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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIPPING Working Group A. Johnston 3 Internet-Draft WorldCom 4 Expires: August 8, 2003 O. Levin 5 RADVISION 6 February 7, 2003 8 Session Initiation Protocol Call Control - Conferencing for User 9 Agents 10 draft-johnston-sipping-cc-conferencing-01 12 Status of this Memo 14 This document is an Internet-Draft and is in full conformance with 15 all provisions of Section 10 of RFC2026. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that other 19 groups may also distribute working documents as Internet-Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at http:// 27 www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on August 8, 2003. 34 Copyright Notice 36 Copyright (C) The Internet Society (2003). All Rights Reserved. 38 Abstract 40 This document defines conferencing call control features for the 41 Session Initiation Protocol (SIP). This document builds on the 42 Conferencing Requirements and Framework documents to define how a 43 tightly coupled SIP conference works. The approach is explored from 44 different user agent (UA) types perspective: conference-unaware, 45 conference-aware and focus UAs. The use of URIs in conferencing, 46 OPTIONS for capabilities discovery, and call control using REFER are 47 covered in detail with example call flow diagrams. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 52 2. Usage of the 'isfocus' Feature Parameter . . . . . . . . . . 3 53 2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2.2 Session Establishment Usage . . . . . . . . . . . . . . . . 4 55 2.3 OPTIONS Usage . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. SIP User Agent Conferencing Capability Types . . . . . . . . 4 57 3.1 Focus UA . . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 3.2 Conference Factory URI . . . . . . . . . . . . . . . . . . . 5 59 3.3 Conference-Unaware UA . . . . . . . . . . . . . . . . . . . 5 60 3.4 Conference-Aware UA . . . . . . . . . . . . . . . . . . . . 6 61 4. SIP Conferencing Primitives . . . . . . . . . . . . . . . . 6 62 4.1 Joining a Conference using the Conference URI - Dial In . . 6 63 4.2 Adding a Participant by the Focus - Dial Out . . . . . . . . 7 64 4.3 Manually Creating a Conference by Dialing into a 65 Conferencing Application . . . . . . . . . . . . . . . . . . 8 66 4.4 Creating a Conference by a Conference-Unaware UA . . . . . . 10 67 4.5 Creating a Conference using Ad-Hoc SIP Methods . . . . . . . 11 68 4.6 Requesting the Focus Add a New Resource to a Conference . . 12 69 4.7 Adding a 3rd Party Using Conference URI . . . . . . . . . . 14 70 4.8 Adding a 3rd Party Using a Dialog Identifier . . . . . . . . 16 71 4.9 Bringing a Point-to-Point Dialog into a Conference . . . . . 17 72 4.10 Requesting the Focus Remove a Participant from a 73 Conference . . . . . . . . . . . . . . . . . . . . . . . . . 17 74 4.11 Discovery of Conferencing Capabilities using OPTIONS . . . . 18 75 5. Security Considerations . . . . . . . . . . . . . . . . . . 20 76 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 77 Normative References . . . . . . . . . . . . . . . . . . . . 21 78 Informative References . . . . . . . . . . . . . . . . . . . 21 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 22 80 Intellectual Property and Copyright Statements . . . . . . . 23 82 1. Introduction 84 This document uses the concepts and definitions from the high level 85 requirements [8] and the SIP conferencing framework [9] documents. 87 The approach described in this document implements key functions in 88 the conferencing framework using SIP primitives only. This allows for 89 conducting simple conferences with defined functionalities using SIP 90 mechanisms and conventions. Many other advanced functions can be 91 implemented using additional means but they are not in the scope of 92 this document. 94 This document presents the basic call control (dial-in and dial-out) 95 conferencing building blocks from the UA perspective. Possible 96 applications include ad-hoc conferences and scheduled conferences. 98 Note that a single conference can bridge participants having 99 different capabilities and who potentially have joined the conference 100 by different means (i.e. dial-in, dial-out, scheduled, and ad-hoc). 102 The call control and dialog manipulation approach is based on the 103 multiparty framework [10] document. That document defines the basic 104 approach of service design adopted for SIP which includes: 106 - Definition of primitives, not services 107 - Signaling model independent 108 - Invoker oriented 109 - Primitives make full use of URIs 110 - Include authentication, authorization, logging, etc. policies 111 - Define graceful fallback to baseline SIP. 113 The use of opaque URIs and the ability to communicate call control 114 context information within a URI (as opposed to service-related 115 header fields), as discussed in RFC 3087 [11], is fundamental to this 116 approach. 