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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: October 9, 2003 O. Levin 5 RADVISION 6 April 10, 2003 8 Session Initiation Protocol Call Control - Conferencing for User 9 Agents 10 draft-ietf-sipping-cc-conferencing-00 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 October 9, 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 | | | 405 |<-------------------| | | 406 | NOTIFY F10 | | | 407 |<-------------------| | | 408 | 200 OK F11 | | | 409 |------------------->| | | 411 Figure 3. A Participant Creates a Conference using an Application. 413 4.4 Creating a Conference by a Conference-Unaware UA 415 It is a requirement that a user (human) be able to use a 416 conference-unaware UA to create and add participants to a conference. 418 A user (human) would choose a conference URI according to system 419 rules and insert it into the Request-URI of the INVITE. This same URI 420 is echoed by a focus adhering to certain addressing conventions 421 (discussed below) in the Contact header by the focus. Additional 422 participants could be added by non-SIP means (publication of the 423 chosen conference URI using web pages, email, IM, etc.). 424 Alternatively, the conference-unaware UA could then add other 425 participants to the conference using SIP call control by establishing 426 a session with them, then transferring [16] them to the conference 427 URI. Note that in this scenario only the user (human) is aware of 428 the conferencing application, and the conference-unaware UA only need 429 support RFC 3261 and optionally call transfer. 431 Making this work does impose certain addressing conventions on a 432 system. As a service/implementation choice, a system could allow the 433 creator of the conference to choose the user portion of the 434 conference URI. However, this requires the URI format to be agreed 435 upon between a user and the system. 437 For example, a service provider might reserve the domain 438 conf.example.com for all conference URIs. Any URI in the domain of 439 conf.example.com would resolve to the focus. The focus could be 440 configured to interpret an unknown user part in the conf.example.com 441 domain as a request for a conference to be created with the 442 conference URI as the Request-URI. For example, an INVITE sent with 443 a Request-URI of sip:k32934208ds72@conf.example.com could be routed 444 to the focus that would then create the conference. This conference 445 URI should be registered by the newly created focus to become 446 routable as a conference URI within the conf.example.com domain. The 447 returned Contact would look as follows: 449 Contact: ;isfocus 451 Note, however, that this approach relies on conventions adopted 452 between the user (human) and the focus. Also, the approach is not 453 robust against collisions in the conference names. If a second user 454 wishing to create a new conference happened to choose the same user 455 part as an existing conference, the result would be that the second 456 user would be added into the existing conference instead of creating 457 a new one. 459 As a result, methods of conference creation in which the conference 460 URI is an opaque URI generated by the focus are preferred. 462 An example call flow is shown in Figure 4. The participant Alice 463 creates the conference URI (using some convention agreed to with the 464 focus domain) and sends an INVITE to that URI which creates the 465 focus. The focus creates the conference and returns the same 466 conference URI in the 200 OK answer to the INVITE (which is ignored 467 by the conference-unaware UA). 469 Alice Focus Bob Carol 470 | | | | 471 | Alice creates the conference and chooses the conference URI. | 472 | | | | 473 | INVITE sip:Conf-ID F1 | | 474 |------------------->| | | 475 | 180 Ringing F2 | | | 476 |<-------------------| | | 477 | 200 OK Contact:Conf-ID;isfocus F3 | | 478 |<-------------------| | | 479 | ACK F4 | | | 480 |------------------->| | | 481 | RTP | | | 482 |<==================>| | | 484 Figure 4. A Conferencing Unaware Participant Creates a Conference 486 4.5 Creating a Conference using Ad-Hoc SIP Methods 488 This section addresses creating a conference by using ad-hoc SIP 489 means. The conference factory URI (as defined in Section 2.4) is 490 used to automatically create the conference in this example. 