.1 Introduction 779
20.2 Privacy Mechanism in SIP 780
20.2.1 Background 780
20.2.2 Varieties of Privacy 781
20.2.3 UA Behavior 782
20.2.4 UA Behavior Constructing Private Messages 782
20.2.5 UA Behavior Expressing Privacy Preferences 783
20.2.6 UA Behavior Routing Requests to Privacy Services 784
20.2.7 UA Behavior Routing Responses to Privacy Services 784
20.2.8 Privacy Service Behavior 785
20.2.9 Location Information Privacy 787
20.2.10 Security Considerations 787
20.3 Asserted and Preferred Identity for Privacy in SIP 788
20.3.1 Background 788
20.3.2 P-Asserted-Identity and P-Preferred-Identity for Privacy 788
20.3.3 Proxy Behavior 789
20.3.4 Hints for Multiple Identities 789
20.3.5 Requesting Privacy 789
20.3.6 UAS Behavior 790
20.3.7 Examples 790
20.3.8 Example of Spec(T 792
20.3.9 Security Considerations 792
20.4 Connected Identity for Privacy in SIP 793
20.4.1 Overview 793
20.4.2 Terminology 793
20.4.3 Overview of Solution 793
20.4.4 UA Behavior outside the Context of an Existing Dialog 794
20.4.5 Behavior of a UA Whose Identity Changes 794
20.4.6 General UA Behavior 795
20.4.7 Authentication Service Behavior 795
20.4.8 Verifier Behavior 795
20.4.9 Proxy Behavior 795
20.4.10 Examples 796
20.4.11 Security Considerations 800
20.5 Guidelines for Using Privacy Mechanism in SIP 801
20.5.1 Definition 801
20.5.2 Semantics of Existing Priv-Values 801
20.5.3 Target for Each Priv-Value 801
20.5.4 Recommended Treatment of User Privacy-Sensitive Information 803
20.6 Anonymity in SIP 810
20.6.1 Overview 810
20.6.2 UA-Driven Anonymity 810
20.6.3 Rejecting Anonymous Requests 813
20.7 Summary 814

Appendix A: ABNF 815
Appendix B: Reference RFCs 821

Index