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Olson 3 Internet-Draft Microsoft 4 Expires: December 19, 2003 June 20, 2003 6 Extensions to the REFER mechanism for Third Party Call Control 7 draft-olson-sipping-refer-extensions-00 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with 12 all provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering 15 Task Force (IETF), its areas, and its working groups. Note that other 16 groups may also distribute working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at http:// 24 www.ietf.org/ietf/1id-abstracts.txt. 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 This Internet-Draft will expire on December 19, 2003. 31 Copyright Notice 33 Copyright (C) The Internet Society (2003). All Rights Reserved. 35 Abstract 37 This document proposes a number of extensions to the REFER method 38 used by the Session Initiation Protocol (SIP) for the purpose of 39 third party call control. 41 Table of Contents 43 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 44 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 45 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 46 4. Use of the application/dialog-info+xml MIME type with REFER . 6 47 5. Extending REFER to SIP response codes . . . . . . . . . . . . 11 48 6. Supressing forking of REFER requests . . . . . . . . . . . . . 18 49 7. Supressing the NOTIFY associated with a REFER . . . . . . . . 19 50 8. Replacing complex Refer-To URIs with a MIME body . . . . . . . 21 51 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 52 9.1 norefersub option tag registration . . . . . . . . . . . . . . 24 53 9.2 refer-response option tag registration . . . . . . . . . . . . 24 54 10. Security Considerations . . . . . . . . . . . . . . . . . . . 25 55 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26 56 References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 57 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 27 58 Intellectual Property and Copyright Statements . . . . . . . . 28 60 1. Terminology 62 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 63 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 64 document are to be interpreted as described in RFC 2119 [1]. 66 To simplify discussions of the REFER method and its extensions, three 67 new terms will be defined: 69 REFER-Issuer: the UA issuing the REFER request 71 REFER-Recipient: the UA receiving the REFER request 73 REFER-Target: the UA designated in the Refer-To URI 75 2. Introduction 77 The REFER [3] extension to SIP [2] defines a basis for third party 78 call control that is useful for implementing features such as call 79 transfer. REFER has a few limitations with respect to implementing 80 more advanced features in conjunction with the dialog info event 81 package [7]. These limitations derive in part from the limited 82 information available in the message/sipfrag [4] content of the 83 associated NOTIFY. Another limiting factor is the requirement to 84 compress the semantics of the referred request in the Refer-To URI by 85 encoding the desired headers as parameters of that URI. Finally, 86 there are situations where the party issuing the REFER request does 87 not need the NOTIFY associated with the REFER, perhaps because that 88 agent is already subscribed to the appropriate dialog package outside 89 of the REFER request. This represents additional unnecessary traffic 90 and state in the REFER-Issuer and REFER-recipient. 92 3. Requirements 94 Requirement 1: Allow the REFER-Issuer to watch the progress of a 95 REFER beyond the end of the INVITE transaction and for the 96 duration of the dialog 98 Requirement 2: Expose the dialog information of the dialog 99 established at the REFER-Recipient as a result of the REFER 101 Requirement 3: Allow specification of much richer detail of the 102 request that is generated at the REFER-Recipient as a result of 103 the REFER beyond just the method and Request-URI. Examples include 104 caller preferences, portions of the SDP, and user defined headers. 106 Requirement 4: Allow one user agent to specify that another user 107 agent answer an incoming call 109 Requirement 5: Prevent forking of a REFER request and allow targeting 110 of that REFER request to a specific device or class of device. 