118 2. Usage of the 'isfocus' Feature Parameter 120 2.1 General 122 The main design guidelines for the development of SIP extensions and 123 conventions for conferencing are to define the minimum number of 124 extensions and to have seamless backwards compatibility with 125 conference-unaware SIP UAs. The minimal requirement for SIP is being 126 able to express that a dialog is a part of a certain conference 127 referenced to by a URI. As a result of these extensions, it is 128 possible to do the following using SIP: 130 - Create a conference 131 - Join a conference 132 - Invite a user to a conference 133 - Expel a user by third party 134 - Discover if a URI is a conference URI 136 The approach taken is to use the feature parameter "isfocus" to 137 express that a SIP dialog belongs to a conference. The use of 138 feature parameters in Contact header fields to describe the 139 characteristics and capabilities of a UA is described in the Caller 140 Preferences and Callee Capabilities [7] document which includes the 141 definition of the "isfocus" feature parameter. 143 2.2 Session Establishment Usage 145 In session establishment, a focus MUST include the "isfocus" feature 146 parameter in the Contact header field unless the focus wishes to hide 147 the fact that it is a focus. To a participant, the feature parameter 148 will be associated with the remote target URI of the dialog. It is 149 an indication to a conference-aware UA that the resulting dialog 150 belongs to a conference identified by the URI in the Contact header 151 field and that the call control conventions defined in this document 152 can be applied. 154 2.3 OPTIONS Usage 156 Currently the only met requirement is: given an opaque URI, being 157 able to recognize whether it belongs to a certain conference (i.e. 158 meaning that it is a conference URI) or not. As with any other 159 OPTIONS request, it can be done either inside an active dialog or 160 outside a dialog. A focus MUST include the "isfocus" feature 161 parameter in a 200 OK response to an OPTIONS unless the focus wishes 162 to hide the fact that it is a focus. 164 3. SIP User Agent Conferencing Capability Types 166 From a conferencing perspective, the framework document outlines a 167 number of possible different SIP components such as 168 conference-unaware participant, conference-aware participant, and 169 focus. 171 This document applies the concepts above to the SIP call control part 172 of the conferencing components. It defines normative behavior of the 173 SIP UAs in various conferencing situations (referred later as 174 "scenarios"). 176 3.1 Focus UA 178 A focus, as defined in the framework, hosts a SIP conference and 179 maintains a SIP signaling relationship with each participant in the 180 conference. A focus contains a conference-aware user agent that 181 supports the conferencing call control conventions as defined in this 182 document. 184 A focus SHOULD support the conference package [5] and indicate so in 185 Allow-Events header fields in requests and responses. A focus MAY 186 include information about the conference in SDP message bodies sent. 188 A user agent with focus capabilities could be implemented in end user 189 equipment and would be used for the creation of ad-hoc conferences. 191 A dedicated conferencing server, whose primary task is to 192 simultaneously host conferences of arbitrary type and size, may 193 allocate and publish a conference factory URI (as defined in the next 194 section) for creating an arbitrary number of ad-hoc conferences (and 195 subsequently their focuses) using SIP call control means. 197 3.2 Conference Factory URI 199 According to the framework, there are many ways in which a conference 200 can be created. These are open to the conferencing server 201 implementation policy and include non-automated means (such as IVR), 202 SIP, and the conference policy control protocol. 204 In order to automatically create an arbitrary number of ad-hoc 205 conferences (and subsequently their focuses) using SIP call control 206 means, a globally routable Conference Factory URI can be allocated 207 and published. 209 A successful attempt to establish a call to this URI would result in 210 the automatic creation a new conference and its focus. As a result, 211 note that the Conference Factory URI and the newly created focus URI 212 MAY resolve to different physical devices. 214 A scenario showing the use of the conference factory URI is shown in 215 Section 4.5. 217 3.3 Conference-Unaware UA 219 The simplest user agent can participate in a conference ignoring all 220 SIP conferencing-related information. The simplest user agent is able 221 to dial into a conference and to be invited to a conference. Any 222 conferencing information is potentially conveyed to it using non-SIP 223 means. Such a user agent would not usually host a conference (at 224 least, not using SIP explicitly). A conference-unaware UA needs only 225 to support RFC 3261 [2]. Call flows for conference-unaware UAs are 226 not shown in general in this document as they would be identical to 227 those in the SIP call flows [13] document. 229 3.4 Conference-Aware UA 231 A conference-aware user agent supports SIP conferencing call control 232 conventions defined in this document as a conference participant, in 233 addition to support of RFC 3261. 