492 The benefit of this approach is that the conference URI need not be 493 known to the user - instead it is created by a focus and used by the 494 participants' UAs. The main difference between this scenario and 495 Section 4.3 is that no user intervention (IVR, web page form, etc.) 496 is required to create the conference. 498 The SIP URI of the conference factory can be provisioned in the UA 499 (as in a "create new conference" button on a SIP phone) or can be 500 discovered using other means. 502 A SIP entity (such as conferencing server) can distinguish this 503 INVITE request as a request to create a new ad-hoc conference from a 504 request to join an existing conference by the Request-URI. 506 Assuming that all security and policy requirements have been met, a 507 new conference will be created with the Contact URI returned in the 508 200 OK being the conference URI. The Contact header field MUST 509 contain the "isfocus" feature parameter to indicate that this URI is 510 for a conference. 512 An example call flow is shown in Figure 5. Note that Conf-Factory is 513 shorthand for the conference factory URI and Conf-ID Is short for the 514 conference URI. In this flow, Alice has a conference-aware UA and 515 creates a conference by sending an INVITE to the conference factory 516 URI. Once the media session is established, Alice subscribes to the 517 conference URI obtained through the Contact in the 200 OK response 518 from the focus. 520 Alice Focus Bob Carol 521 | | | | 522 | Alice creates the conference. | | 523 | | | | 524 | INVITE sip:Conf-Factory F1 | | 525 |------------------->| | | 526 | 180 Ringing F2 | | | 527 |<-------------------| | | 528 | 200 OK Contact:Conf-ID;isfocus F3 | | 529 |<-------------------| | | 530 | ACK F4 | | | 531 |------------------->| | | 532 | RTP | | | 533 |<==================>| | | 534 | | | | 535 | Alice subscribes to the conference URI. | | 536 | | | | 537 | SUBSCRIBE sip:Conf-ID F5 | | 538 |------------------->| | | 539 | 200 OK F6 | | | 540 |<-------------------| | | 541 | NOTIFY F7 | | | 542 |<-------------------| | | 543 | 200 OK F8 | | | 544 |------------------->| | | 546 Figure 5. Creation of a Conference using SIP Ad-Hoc Methods. 548 4.6 Requesting the Focus Add a New Resource to a Conference 550 A SIP conference URI can be used to inject different kinds of 551 information into the conference. Examples include new participants, 552 new real-time media sources, new IM messages, and pointers to passive 553 information references (such as HTTP URIs). 555 To request the focus add a new information resource to the specified 556 conference, any SIP UA can send a REFER to the conference URI with a 557 Refer-To containing the URI of the new resource. Since this REFER is 558 sent to the conference URI and not the conference factory URI, the 559 semantics to the focus are to bring the resource into the conference 560 and make it visible to the conference participants. The resultant 561 focus procedures are dependant both on the nature of the new resource 562 (as expressed by its URI) and the own focus policies regarding IM, 563 central vs. distributed real time media processing, etc. 565 The scenario for adding a new UA participant is important to support 566 because it works even if the new participant does not support REFER 567 and transfer call control - only the requesting participant and the 568 focus need to support the REFER and transfer call control. 570 Upon receipt of the REFER containing a Refer-To header with a SIP 571 URI, the focus SHOULD send an INVITE to the new participant 572 identified by the Refer-To SIP URI containing a Contact header field 573 with the conference URI and the "isfocus" feature parameter. 575 A conference-unaware UA would simply ignore the conferencing 576 information and treat the session (from a SIP perspective) as a point 577 to point session. 