112 Requirement 6: Reduce the number of messages exchanged to perform a 113 REFER when the REFER-Issuer and REFER-Recipient support richer 114 call control primitives such as the dialog and conference event 115 packages. 117 4. Use of the application/dialog-info+xml MIME type with REFER 119 The REFER specification [3] mandates the use of the message/sipfrag 120 [4] MIME type for NOTIFYs of the refer event package. These NOTIFYs 121 are sent as part of the implicit subscription created by the REFER. 122 The purpose of the NOTIFY is to communicate the state of the 123 transaction between the REFER-Recipient and the REFER-Target that is 124 created as a result of the REFER 126 Where the purpose of sending the REFER is actually to establish a 127 dialog, for example through an INVITE in the Refer-To, the derivative 128 state of interest is actually the dialog state. While this may be 129 inferred from the contents of the message/sipfrag body it is a bit 130 clumsy and in many cases the REFER-Recipient will omit important 131 details of the dialog in the message/sipfrag body. Two things that 132 are generally lacking from the message/sipfrag content are the dialog 133 identifier (Call-ID plus local and remote tags) and the state of the 134 dialog. Not coincidentally, this is the same information available in 135 the application/dialog-info+xml MIME type [7] used for the dialog 136 event package. 138 To address this shortcoming, it is proposed to extend REFER to allow 139 the (optional) use of the application/dialog-info+xml MIME type in 140 place of message/sipfrag. The REFER-Issuer specifies support for this 141 by including this MIME type in an Accept header. The REFER-Issuer 142 MUST also support message/sipfrag for compatibility with strictly 143 RFC3515 compliant implementations. If the REFER-Recipient supports 144 this extension, it may choose to honor the request or default to 145 using message/sipfrag dependent on local policy. It is OPTIONAL for 146 the REFER-Recipient to maintain the subscription for the duration of 147 the resultant (INVITE) dialog and RECOMMENDED that the subscription 148 be maintained at least until the dialog is in "confirmed" state. 150 Figure 1: Example of using application/dialog-info+xml 152 REFER-Issuer REFER-Recipient REFER-Target 153 | | | 154 | M1 REFER (INVITE) | | 155 |--------------------------->| | 156 | M2 202 Accepted | | 157 |<---------------------------| | 158 | M3 NOTIFY | | 159 |<---------------------------| | 160 | M4 200 OK | | 161 |--------------------------->| | 162 | | M5 INVITE | 163 | |----------------------->| 164 | M6 NOTIFY | | 165 |<---------------------------| | 166 | M7 200 OK | | 167 |--------------------------->| | 168 | | M8 180 Ringing | 169 | |<-----------------------| 170 | M9 NOTIFY | | 171 |<---------------------------| | 172 | M10 200 OK | | 173 |--------------------------->| | 174 | | M11 200 OK | 175 | |<-----------------------| 176 | M12 NOTIFY | | 177 |<---------------------------| | 178 | M13 200 OK | | 179 |--------------------------->| M14 ACK | 180 | |----------------------->| 182 Message flow: 184 M1: The REFER-Issuer creates a REFER, specifying a preference for the 185 application/dialog-info+xml MIME type. 187 REFER sip:b@tradewind.com SIP/2.0 188 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 189 From: ;tag=1ab 190 To: 191 Call-ID: 1@issuer.tradewind.com 192 CSeq: 234234 REFER 193 Max-Forwards: 70 194 Refer-To: sip:c@tradewind.com;method=INVITE 195 Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1 196 Contact: sip:a@issuer.tradewind.com 197 Content-Length: 0 199 M5: The REFER-Recipient creates an appropriate INVITE based on the 200 REFER and sends it to the REFER-Target. 202 INVITE sip:c@tradewind.com SIP/2.0 203 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-1 204 From: ;tag=1bc 205 To: 206 Call-ID: 1@recipient.tradewind.com 207 CSeq: 1234567 INVITE 208 Max-Forwards: 70 209 Contact: sip:b@recipient.tradewind.com 210 Content-Length: ... 212 214 M6: The REFER-Recipient sends a NOTIFY triggered by the INVITE being 215 sent to the REFER-Target. The body of the NOTIFY contains the 216 dialog identifier and current state ("trying"). 