235 A conference-aware UA MUST recognize the "isfocus" feature parameter. 236 A conference-aware UA SHOULD support REFER [3], SIP events [4], and 237 the conferencing package [5]. 239 A conference-aware UA SHOULD subscribe to the conference package if 240 the "isfocus" parameter is in the remote target URI of a dialog and 241 if the conference package is listed by a focus in an Allow-Events 242 header field. 244 A conference-aware UA MAY render to the user any information about 245 the conference obtained from the SIP header fields and SDP fields 246 from the focus. 248 4. SIP Conferencing Primitives 250 The SIP conferencing call control flows presented in this section are 251 the call control building blocks for various SIP tight conferencing 252 applications as described in the conferencing requirements [8] and 253 framework [9] documents. The major design goal is that the same SIP 254 conferencing primitives would be used by user agents having different 255 conferencing capabilities and comprising different applications. 257 4.1 Joining a Conference using the Conference URI - Dial In 259 In this section a user knows the conference URI and "dials in" to 260 join this conference. 262 If the UA is the first participant of the conference to dial in, it 263 is likely that this INVITE will create the focus and hence the 264 conference. However, the conference URI must have been reserved 265 prior to its use. 267 If the conference is up and running already, the dialing-in 268 participant is joined to the conference by its focus. 270 To join an existing specific conference a UA SHOULD send an INVITE 271 with the Request-URI set to the conference URI. The focus MUST 272 include the "isfocus" feature parameter in the Contact header field 273 of the 200 OK response to the INVITE. 275 An example call flow is shown in Figure 1. 277 Alice Focus Bob Carol 278 | | | 279 | | Carol joins the conference | 280 | | | 281 | | INVITE sip:Conf-ID F1 | 282 | |<----------------------------------------| 283 | | 180 Ringing F2 | 284 | |---------------------------------------->| 285 | | 200 OK Contact:Conf-ID;isfocus F3 | 286 | |---------------------------------------->| 287 | | ACK F4 | 288 | |<----------------------------------------| 289 | | RTP | 290 | |<=======================================>| 291 | | SUBSCRIBE sip:Conf-ID F5 | 292 | |<----------------------------------------| 293 | | 200 OK F6 | 294 | |---------------------------------------->| 295 | | NOTIFY F7 | 296 | |---------------------------------------->| 297 | | 200 OK F8 | 298 | |<----------------------------------------| 300 Figure 1. A Participant Joins a Conference using the Conference URI. 302 4.2 Adding a Participant by the Focus - Dial Out 304 To directly add a participant to a conference, a focus SHOULD send an 305 INVITE to the participant containing a Contact header field with the 306 conference URI and the "isfocus" feature parameter. 308 Note that a conference-unaware UA would simply ignore the 309 conferencing information and treat the session (from a SIP 310 perspective) as a point to point session. 312 An example call flow is shown in Figure 2. It is assumed that Alice 313 is already a participant of the conference. The focus invites Carol 314 to the conference by sending an INVITE. After the session is 315 established, Carol subscribes to the conference URI. It is important 316 to note that there is no dependency on Carol's SUBSCRIBE (F5) and the 317 NOTIFY to Alice (F9) - they occur asynchronously and independently. 319 Alice Focus Bob Carol 320 | | | | 321 |<==================>| | | 322 | | | 323 | Focus "dials out" to add Carol to the conference | 324 | | | 325 | | INVITE Contact:Conf-ID;isfocus F1 | 326 | |---------------------------------------->| 327 | | 180 Ringing F2 | 328 | |<----------------------------------------| 329 | | 200 OK F3 | 330 | |<----------------------------------------| 331 | | ACK F4 | 332 | |---------------------------------------->| 333 | | RTP | 334 | |<=======================================>| 335 | | SUBSCRIBE sip:Conf-ID F5 | 336 | |<----------------------------------------| 337 | | 200 OK F6 | 338 | |---------------------------------------->| 339 | | NOTIFY F7 | 340 | |---------------------------------------->| 341 | | 200 OK F8 | 342 | |<----------------------------------------| 343 | NOTIFY F9 | | 344 |<-------------------| | 345 | 200 OK F10 | | 346 |------------------->| | 348 Figure 2. A Focus "dials out" to Add a Participant to the Conference. 350 4.3 Manually Creating a Conference by Dialing into a Conferencing 351 Application 353 In this section, a user sends an INVITE to a conference server 354 application. The application (such as an IVR system or a web page) 355 is implemented because the system requires additional input from the 356 user before it is able to create a conference. After a normal dialog 357 is established, additional information is received and the conference 358 together with its focus are created. At this point the conference 359 server MUST re-INVITE the user with the conference URI in Contact 360 with the "isfocus" feature parameter. 