579 An example call flow is shown in Figure 6. It is assumed that Alice 580 is already a participant of the conference. Alice sends a REFER to 581 the conference URI. The focus invites Carol to the conference by 582 sending an INVITE. After the session is established, Carol 583 subscribes to the conference URI. It is important to note that 584 there is no dependency on Carol's SUBSCRIBE (F11) and the NOTIFY to 585 Alice (F15) - they occur asynchronously and independently. 587 Alice Focus Bob Carol 588 | | | | 589 |<==================>| | | 590 | REFER sip:Conf-ID Refer-To:Carol F1 | | 591 |------------------->| | 592 | 202 Accepted F2 | | 593 |<-------------------| | 594 | NOTIFY (Trying) F3 | 595 |<-------------------| | 596 | 200 OK F4 | | 597 |------------------->| | 598 | | | 599 | Focus "dials out" to join Carol to the conference | 600 | | | 601 | | INVITE Contact:Conf-ID;isfocus F5 | 602 | |---------------------------------------->| 603 | | 180 Ringing F6 | 604 | |<----------------------------------------| 605 | | 200 OK F7 | 606 | |<----------------------------------------| 607 | | ACK F8 | 608 | |---------------------------------------->| 609 | | RTP | 610 | |<=======================================>| 611 | NOTIFY (OK) F9 | | 612 |<-------------------| | 613 | 200 OK F10 | | 614 |------------------->| | 615 | | SUBSCRIBE sip:Conf-ID F11 | 616 | |<----------------------------------------| 617 | | 200 OK F12 | 618 | |---------------------------------------->| 619 | | NOTIFY F13 | 620 | |---------------------------------------->| 621 | | 200 OK F14 | 622 | |<----------------------------------------| 623 | NOTIFY F15 | | 624 |<-------------------| | 625 | 200 OK F16 | | 626 |------------------->| | 628 Figure 6. Participant Requests Focus add a Participant to the Conference. 630 4.7 Adding a 3rd Party Using Conference URI 632 A participant wishing to add a new participant will request this 633 participant to send an INVITE to the conference URI. This can be 634 done using a non-SIP means (such as passing or publishing the 635 conference URI in an email, IM, or web page). If a non-SIP means is 636 used, then the flow and requirements are identical to Section 4.1. 638 The SIP mechanism to do this utilizes the REFER method. 640 A UA wishing to add a new participant SHOULD send a REFER request to 641 the participant with a Refer-To header containing the conference URI 642 and the "isfocus" feature parameter. 644 The requirements are then identical to the "dial in" case of Section 645 4.1. The inviting participant MAY receive notification through the 646 REFER action that the new participant has been added in addition to 647 the notification received through the conference package. 649 An example is shown in Figure 7. In this call flow, it is assumed 650 that Alice is already a participant of the conference. Alice sends 651 Bob an "out of band" REFER - that is, a REFER outside of an 652 established dialog. Should Bob reject the REFER, Alice might try 653 sending an INVITE to Bob to establish a session first, then send a 654 REFER within the dialog, effectively transferring Bob into the 655 conference [16]. 657 Alice Focus Bob Carol 658 | | | | 659 |<==================>| | | 660 | | | | 661 | Alice adds Bob into conference | | 662 | | | | 663 | REFER Refer-To:Conf-ID F1 | | 664 |---------------------------------------->| | 665 | 202 Accepted F2 | | | 666 |<----------------------------------------| | 667 | NOTIFY (Trying) F3| | | 668 |<----------------------------------------| | 669 | 200 OK F4 | | | 670 |---------------------------------------->| | 671 | | INVITE sip:Conf-ID F5 | 672 | |<-------------------| | 673 | | 180 Ringing F6 | | 674 | |------------------->| | 675 | | 200 OK Contact:Conf-ID;isfocus F7 | 676 | |------------------->| | 677 | | ACK F8 | | 678 | |<-------------------| | 679 | | RTP | | 680 | |<==================>| | 681 | NOTIFY (OK) F9 | | | 682 |<----------------------------------------| | 683 | 200 OK F10 | | | 684 |---------------------------------------->| | 685 | NOTIFY F11 | | | 686 |<-------------------| | | 687 | 200 OK F12 | | | 688 |------------------->| | | 689 | | SUBSCRIBE sip:Conf-ID F13 | 690 | |<-------------------| | 691 | | 200 OK F14 | | 692 | |------------------->| | 693 | | NOTIFY F15 | | 694 | |------------------->| | 695 | | 200 OK F16 | | 696 | |<-------------------| | 698 Figure 7. Adding a Participant to an Existing Conference. 