218 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 219 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-2 220 From: ;tag=1ab 221 To: ;tag=1ba 222 Call-ID: 1@issuer.tradewind.com 223 CSeq: 1278784 NOTIFY 224 Max-Forwards: 70 225 Event: refer;id=234234 226 Subscription-State: active;expires=3600 227 Contact: sip:b@recipient.tradewind.com 228 Content-Type: application/dialog-info+xml 229 Content-Length: ... 231 232 236 239 trying 240 241 243 M9: The REFER-Recipient sends a NOTIFY triggered by the 180 Ringing 244 received from the REFER-Target. The body of the NOTIFY contains 245 the dialog identifier and current state ("early"). 247 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 248 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-3 249 From: ;tag=1ab 250 To: ;tag=1ba 251 Call-ID: 1@issuer.tradewind.com 252 CSeq: 1278785 NOTIFY 253 Max-Forwards: 70 254 Event: refer;id=234234 255 Subscription-State: active;expires=3600 256 Contact: sip:b@recipient.tradewind.com 257 Content-Type: application/dialog-info+xml 258 Content-Length: ... 260 261 265 268 early 269 270 272 M12: The REFER-Recipient sends a NOTIFY triggered by the 200 OK 273 received from the REFER-Target. The body of the NOTIFY contains 274 the dialog identifier and current state ("confirmed"). 276 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 277 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4 278 From: ;tag=1ab 279 To: ;tag=1ba 280 Call-ID: 1@issuer.tradewind.com 281 CSeq: 1278786 NOTIFY 282 Max-Forwards: 70 283 Event: refer;id=234234 284 Subscription-State: active;expires=3600 285 Contact: sip:b@recipient.tradewind.com 286 Content-Type: application/dialog-info+xml 287 Content-Length: ... 289 290 294 297 confirmed 298 299 301 5. Extending REFER to SIP response codes 303 REFER is currently defined to trigger the sending of a request from 304 the REFER-Recipient to the REFER-Target. The intention is most often 305 to initiate a dialog from the REFER-Recipient to the REFER-Target. 306 This is an excellent way to generate an action at the REFER-Recipient 307 based on an event or action that takes places at the REFER-Issuer. 308 The classic example is call transfer as a result of a user at the 309 REFER-Issuer taking some action. 311 With the use of the dialog event package, it is possible for one UA 312 to monitor events at another UA related to a dialog, such as the 313 receipt of an INVITE to establish a new dialog. What is lacking is a 314 way for the watcher to indicate what should be the response to such 315 an INVITE request. For example, the dialog watcher would like the 316 recipient of the session initiation request to accept the initiation 317 (send a 200 OK response to the INVITE request). One motivating 318 scenario for this is a set of co-operating User Agents (devices) that 319 belong to the same user. The user, while using one SIP device, wishes 320 to answer a call that is being received on another of that user's SIP 321 devices. This gives the user a single UI focus for control while 322 allowing multiple devices with differing capabilities. 324 To enable such a scenario, this document proposes an extension to the 325 SIP(S) URI syntax as defined in SIP [2] The extension is analogous to 326 the "method" uri-parameter that currently exists to communicate a 327 method for use in the Refer-To header. A new uri-parameter, 328 "response", is proposed that is used in conjunction with the "method" 329 uri-parameter and associated call-id, local tag, and remote tag to 330 request that the REFER-Recipient send a response within the 331 identified SIP transaction to the REFER-Target. 333 The REFER-Issuer MUST specify a "method" parameter in addition to the 334 "response" parameter. The REFER-Issuer MUST also include the 335 appropriate local-uri, local-tag, remote-uri, and remote-tag encoded 336 as From and To headers in the Refer-To URI (or using a message/ 337 sipfrag body as defined later in this document). Note that in order 338 to satisfy this requirement, the REFER-Issuer must have access to 339 this information. In particular, it is assumed that the REFER-Issuer 340 receives the local-uri and remote-uri in the NOTIFY for the dialog 341 event package from the REFER-Recipient. These elements are optional 342 in the XML schema. It is anticipated that User Agents that support 343 these REFER extensions will also include these optional elements in 344 the application/dialog-info+xml payload (as privacy concerns allow). 