362 Alternatively, the additional information MAY be provided by the user 363 during an early dialog established. This could be accomplished by a 364 183 Session Progress response sent by the conferencing application. 365 After the conference is created, the conference URI MUST then be 366 returned in a Contact in the 200 OK. 368 An example call flow is shown in Figure 3. In this example, Alice 369 uses a conference application which is triggered when Alice sends an 370 INVITE to the conference application. In this example, Conf-App is 371 used to represent the conference application URI. Alice's 372 conference-aware UA learns of the existence of the conference from 373 the "isfocus" feature parameter and subscribes to the conference 374 package to receive notifications of the conference state. 376 Alice Focus Bob Carol 377 | | | | 378 | Alice establishes session with conference application. | 379 | | | | 380 | INVITE sip:Conf-App F1 | | 381 |------------------->| | | 382 | 180 Ringing F2 | | | 383 |<-------------------| | | 384 | 200 OK F3 | | | 385 |<-------------------| | | 386 | ACK F4 | | | 387 |------------------->| | | 388 | RTP | | | 389 |<==================>| | | 390 | | | | 391 | Alice uses the application to create the conference. | 392 | | | | 393 | INVITE Contact:Conf-ID;isfocus F5 | | 394 |<-------------------| | | 395 | 200 OK F6 | | | 396 |------------------->| | | 397 | ACK F7 | | | 398 |<-------------------| | | 399 | RTP | | | 400 |<==================>| | | 401 | | | | 402 | SUBSCRIBE sip:Conf-ID F8 | | 403 |------------------->| | | 404 | 200 OK F9 | | | 406 |<-------------------| | | 407 | NOTIFY F10 | | | 408 |<-------------------| | | 409 | 200 OK F11 | | | 410 |------------------->| | | 412 Figure 3. A Participant Creates a Conference using an Application. 414 4.4 Creating a Conference by a Conference-Unaware UA 416 It is a requirement that a user (human) be able to use a 417 conference-unaware UA to create and add participants to a conference. 419 A user (human) would choose a conference URI according to system 420 rules and insert it into the Request-URI of the INVITE. This same URI 421 is echoed by a focus adhering to certain addressing conventions 422 (discussed below) in the Contact header by the focus. Additional 423 participants could be added by non-SIP means (publication of the 424 chosen conference URI using web pages, email, IM, etc.). 425 Alternatively, the conference-unaware UA could then add other 426 participants to the conference using SIP call control by establishing 427 a session with them, then transferring [16] them to the conference 428 URI. Note that in this scenario only the user (human) is aware of 429 the conferencing application, and the conference-unaware UA only need 430 support RFC 3261 and optionally call transfer. 432 Making this work does impose certain addressing conventions on a 433 system. As a service/implementation choice, a system could allow the 434 creator of the conference to choose the user portion of the 435 conference URI. However, this requires the URI format to be agreed 436 upon between a user and the system. 438 For example, a service provider might reserve the domain 439 conf.example.com for all conference URIs. Any URI in the domain of 440 conf.example.com would resolve to the focus. The focus could be 441 configured to interpret an unknown user part in the conf.example.com 442 domain as a request for a conference to be created with the 443 conference URI as the Request-URI. For example, an INVITE sent with 444 a Request-URI of sip:k32934208ds72@conf.example.com could be routed 445 to the focus that would then create the conference. This conference 446 URI should be registered by the newly created focus to become 447 routable as a conference URI within the conf.example.com domain. The 448 returned Contact would look as follows: 450 Contact: ;isfocus 452 Note, however, that this approach relies on conventions adopted 453 between the user (human) and the focus. Also, the approach is not 454 robust against collisions in the conference names. If a second user 455 wishing to create a new conference happened to choose the same user 456 part as an existing conference, the result would be that the second 457 user would be added into the existing conference instead of creating 458 a new one. 460 As a result, methods of conference creation in which the conference 461 URI is an opaque URI generated by the focus are preferred. 463 An example call flow is shown in Figure 4. The participant Alice 464 creates the conference URI (using some convention agreed to with the 465 focus domain) and sends an INVITE to that URI which creates the 466 focus. The focus creates the conference and returns the same 467 conference URI in the 200 OK answer to the INVITE (which is ignored 468 by the conference-unaware UA). 470 Alice Focus Bob Carol 471 | | | | 472 | Alice creates the conference and chooses the conference URI. | 473 | | | | 474 | INVITE sip:Conf-ID F1 | | 475 |------------------->| | | 476 | 180 Ringing F2 | | | 477 |<-------------------| | | 478 | 200 OK Contact:Conf-ID;isfocus F3 | | 479 |<-------------------| | | 480 | ACK F4 | | | 481 |------------------->| | | 482 | RTP | | | 483 |<==================>| | | 485 Figure 4. A Conferencing Unaware Participant Creates a Conference 487 4.5 Creating a Conference using Ad-Hoc SIP Methods 489 This section addresses creating a conference by using ad-hoc SIP 490 means. The conference factory URI (as defined in Section 2.4) is 491 used to automatically create the conference in this example. 