700 4.8 Adding a 3rd Party Using a Dialog Identifier 702 Under some circumstances, a participant wanting to join a conference 703 may only know a dialog identifier of one of the legs of the 704 conference and the conference factory URI, instead of the conference 705 URI. The information may have been learned using the dialog package 706 [17] or some non-SIP means to retrieve this information from a 707 conference participant. 709 A UA can request to be added to a conference by sending a request to 710 the focus containing a Join [6] header field containing a dialog ID 711 of one leg of the conference (a dialog between a participant and the 712 focus). 714 There are other scenarios in which a UA can use the Join header for 715 certain conferencing call control scenarios. See [6] for further 716 examples and details. 718 An example is shown in Figure 8. It is assumed that Alice is a 719 participant of the conference. The dialog identifier between Alice 720 and the focus is abbreviated as A-F and is known by Bob. Bob 721 requests to be added to the conference by sending an INVITE message 722 F1 to the focus containing a Join header which contains the dialog 723 identifier A-F. Note that this dialog identifier could be learned 724 through some non-SIP mechanism, or by use of SUBSCRIBE/NOTIFY and the 725 dialog event package [17]. Bob is added into the conference by the 726 focus. 728 Alice Focus Bob Carol 729 | | | | 730 |<==================>| | | 731 | | | | 732 | Bob requests to be added to the conference. | 733 | | | | 734 | | INVITE Join:A-F F1| | 735 | |<-------------------| | 736 | | 180 Ringing F2 | | 737 | |------------------->| | 738 | | 200 OK Contact:Conf-ID;isfocus F3 | 739 | |------------------->| | 740 | | ACK F4 | | 741 | |<-------------------| | 742 | | RTP | | 743 | |<==================>| | 744 | | SUBSCRIBE sip:Conf-ID F5 | 745 | |<-------------------| | 746 | | 200 OK F6 | | 747 | |------------------->| | 748 | | NOTIFY F7 | | 749 | |------------------->| | 750 | | 200 OK F8 | | 751 | |<-------------------| | 753 Figure 8. Adding a Participant to an Existing Conference using Join. 755 4.9 Bringing a Point-to-Point Dialog into a Conference 757 A focus is capable of bringing an existing point-to-point dialog with 758 another UA to a conference that the focus hosts. The focus would do 759 it by sending re-INVITE changing the Contact URI to the conference 760 URI with the "isfocus" feature parameter. By doing this, the focus 761 signals to the UA that it becomes a participant of the conference, 762 specified in the Contact header. 764 Currently, there is no way for a UA, being in an active 765 point-to-point call with a focus, to express by SIP call control 766 means a request to bridge its dialog with a specific conference or to 767 create a new conference and include the dialog in this conference. 768 Instead, a new dialog will need to be created. Even if the UA 769 discovers that the other side has focus capabilities, the UA needs to 770 close the old session and to establish a new session/dialog with the 771 focus. 773 4.10 Requesting the Focus Remove a Participant from a Conference 774 To request the focus remove a participant from the specified 775 conference, a properly authorized SIP UA (typically the conference 776 owner) can send a REFER to the conference URI with a Refer-To 777 containing the URI of the participant and with the method set to BYE. 778 The requestor does not need to know the dialog information about the 779 dialog between the focus and the participant who will be removed - 780 the focus knows this information and fills it when it generates the 781 BYE request. 783 An example call flow is shown in Figure 9. It is assumed that Alice 784 and Carol are already participants of the conference and that Alice 785 is authorized to remove members from the conference. Alice sends a 786 REFER to the conference URI with a Refer-To header containing a URI 787 of the form . 