346 To ensure the REFER-Recipient conformant with RFC3515 does not 347 misintepret this as a REFER to send a request of the specified 348 method, the REFER-Issuer MUST also include a Require: refer-response 349 header in the REFER request. REFER-Recipients which do not understand 350 this extension will return a 420 response. The REFER-Target does not 351 need to understand this extension for this to work. Support for this 352 extension can be queried in advance using a standard OPTIONS request. 354 The REFER-Issuer MUST request the use of the application/ 355 dialog-info+xml MIME type in NOTIFYs associated with a REFER request 356 which uses this "response" extension. 358 The proposed syntax modification follows 360 uri-parameters = *( ";" uri-parameter) 361 uri-parameter = transport-param / user-param / method-param 362 / ttl-param / maddr-param / lr-param / response-param / other-param 363 response-param = "response=" 1*3DIGIT 365 An example call flow follows: 367 Figure 2: Example of using the "response" uri-parameter in the 368 Refer-To header 370 REFER-Issuer REFER-Recipient REFER-Target 371 | | | 372 | N1 SUBSCRIBE (dialog) | | 373 |--------------------------->| | 374 | N2 202 Accepted | | 375 |<---------------------------| | 376 | N3 NOTIFY | | 377 |<---------------------------| | 378 | N4 200 OK | | 379 |--------------------------->| | 380 | | N5 INVITE | 381 | |<-----------------------| 382 | N6 NOTIFY | | 383 |<---------------------------| | 384 | N7 200 OK | | 385 |--------------------------->| | 386 | | N8 180 Trying | 387 | |----------------------->| 388 | N9 NOTIFY | | 389 |<---------------------------| | 390 | N10 200 OK | | 391 |--------------------------->| | 392 | | | 393 | N11 REFER (200) | | 394 |--------------------------->| | 395 | N12 200 OK | | 396 |<---------------------------| N13 200 | 397 | |----------------------->| 398 | N14 NOTIFY | | 399 |<---------------------------| | 400 | N15 200 OK | | 401 |--------------------------->| | 403 Message flow: 405 N1: The REFER-Issuer subscribes to the dialog event package at the 406 REFER-Recipient. 408 SUBSCRIBE sip:b@tradewind.com SIP/2.0 409 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 410 From: ;tag=1ab 411 To: 412 Call-ID: 1@issuer.tradewind.com 413 CSeq: 234234 SUBSCRIBE 414 Max-Forwards: 70 415 Event: dialog 416 Accept: application/dialog-info+xml 417 Contact: sip:a@issuer.tradewind.com 418 Content-Length: 0 420 N5: The REFER-Recipient receives an INVITE from the REFER-Target to 421 start a new call. 423 INVITE sip:b@tradewind.com SIP/2.0 424 Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-c-1 425 From: ;tag=1cb 426 To: 427 Call-ID: 1@target.tradewind.com 428 CSeq: 1234567 INVITE 429 Max-Forwards: 70 430 Contact: sip:c@target.tradewind.com 431 Content-Length: ... 433 435 N6: The REFER-Recipient sends a NOTIFY triggered by the INVITE 436 received from the REFER-Target. The body of the NOTIFY contains 437 the dialog identifier and current state ("trying"). 439 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 440 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-2 441 From: ;tag=1ab 442 To: ;tag=1ba 443 Call-ID: 1@issuer.tradewind.com 444 CSeq: 454545 NOTIFY 445 Max-Forwards: 70 446 Event: dialog;id=234234 447 Subscription-State: active;expires=3600 448 Contact: sip:b@recipient.tradewind.com 449 Content-Type: application/dialog-info+xml 450 Content-Length: ... 452 453 457 460 b@tradewind.com 461 c@tradewind.com 462 trying 463 464 466 N8: The REFER-Recipient sends a 180 Ringing response to the 467 REFER-Target. 469 SIP/2.0 180 Ringing 470 Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-b-3 471 From: ;tag=1cb 472 To: ;tag=1bc 473 Call-ID: 1@target.tradewind.com 474 CSeq: 1234567 INVITE 475 Contact: sip:b@recipient.tradewind.com 476 Content-Length: 0 478 N9: The REFER-Recipient sends a NOTIFY triggered by the 180 Ringing 479 sent to the REFER-Target. The body of the NOTIFY contains the 480 dialog identifier and current state ("early"). 482 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 483 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4 484 From: ;tag=1ab 485 To: ;tag=1ba 486 Call-ID: 1@issuer.tradewind.com 487 CSeq: 454546 NOTIFY 488 Max-Forwards: 70 489 Event: dialog;id=234234 490 Subscription-State: active;expires=3600 491 Contact: sip:b@recipient.tradewind.com 492 Content-Type: application/dialog-info+xml 493 Content-Length: ... 