493 The benefit of this approach is that the conference URI need not be 494 known to the user - instead it is created by a focus and used by the 495 participants� UAs. The main difference between this scenario and 496 Section 4.3 is that no user intervention (IVR, web page form, etc.) 497 is required to create the conference. 499 The SIP URI of the conference factory can be provisioned in the UA 500 (as in a "create new conference" button on a SIP phone) or can be 501 discovered using other means. 503 A SIP entity (such as conferencing server) can distinguish this 504 INVITE request as a request to create a new ad-hoc conference from a 505 request to join an existing conference by the Request-URI. 507 Assuming that all security and policy requirements have been met, a 508 new conference will be created with the Contact URI returned in the 509 200 OK being the conference URI. The Contact header field MUST 510 contain the "isfocus" feature parameter to indicate that this URI is 511 for a conference. 513 An example call flow is shown in Figure 5. Note that Conf-Factory is 514 shorthand for the conference factory URI and Conf-ID Is short for the 515 conference URI. In this flow, Alice has a conference-aware UA and 516 creates a conference by sending an INVITE to the conference factory 517 URI. Once the media session is established, Alice subscribes to the 518 conference URI obtained through the Contact in the 200 OK response 519 from the focus. 521 Alice Focus Bob Carol 522 | | | | 523 | Alice creates the conference. | | 524 | | | | 525 | INVITE sip:Conf-Factory F1 | | 526 |------------------->| | | 527 | 180 Ringing F2 | | | 528 |<-------------------| | | 529 | 200 OK Contact:Conf-ID;isfocus F3 | | 530 |<-------------------| | | 531 | ACK F4 | | | 532 |------------------->| | | 533 | RTP | | | 534 |<==================>| | | 535 | | | | 536 | Alice subscribes to the conference URI. | | 537 | | | | 538 | SUBSCRIBE sip:Conf-ID F5 | | 539 |------------------->| | | 540 | 200 OK F6 | | | 541 |<-------------------| | | 542 | NOTIFY F7 | | | 543 |<-------------------| | | 544 | 200 OK F8 | | | 545 |------------------->| | | 547 Figure 5. Creation of a Conference using SIP Ad-Hoc Methods. 549 4.6 Requesting the Focus Add a New Resource to a Conference 551 A SIP conference URI can be used to inject different kinds of 552 information into the conference. Examples include new participants, 553 new real-time media sources, new IM messages, and pointers to passive 554 information references (such as HTTP URIs). 556 To request the focus add a new information resource to the specified 557 conference, any SIP UA can send a REFER to the conference URI with a 558 Refer-To containing the URI of the new resource. Since this REFER is 559 sent to the conference URI and not the conference factory URI, the 560 semantics to the focus are to bring the resource into the conference 561 and make it visible to the conference participants. The resultant 562 focus procedures are dependant both on the nature of the new resource 563 (as expressed by its URI) and the own focus policies regarding IM, 564 central vs. distributed real time media processing, etc. 566 The scenario for adding a new UA participant is important to support 567 because it works even if the new participant does not support REFER 568 and transfer call control - only the requesting participant and the 569 focus need to support the REFER and transfer call control. 571 Upon receipt of the REFER containing a Refer-To header with a SIP 572 URI, the focus SHOULD send an INVITE to the new participant 573 identified by the Refer-To SIP URI containing a Contact header field 574 with the conference URI and the "isfocus" feature parameter. 576 A conference-unaware UA would simply ignore the conferencing 577 information and treat the session (from a SIP perspective) as a point 578 to point session. 580 An example call flow is shown in Figure 6. It is assumed that Alice 581 is already a participant of the conference. Alice sends a REFER to 582 the conference URI. The focus invites Carol to the conference by 583 sending an INVITE. After the session is established, Carol 584 subscribes to the conference URI. It is important to note that 585 there is no dependency on Carol's SUBSCRIBE (F11) and the NOTIFY to 586 Alice (F15) - they occur asynchronously and independently. 