789 Alice Focus Bob Carol 790 | | | | 791 |<==================>| | | 792 | REFER sip:Conf-ID Refer-To:Carol?method=BYE F1 | 793 |------------------->| | 794 | 202 Accepted F2 | | 795 |<-------------------| | 796 | NOTIFY (Trying) F3 | 797 |<-------------------| | 798 | 200 OK F4 | | 799 |------------------->| | 800 | | | 801 | Focus removes Carol from the conference | 802 | | | 803 | | BYE sip:Carol F5 | 804 | |---------------------------------------->| 805 | | 200 OK F6 | 806 | |<----------------------------------------| 807 | NOTIFY (OK) F7 | | 808 |<-------------------| | 809 | 200 OK F8 | | 810 |------------------->| | 811 | NOTIFY F9 | | 812 |<-------------------| | 813 | 200 OK F10 | | 814 |------------------->| | 816 Figure 9. Participant Requests Focus Remove a Participant from the Conference. 818 4.11 Discovery of Conferencing Capabilities using OPTIONS 820 A UA MAY send an OPTIONS request to discover if an opaque URI is a 821 conference URI (resolves to a focus). In addition, the reply to the 822 OPTIONS request can also indicate support for various SIP call 823 control extensions used in this document. 825 Note that the Allow, Accept, Allow-Events, and Supported header 826 fields should be present in an INVITE from a focus or a 200 OK answer 827 from the focus to an INVITE as a part of a normal dialog 828 establishment process. 830 An example is shown in Figure 10 where Alice sends an OPTIONS to a 831 URI which resolves to a focus. 833 Alice Focus Bob Carol 834 | | | | 835 | OPTIONS sip:Conf-ID F1 | | 836 |------------------->| | | 837 | 200 OK Contact:Conf-ID;isfocus F2 | | 838 |<-------------------| | | 840 Figure 10. Participant Queries Capabilities of URI which resolves to a Focus. 842 Following is an example message detail of message F2 in Figure 10. 843 Based on the response, Alice's UA learns that the URI is a conference 844 URI and that the responding UA is focus that supports a number of SIP 845 call control extensions. 847 The response details are as follows: 849 SIP/2.0 200 OK 850 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKhjhs8ass877 851 ;received=192.0.2.4 852 To: ;tag=93810874 853 From: Alice ;tag=1928301774 854 Call-ID: a84b4c76e66710 855 CSeq: 63104 OPTIONS 856 Contact: ;isfocus 857 Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, 858 SUBSCRIBE, NOTIFY 859 Allow-Events: refer, conference 860 Accept: application/sdp, application/conference-info+xml, 861 message/sipfrag 862 Accept-Language: en 863 Supported: replaces 864 Content-Type: application/sdp 865 Content-Length: 274 867 (SDP not shown) 869 Useful information from each of these headers is detailed in the next 870 sections. 872 Allow. The support of methods such as REFER, SUBSCRIBE, and NOTIFY 873 indicate that the user agent supports call control and SIP Events. 875 Accept. The support of bodies such as message/sipfrag [12], 876 application/conference-info+xml [5] also indicates support of call 877 control and conferencing. 879 Allow-Events. The support of event packages such as refer [3], 880 conference [5]. 882 Supported. The support of extensions such as replaces [15]. 884 Contact. The presence of the "isfocus" feature parameter in the 885 Contact header indicates that the URI is a conference URI and that 886 the UA is a focus. 888 5. Security Considerations 890 This document discusses call control for SIP conferencing. Both call 891 control and conferencing have specific security requirements which 892 will be summarized here. Conferences generally have authorization 893 rules about who may or may not join a conference, what type of media 894 may or may not be used, etc. This information is used by the Focus 895 to admit or deny participation in a conference. It is recommended 896 that these types of authorization rules be used to provide security 897 for a SIP conference. For this authorization information to be used, 898 the focus needs to be able to authenticate potential participants. 