495 496 500 503 b@tradewind.com 504 c@tradewind.com 505 early 506 507 509 N11: The REFER-Issuer creates a REFER, specifying that the 510 REFER-Recipient should send a 200 OK to accept the session 511 invitation. The From and To headers of the 200 OK are encoded in 512 the Refer-To URI. The local and remote tags for this are 513 determined from the information provided in the NOTIFY for the 514 dialog package. This allows the REFER-Issuer to specify a 515 particular dialog. Combined with the "method" parameter, this 516 identifies a specific transaction within the dialog. 518 REFER sip:b@tradewind.com SIP/2.0 519 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-2 520 From: ;tag=1ab 521 To: 522 Call-ID: 2@issuer.tradewind.com 523 CSeq: 818181 REFER 524 Max-Forwards: 70 525 Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1 526 Require: refer-response 527 Refer-To: sip:c@tradewind.com;method=INVITE;response=200?Call-ID=1%40target.tradewind.com& 528 To=b%40tradewind.com;tag=1bc&From=c%40tradewind.com;tag=1cb 529 Contact: sip:a@issuer.tradewind.com 530 Content-Length: 0 532 N13: The REFER-Recipient sends a 200 OK to the REFER-Target 533 constructed using the information in the Refer-To header. 535 SIP/2.0 200 OK 536 Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-c-1 537 From: ;tag=1cb 538 To: ;tag=1bc 539 Call-ID: 1@target.tradewind.com 540 Supported: refer-response 541 CSeq: 1234567 INVITE 542 Contact: sip:b@recipient.tradewind.com 543 Content-Length: 0 545 N14: The REFER-Recipient sends a NOTIFY triggered by the 200 OK sent 546 to the REFER-Target. The body of the NOTIFY contains the dialog 547 identifier and current state ("confirmed"). 549 NOTIFY sip:a@issuer.tradewind.com SIP/2.0 550 Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4 551 From: ;tag=1ab 552 To: ;tag=1ba 553 Call-ID: 1@issuer.tradewind.com 554 CSeq: 454547 NOTIFY 555 Max-Forwards: 70 556 Event: dialog;id=234234 557 Subscription-State: active;expires=3600 558 Contact: sip:b@recipient.tradewind.com 559 Content-Type: application/dialog-info+xml 560 Content-Length: ... 562 563 567 570 b@tradewind.com 571 c@tradewind.com 572 confirmed 573 574 576 6. Supressing forking of REFER requests 578 The REFER specification allows for the possibility of forking a REFER 579 request which is sent outside of an existing dialog. In some 580 situations forking a REFER may result in absolutely incorrect 581 behavior. This is especially true when the REFER is intended to 582 target a specific device, perhaps with specific capabilities. 583 Fortunately, there is already a mechanism which can address the need 584 to indicate that a REFER should NOT be forked. The caller preferences 585 extension [8] defines both a way to indicate a request not be forked 586 and a means to target a specific device with the desired 587 capabilities. No further extension is required. An example of the 588 usage of caller preferences in conjunction with REFER is shown below. 590 Example of REFER using Caller Preferences extension 592 REFER sip:b@tradewind.com SIP/2.0 593 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 594 From: ;tag=1a 595 To: 596 Call-ID: 1@issuer.tradewind.com 597 CSeq: 234234 REFER 598 Max-Forwards: 70 599 Refer-To: sip:c@tradewind.com;method=INVITE 600 Require: pref 601 Request-Dispostion: no-fork 602 Accept-Contact: *;audio;require 603 Reject-Contact: *;video 604 Contact: sip:a@issuer.tradewind.com 605 Content-Length: 0 607 7. Supressing the NOTIFY associated with a REFER 609 The REFER specification mandates that every REFER create an implicit 610 subscription between the REFER-Issuer and the REFER-Recipient. This 611 subscription results in at least one NOTIFY being sent from the 612 REFER-Recipient to the REFER-Issuer. The REFER-Recipient may choose 613 to cancel the implicit subscription with this NOTIFY. The 614 REFER-Issuer may choose to cancel this implicit subscription with an 615 explicit SUBSCRIBE (Expires: 0) after receipt of the initial NOTIFY 616 or by sending a 481 response to this initial NOTIFY request. 