588 Alice Focus Bob Carol 589 | | | | 590 |<==================>| | | 591 | REFER sip:Conf-ID Refer-To:Carol F1 | | 592 |------------------->| | 593 | 202 Accepted F2 | | 594 |<-------------------| | 595 | NOTIFY (Trying) F3 | 596 |<-------------------| | 597 | 200 OK F4 | | 598 |------------------->| | 599 | | | 600 | Focus "dials out" to join Carol to the conference | 601 | | | 602 | | INVITE Contact:Conf-ID;isfocus F5 | 603 | |---------------------------------------->| 604 | | 180 Ringing F6 | 605 | |<----------------------------------------| 606 | | 200 OK F7 | 607 | |<----------------------------------------| 608 | | ACK F8 | 609 | |---------------------------------------->| 610 | | RTP | 611 | |<=======================================>| 612 | NOTIFY (OK) F9 | | 613 |<-------------------| | 614 | 200 OK F10 | | 615 |------------------->| | 616 | | SUBSCRIBE sip:Conf-ID F11 | 617 | |<----------------------------------------| 618 | | 200 OK F12 | 619 | |---------------------------------------->| 620 | | NOTIFY F13 | 621 | |---------------------------------------->| 622 | | 200 OK F14 | 623 | |<----------------------------------------| 624 | NOTIFY F15 | | 625 |<-------------------| | 626 | 200 OK F16 | | 627 |------------------->| | 629 Figure 6. Participant Requests Focus add a Participant to the Conference. 631 4.7 Adding a 3rd Party Using Conference URI 633 A participant wishing to add a new participant will request this 634 participant to send an INVITE to the conference URI. This can be 635 done using a non-SIP means (such as passing or publishing the 636 conference URI in an email, IM, or web page). If a non-SIP means is 637 used, then the flow and requirements are identical to Section 4.1. 639 The SIP mechanism to do this utilizes the REFER method. 641 A UA wishing to add a new participant SHOULD send a REFER request to 642 the participant with a Refer-To header containing the conference URI 643 and the "isfocus" feature parameter. 645 The requirements are then identical to the "dial in" case of Section 646 4.1. The inviting participant MAY receive notification through the 647 REFER action that the new participant has been added in addition to 648 the notification received through the conference package. 650 An example is shown in Figure 7. In this call flow, it is assumed 651 that Alice is already a participant of the conference. Alice sends 652 Bob an "out of band" REFER - that is, a REFER outside of an 653 established dialog. Should Bob reject the REFER, Alice might try 654 sending an INVITE to Bob to establish a session first, then send a 655 REFER within the dialog, effectively transferring Bob into the 656 conference [16]. 658 Alice Focus Bob Carol 659 | | | | 660 |<==================>| | | 661 | | | | 662 | Alice adds Bob into conference | | 664 | | | | 665 | REFER Refer-To:Conf-ID F1 | | 666 |---------------------------------------->| | 667 | 202 Accepted F2 | | | 668 |<----------------------------------------| | 669 | NOTIFY (Trying) F3| | | 670 |<----------------------------------------| | 671 | 200 OK F4 | | | 672 |---------------------------------------->| | 673 | | INVITE sip:Conf-ID F5 | 674 | |<-------------------| | 675 | | 180 Ringing F6 | | 676 | |------------------->| | 677 | | 200 OK Contact:Conf-ID;isfocus F7 | 678 | |------------------->| | 679 | | ACK F8 | | 680 | |<-------------------| | 681 | | RTP | | 682 | |<==================>| | 683 | NOTIFY (OK) F9 | | | 684 |<----------------------------------------| | 685 | 200 OK F10 | | | 686 |---------------------------------------->| | 687 | NOTIFY F11 | | | 688 |<-------------------| | | 689 | 200 OK F12 | | | 690 |------------------->| | | 691 | | SUBSCRIBE sip:Conf-ID F13 | 692 | |<-------------------| | 693 | | 200 OK F14 | | 694 | |------------------->| | 695 | | NOTIFY F15 | | 696 | |------------------->| | 697 | | 200 OK F16 | | 698 | |<-------------------| | 700 Figure 7. Adding a Participant to an Existing Conference. 702 4.8 Adding a 3rd Party Using a Dialog Identifier 704 Under some circumstances, a participant wanting to join a conference 705 may only know a dialog identifier of one of the legs of the 706 conference and the conference factory URI, instead of the conference 707 URI. The information may have been learned using the dialog package 708 [17] or some non-SIP means to retrieve this information from a 709 conference participant. 711 A UA can request to be added to a conference by sending a request to 712 the focus containing a Join [6] header field containing a dialog ID 713 of one leg of the conference (a dialog between a participant and the 714 focus). 716 There are other scenarios in which a UA can use the Join header for 717 certain conferencing call control scenarios. See [6] for further 718 examples and details. 720 An example is shown in Figure 8. It is assumed that Alice is a 721 participant of the conference. The dialog identifier between Alice 722 and the focus is abbreviated as A-F and is known by Bob. Bob 723 requests to be added to the conference by sending an INVITE message 724 F1 to the focus containing a Join header which contains the dialog 725 identifier A-F. Note that this dialog identifier could be learned 726 through some non-SIP mechanism, or by use of SUBSCRIBE/NOTIFY and the 727 dialog event package [17]. Bob is added into the conference by the 728 focus. 