899 Normal SIP mechanisms including Digest authentication and 900 certificates can be used. These conference specific security 901 requirements are discussed further in the requirements and framework 902 documents. 904 For call control security, a user agent must maintain local policy on 905 who is permitted to perform call control operations, initiate REFERs, 906 and replace dialogs. Normal SIP authentication mechanisms are also 907 appropriate here. The specific authentication and authorization 908 schemes are described in the multiparty call control framework 909 document. 911 6. Contributors 913 We would like to thank Rohan Mahy, Jonathan Rosenberg, Roni Even, 914 Petri Koskelainen, Brian Rosen, Paul Kyzivat, Eric Burger, and others 915 in list discussions. 917 Normative References 919 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 920 Levels", BCP 14, RFC 2119, March 1997. 922 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 923 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 924 Session Initiation Protocol", RFC 3261, June 2002. 926 [3] Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07 927 (work in progress), December 2002. 929 [4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 930 Notification", RFC 3265, June 2002. 932 [5] Rosenberg, J. and H. Schulzrinne, "A Session Initiation Protocol 933 (SIP) Event Package for Conference State", 934 draft-ietf-sipping-conference-package-00 (work in progress), 935 June 2002. 937 [6] Mahy, R. and D. Petrie, "The Session Inititation Protocol (SIP) 938 'Join' Header", draft-ietf-sip-join-01 (work in progress), March 939 2003. 941 [7] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller 942 Preferences and Callee Capabilities for the Session Initiation 943 Protocol (SIP)", draft-ietf-sip-callerprefs-08 (work in 944 progress), March 2003. 946 Informative References 948 [8] Levin, O. and R. Even, "High Level Requirements for Tightly 949 Coupled SIP Conferencing", 950 draft-levin-sipping-conferencing-requirements-03 (work in 951 progress), March 2003. 953 [9] Rosenberg, J., "A Framework for Conferencing with the Session 954 Initiation Protocol", 955 draft-rosenberg-sipping-conferencing-framework-01 (work in 956 progress), February 2003. 958 [10] Mahy, R., "A Call Control and Multi-party usage framework for 959 the Session Initiation Protocol (SIP)", 960 draft-ietf-sipping-cc-framework-02 (work in progress), March 961 2003. 963 [11] Campbell, B. and R. Sparks, "Control of Service Context using 964 SIP Request-URI", RFC 3087, April 2001. 966 [12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, 967 November 2002. 969 [13] Johnston, A., "Session Initiation Protocol Basic Call Flow 970 Examples", draft-ietf-sipping-basic-call-flows-02 (work in 971 progress), April 2003. 973 [14] Johnston, A. and S. Donovan, "Session Initiation Protocol 974 Service Examples", draft-ietf-sipping-service-examples-04 (work 975 in progress), March 2003. 977 [15] Dean, R., Biggs, B. and R. Mahy, "The Session Inititation 978 Protocol (SIP) 'Replaces' Header", draft-ietf-sip-replaces-03 979 (work in progress), March 2003. 981 [16] Sparks, R. and A. Johnston, "Session Initiation Protocol Call 982 Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in 983 progress), February 2003. 985 [17] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog 986 Event Package for the Session Initiation Protocol (SIP", 987 draft-ietf-sipping-dialog-package-01 (work in progress), March 988 2003. 990 Authors' Addresses 992 Alan Johnston 993 WorldCom 994 100 South 4th Street 995 St. Louis, MO 63104 997 EMail: alan.johnston@wcom.com 999 Orit Levin 1000 RADVISION 1001 266 Harristown Road 1002 Glen Rock, NJ 75024 1004 EMail: orit@radvision.com 1006 Intellectual Property Statement 1008 The IETF takes no position regarding the validity or scope of any 1009 intellectual property or other rights that might be claimed to 1010 pertain to the implementation or use of the technology described in 1011 this document or the extent to which any license under such rights 1012 might or might not be available; neither does it represent that it 1013 has made any effort to identify any such rights. 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