618 The purpose of requiring the implicit subcription and initial NOTIFY 619 is to allow for the situation where the REFER request gets forked and 620 the REFER-Issuer needs a way to see the multiple dialogs that may be 621 established as a result of the forked REFER. This is the same 622 approach used to handle forking of SUBSCRIBE [5] requests. Where the 623 REFER-Issuer explicitly specifies that forking not occur via caller 624 preferences, or implicitly does not allow for forking by sending the 625 REFER within an existing dialog, the requirement that an implicit 626 subscription be established is unnecessary. 628 Another purpose of the NOTIFY is to inform the REFER-Issuer of the 629 progress of the SIP transaction that results from the REFER at the 630 REFER-Recipient. In the case where the REFER-Issuer is already aware 631 of the progress of the SIP transaction, such as when the REFER-Issuer 632 has an explicit subscription to the dialog event package at the 633 REFER-Recipient, the implicit subscription and resultant NOTIFY 634 traffic related to the REFER is superfluous and unnecessary network 635 overhead. 637 To avoid this unnecessary overhead, it is proposed that the 638 REFER-Issuer insert a Supported: norefersub header in the REFER 639 request to indicate to the REFER-Recipient that no implicit 640 subscription or NOTIFY is needed with respect to this REFER request. 641 This MUST be used by the REFER-Issuer only when the REFER-Issuer can 642 be certain that the REFER request will not be forked. The 643 REFER-Recipient MUST signal support for this extension by inserting a 644 Supported: norefersub header in the 2xx response to the REFER 645 request. 647 Example of REFER which supresses the implicit subscription 649 REFER sip:b@tradewind.com SIP/2.0 650 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 651 From: ;tag=1a 652 To: 653 Call-ID: 1@issuer.tradewind.com 654 CSeq: 234234 REFER 655 Max-Forwards: 70 656 Refer-To: sip:c@tradewind.com;method=INVITE 657 Request-Dispostion: no-fork 658 Accept-Contact: *;audio;require 659 Supported: norefersub 660 Contact: sip:a@issuer.tradewind.com 661 Content-Length: 0 663 8. Replacing complex Refer-To URIs with a MIME body 665 Much of the semantics of the REFER request is encapsulated in the 666 Refer-To URI. This URI will commonly encode the method, From, To, 667 Call-ID, Accept-Disposition, Accept-Contact [8], and other headers 668 all in a properly escaped URI. Such a URI can become long, difficult 669 to debug, and prone to URI escaping errors in SIP implementations. 670 The situation becomes more complex if the method is itself a REFER 671 complete with a Refer-To which must contain URI-escaped characters. 672 This double escaping obfuscates things even more and increases the 673 chances of improperly escaping/unescaping of the Refer-To URI. 675 The alternative that is proposed is to make use of the currently 676 unused REFER body to encapsulate all the information that is 677 potentially escaped in the Refer-To URI. The possible information 678 includes the method, headers, and body of the request which is 679 desired to be created by the REFER-Recipient. The ideal candidate for 680 this is the message/sipfrag MIME type. This MIME type can express all 681 of these: the method, Request-URI, headers, and body. This applies to 682 requests as well as responses (as an extension). 684 There are two possibilities: use a cid URI [6] in the Refer-To header 685 to reference the message/sipfrag content in the body of the REFER or 686 continue to place the URI of the REFER-Target in the Refer-To header 687 and place all other facets of the request in the message/sipfrag body 688 with the understanding that the presence of the body implies its 689 usage for this purpose. A few examples should illuminate things. 691 Before 693 REFER sip:b@tradewind.com SIP/2.0 694 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 695 From: ;tag=1a 696 To: 697 Call-ID: 1@issuer.tradewind.com 698 CSeq: 234234 REFER 699 Max-Forwards: 70 700 Refer-To: sip:c@tradewind.com;method=INVITE;response=200?Call-ID=1%40target.tradewind.com& 701 From=b%40tradewind.com;tag=2b&To=c%40tradewind.com;tag=1c 702 Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1 703 Require: refer-response 704 Contact: sip:a@issuer.tradewind.com 705 Content-Length: 0 706 After (placing the REFER-Target in the Refer-To) 708 REFER sip:b@tradewind.com SIP/2.0 709 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 710 From: ;tag=1a 711 To: 712 Call-ID: 1@issuer.tradewind.com 713 CSeq: 234234 REFER 714 Max-Forwards: 70 715 Refer-To: sip:c@tradewind.com;method=INVITE 716 Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1 717 Require: refer-response 718 Contact: sip:a@issuer.tradewind.com 719 Content-Type: message/sipfrag 720 Content-Length: ... 