730 Alice Focus Bob Carol 731 | | | | 732 |<==================>| | | 733 | | | | 734 | Bob requests to be added to the conference. | 735 | | | | 736 | | INVITE Join:A-F F1| | 737 | |<-------------------| | 738 | | 180 Ringing F2 | | 739 | |------------------->| | 740 | | 200 OK Contact:Conf-ID;isfocus F3 | 741 | |------------------->| | 742 | | ACK F4 | | 743 | |<-------------------| | 744 | | RTP | | 745 | |<==================>| | 746 | | SUBSCRIBE sip:Conf-ID F5 | 747 | |<-------------------| | 748 | | 200 OK F6 | | 749 | |------------------->| | 750 | | NOTIFY F7 | | 751 | |------------------->| | 752 | | 200 OK F8 | | 753 | |<-------------------| | 755 Figure 8. Adding a Participant to an Existing Conference using Join. 757 4.9 Bringing a Point-to-Point Dialog into a Conference 759 A focus is capable of bringing an existing point-to-point dialog with 760 another UA to a conference that the focus hosts. The focus would do 761 it by sending re-INVITE changing the Contact URI to the conference 762 URI with the "isfocus" feature parameter. By doing this, the focus 763 signals to the UA that it becomes a participant of the conference, 764 specified in the Contact header. 766 Currently, there is no way for a UA, being in an active 767 point-to-point call with a focus, to express by SIP call control 768 means a request to bridge its dialog with a specific conference or to 769 create a new conference and include the dialog in this conference. 770 Instead, a new dialog will need to be created. Even if the UA 771 discovers that the other side has focus capabilities, the UA needs to 772 close the old session and to establish a new session/dialog with the 773 focus. 775 4.10 Requesting the Focus Remove a Participant from a Conference 776 To request the focus remove a participant from the specified 777 conference, a properly authorized SIP UA (typically the conference 778 owner) can send a REFER to the conference URI with a Refer-To 779 containing the URI of the participant and with the method set to BYE. 780 The requestor does not need to know the dialog information about the 781 dialog between the focus and the participant who will be removed - 782 the focus knows this information and fills it when it generates the 783 BYE request. 785 An example call flow is shown in Figure 9. It is assumed that Alice 786 and Carol are already participants of the conference and that Alice 787 is authorized to remove members from the conference. Alice sends a 788 REFER to the conference URI with a Refer-To header containing a URI 789 of the form <sip:carol@chicago.example.com&method=BYE>. 791 Alice Focus Bob Carol 792 | | | | 793 |<==================>| | | 794 | REFER sip:Conf-ID Refer-To:Carol?method=BYE F1 | 795 |------------------->| | 796 | 202 Accepted F2 | | 797 |<-------------------| | 798 | NOTIFY (Trying) F3 | 799 |<-------------------| | 800 | 200 OK F4 | | 801 |------------------->| | 802 | | | 803 | Focus removes Carol from the conference | 804 | | | 805 | | BYE sip:Carol F5 | 806 | |---------------------------------------->| 807 | | 200 OK F6 | 808 | |<----------------------------------------| 809 | NOTIFY (OK) F7 | | 810 |<-------------------| | 811 | 200 OK F8 | | 812 |------------------->| | 813 | NOTIFY F9 | | 814 |<-------------------| | 815 | 200 OK F10 | | 816 |------------------->| | 818 Figure 9. Participant Requests Focus Remove a Participant from the Conference. 820 4.11 Discovery of Conferencing Capabilities using OPTIONS 822 A UA MAY send an OPTIONS request to discover if an opaque URI is a 823 conference URI (resolves to a focus). In addition, the reply to the 824 OPTIONS request can also indicate support for various SIP call 825 control extensions used in this document. 827 Note that the Allow, Accept, Allow-Events, and Supported header 828 fields should be present in an INVITE from a focus or a 200 OK answer 829 from the focus to an INVITE as a part of a normal dialog 830 establishment process. 832 An example is shown in Figure 10 where Alice sends an OPTIONS to a 833 URI which resolves to a focus. 835 Alice Focus Bob Carol 836 | | | | 837 | OPTIONS sip:Conf-ID F1 | | 838 |------------------->| | | 839 | 200 OK Contact:Conf-ID;isfocus F2 | | 840 |<-------------------| | | 842 Figure 10. Participant Queries Capabilities of URI which resolves to a Focus. 844 Following is an example message detail of message F2 in Figure 10. 845 Based on the response, Alice's UA learns that the URI is a conference 846 URI and that the responding UA is focus that supports a number of SIP 847 call control extensions. 849 The response details are as follows: 851 SIP/2.0 200 OK 852 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877 853 ;received=192.0.2.4 854 To: ;tag=93810874 855 From: Alice ;tag=1928301774 856 Call-ID: a84b4c76e66710 857 CSeq: 63104 OPTIONS 858 Contact: ;isfocus 859 Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, 860 SUBSCRIBE, NOTIFY 861 Allow-Events: refer, conference 862 Accept: application/sdp, application/conference-info+xml, 863 message/sipfrag 864 Accept-Language: en 865 Supported: replaces 866 Content-Type: application/sdp 867 Content-Length: 274 869 (SDP not shown) 871 Useful information from each of these headers is detailed in the next 872 sections. 874 Allow. The support of methods such as REFER, SUBSCRIBE, and NOTIFY 875 indicate that the user agent supports call control and SIP Events. 877 Accept. The support of bodies such as message/sipfrag [12], 878 application/conference-info+xml [5] also indicates support of call 879 control and conferencing. 