722 SIP/2.0 200 OK 723 Call-ID: 1@target.tradewind.com 724 From: b@tradewind.com;tag=2b 725 To: c@tradewind.com;tag=1c 727 After (using a cid URI) 729 REFER sip:b@tradewind.com SIP/2.0 730 Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1 731 From: ;tag=1a 732 To: 733 Call-ID: 1@issuer.tradewind.com 734 CSeq: 234234 REFER 735 Max-Forwards: 70 736 Refer-To: cid:1239103912039@issuer.tradewind.com 737 Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1 738 Require: refer-response 739 Contact: sip:a@issuer.tradewind.com 740 Content-Type: message/sipfrag 741 Content-Id: <1239103912039@issuer.tradewind.com> 742 Content-Length: ... 744 SIP/2.0 200 OK 745 Call-ID: 1@target.tradewind.com 746 From: b@tradewind.com;tag=2b 747 To: c@tradewind.com;tag=1c 749 Note that the target URI and method are still included in the 750 Refer-To header. 752 If the REFER-Recipient does not understand this extension, it MUST 753 return a 4xx response to the REFER request. The REFER-Issuer SHOULD 754 re-issue the REFER request using URI escaping and only the Refer-To: 755 URI to convey the same information if possible. 757 9. IANA Considerations 759 9.1 norefersub option tag registration 761 This document defines a new option tag, norefersub, which specifies 762 that an implicit subscription for event package refer should not be 763 created as a result of accepting this REFER request. This option tag 764 is only meaningful for the REFER request defined in RFC3515. 766 9.2 refer-response option tag registration 768 This document defines a new option tag, refer-response, which 769 specifies that the recipient of the REFER request is expected to 770 issue a response for the SIP transaction identified within the 771 Refer-To URI. This option tag is only meaningful for the REFER 772 request defined in RFC3515. 774 10. Security Considerations 776 No additional concerns above what is defined in RFC3515. 778 11. Acknowledgements 780 The author would like to thank Rohan Mahy for his insightful 781 comments. The author would also like to thank Sriram Parameswar for 782 his draft-parameswar-sipping-norefersub-00 proposal which is the 783 basis for the norefersub option tag proposed in this draft. 785 References 787 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 788 Levels", BCP 14, RFC 2119, March 1997. 790 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 791 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 792 Session Initiation Protocol", RFC 3261, June 2002. 794 [3] Sparks, R., "The Session Initiation Protocol (SIP) Refer 795 Method", RFC 3515, April 2003. 797 [4] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, 798 November 2002. 800 [5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 801 Notification", RFC 3265, June 2002. 803 [6] Levinson, E., "Content-ID and Message-ID Uniform Resource 804 Locators", RFC 2392, August 1998. 806 [7] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog 807 Event Package for the Session Initiation Protocol (SIP", 808 draft-ietf-sipping-dialog-package-01 (work in progress), March 809 2003. 811 [8] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller 812 Preferences and Callee Capabilities for the Session Initiation 813 Protocol (SIP)", draft-ietf-sip-callerprefs-08 (work in 814 progress), March 2003. 816 Author's Address 818 Sean Olson 819 Microsoft 820 One Microsoft Way 821 Redmond, WA 98052 822 US 824 Phone: +1-425-707-2846 825 EMail: seanol@microsoft.com 826 URI: http://www.microsoft.com/rtc 828 Intellectual Property Statement 830 The IETF takes no position regarding the validity or scope of any 831 intellectual property or other rights that might be claimed to 832 pertain to the implementation or use of the technology described in 833 this document or the extent to which any license under such rights 834 might or might not be available; neither does it represent that it 835 has made any effort to identify any such rights. Information on the 836 IETF's procedures with respect to rights in standards-track and 837 standards-related documentation can be found in BCP-11. 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