881 Allow-Events. The support of event packages such as refer [3], 882 conference [5]. 884 Supported. The support of extensions such as replaces [15]. 886 Contact. The presence of the "isfocus" feature parameter in the 887 Contact header indicates that the URI is a conference URI and that 888 the UA is a focus. 890 5. Security Considerations 892 This document discusses call control for SIP conferencing. Both call 893 control and conferencing have specific security requirements which 894 will be summarized here. Conferences generally have authorization 895 rules about who may or may not join a conference, what type of media 896 may or may not be used, etc. This information is used by the Focus 897 to admit or deny participation in a conference. It is recommended 898 that these types of authorization rules be used to provide security 899 for a SIP conference. For this authorization information to be used, 900 the focus needs to be able to authenticate potential participants. 901 Normal SIP mechanisms including Digest authentication and 902 certificates can be used. These conference specific security 903 requirements are discussed further in the requirements and framework 904 documents. 906 For call control security, a user agent must maintain local policy on 907 who is permitted to perform call control operations, initiate REFERs, 908 and replace dialogs. Normal SIP authentication mechanisms are also 909 appropriate here. The specific authentication and authorization 910 schemes are described in the multiparty call control framework 911 document. 913 6. Contributors 915 We would like to thank Rohan Mahy, Jonathan Rosenberg, Roni Even, 916 Petri Koskelainen, Brian Rosen, Paul Kyzivat, Eric Burger, and others 917 in list discussions. 919 Normative References 921 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 922 Levels", BCP 14, RFC 2119, March 1997. 924 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 925 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 926 Session Initiation Protocol", RFC 3261, June 2002. 928 [3] Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07 929 (work in progress), December 2002. 931 [4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 932 Notification", RFC 3265, June 2002. 934 [5] Rosenberg, J. and H. Schulzrinne, "A Session Initiation Protocol 935 (SIP) Event Package for Conference State", 936 draft-ietf-sipping-conference-package-00 (work in progress), 937 June 2002. 939 [6] Mahy, R. and D. Petrie, "The Session Inititation Protocol (SIP) 940 'Join' Header", draft-ietf-sip-join-00 (work in progress), 941 October 2002. 943 [7] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol 944 (SIP) Caller Preferences and Callee Capabilities", 945 draft-ietf-sip-callerprefs-07 (work in progress), November 2002. 947 Informative References 949 [8] Levin, O., "Requirements for Tightly Coupled SIP Conferencing", 950 draft-levin-sipping-conferencing-requirements-02 (work in 951 progress), November 2002. 953 [9] Rosenberg, J., "A Framework for Conferencing with the Session 954 Initiation Protocol", 955 draft-rosenberg-sipping-conferencing-framework-00 (work in 956 progress), November 2002. 958 [10] Mahy, R., "A Multi-party Application Framework for SIP", 959 draft-ietf-sipping-cc-framework-01 (work in progress), July 960 2002. 962 [11] Campbell, B. and R. Sparks, "Control of Service Context using 963 SIP Request-URI", RFC 3087, April 2001. 965 [12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, 966 November 2002. 968 [13] Johnston, A., "Session Initiation Protocol Basic Call Flow 969 Examples", draft-ietf-sipping-basic-call-flows-01 (work in 970 progress), October 2002. 972 [14] Johnston, A. and S. Donovan, "Session Initiation Protocol 973 Service Examples", draft-ietf-sipping-service-examples-03 (work 974 in progress), November 2002. 976 [15] Dean, R., Biggs, B. and R. Mahy, "The Session Inititation 977 Protocol (SIP) 'Replaces' Header", draft-ietf-sip-replaces-02 978 (work in progress), May 2002. 980 [16] Sparks, R. and A. Johnston, "Session Initiation Protocol Call 981 Control - Transfer", draft-ietf-sipping-cc-transfer-00 (work in 982 progress), October 2002. 984 [17] Rosenberg, J. and H. Schulzrinne, "A Session Initiation 985 Protocol (SIP) Event Package for Dialog State", 986 draft-ietf-sipping-dialog-package-00 (work in progress), June 987 2002. 989 Authors' Addresses 991 Alan Johnston 992 WorldCom 993 100 South 4th Street 994 St. Louis, MO 63104 996 EMail: alan.johnston@wcom.com 998 Orit Levin 999 RADVISION 1000 266 Harristown Road 1001 Glen Rock, NJ 75024 1003 EMail: orit@radvision.com 1005 Intellectual Property Statement 1007 The IETF takes no position regarding the validity or scope of any 1008 intellectual property or other rights that might be claimed to 1009 pertain to the implementation or use of the technology described in 1010 this document or the extent to which any license under such rights 1011 might or might not be available; neither does it represent that it 1012 has made any effort to identify any such rights. 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