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Kyzivat 5 Expires: September 2008 Cisco Systems, Inc. 6 March 31, 2008 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-07.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that 14 any applicable patent or other IPR claims of which he or she is 15 aware have been or will be disclosed, and any of which he or she 16 becomes aware will be disclosed, in accordance with Section 6 of 17 BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six 25 months and may be updated, replaced, or obsoleted by other 26 documents at any time. It is inappropriate to use Internet-Drafts 27 as reference material or to cite them other than as "work in 28 progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html 36 This Internet-Draft will expire on November 30, 2007. 38 Abstract 40 The Session Initiation Protocol (SIP) utilizes the offer/answer 41 model to establish and update multimedia sessions using the Session 42 Description Protocol (SDP). The description of the offer/answer 43 model in SIP is dispersed across multiple RFCs. This document 44 summarizes all the current usages of the offer/answer model in SIP 45 communication. 47 Table of Contents 49 1. Introduction................................................... 3 50 2. Summary of SIP usage of the Offer/Answer Model................. 3 51 2.1. Offer/Answer Exchange Pairs in SIP Messages............... 3 52 2.2. Rejection of an Offer..................................... 5 53 2.3. Session Description which is not Offer nor Answer......... 6 54 3. Detailed Discussion of the Offer/Answer Model for SIP.......... 6 55 3.1. Offer/Answer for the INVITE method with 100rel extension.. 6 56 3.1.1. INVITE Request with SDP.............................. 7 57 3.1.2. INVITE request without SDP........................... 9 58 3.2. Offer/Answer Exchange in Early Dialog.................... 10 59 3.3. Offer/Answer Exchange in an Established Dialog........... 10 60 4. Exceptional Case Handling..................................... 11 61 4.1. Message Crossing Case Handling........................... 11 62 4.2. Glare Case Handling...................................... 13 63 5. Content of Offers and Answers................................. 14 64 5.1. General Principle for Constructing Offers and Answers.... 14 65 5.2. Choice of Media Types and Formats to Include and Exclude. 15 66 5.2.1. Sending an Initial INVITE with Offer................ 15 67 5.2.2. Responding with an Offer when the Initial INVITE has 68 no Offer................................................... 15 69 5.2.3. Answering an Initial INVITE with Offer.............. 16 70 5.2.4. Answering when the Initial INVITE had no Offer...... 17 71 5.2.5. Subsequent Offers and Answers....................... 17 72 5.3. Hold and Resume of media................................. 18 73 5.4. Behavior on receiving SDP with c=0.0.0.0................. 19 74 6. Remaining Issues or Best Practices on Offer/Answer............ 19 75 6.1. Rejecting PRACK Offer.................................... 20 76 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 77 Transaction................................................... 21 78 6.3. Offer in a Reliable Response............................. 22 79 6.4. Requesting Hold while already on Hold.................... 23 80 7. Add New Offer/Answer Usage in SIP............................. 23 81 7.1. Explicit Usage........................................... 23 82 7.2. Rejection of an Offer.................................... 23 83 7.3. Backward Compatibility................................... 23 84 7.4. Exceptional Case Handling................................ 23 85 8. IANA Considerations........................................... 24 86 9. Security Considerations....................................... 24 87 10. References................................................... 24 88 10.1. Normative References.................................... 24 89 10.2. Informative References.................................. 24 90 Author's Addresses............................................... 25 91 Full Copyright Statement......................................... 25 92 Intellectual Property Statement.................................. 25 93 Acknowledgment................................................... 26 95 1. Introduction 97 SIP utilizes the offer/answer model to establish and update the 98 session. The rules to govern the offer/answer behaviors in SIP are 99 described in the several RFCs. 101 The primary purpose of this document is to describe all forms of 102 SIP usage of the offer/answer model in one document to help the 103 readers to fully understand it. Also, this document tries to 104 incorporate the results of the discussions on the controversial 105 issues to avoid repeating the same discussions later. 107 This document is not intended to make normative changes. Rather, it 108 makes the remaining open issues clear and leaves them for further 109 study. 111 2. Summary of SIP usage of the Offer/Answer Model 113 The offer/answer model itself is independent from the higher layer 114 application protocols which utilize it. SIP is one of the 115 applications using the offer/answer model. RFC 3264 [3] defines the 116 offer/answer model, but does not specify which SIP messages should 117 convey an offer or an answer. This should be defined in the SIP 118 core and extensions RFCs. 120 In theory, any SIP message can include a session description in its 121 body. But a session description in a SIP message is not necessarily 122 an offer or an answer. Only certain session description usages that 123 conform to the rules described in standards-track RFCs can be 124 interpreted as an offer or an answer. The rules for how to handle 125 the offer/answer model are currently defined in several RFCs. 127 The offer/answer model defines a mechanism for update of sessions. 128 In SIP, a dialog is used to associate an offer/answer exchange with 129 the session which it is to update. In other words, only the 130 offer/answer exchange in the SIP dialog can update the session 131 which is managed by that dialog. 133 2.1. Offer/Answer Exchange Pairs in SIP Messages 135 Currently, the rules on the offer/answer model are defined in RFC 136 3261 [1], RFC 3262 [2] and RFC 3311 [4]. In these RFCs, only the 137 six patterns shown in Table 1 are defined for exchanging an offer 138 and an answer with SIP messages. 140 Note that an offer/answer exchange initiated by an INVITE request 141 must follow exactly one of the patterns 1, 2, 3, 4. When an initial 142 INVITE causes multiple dialogs due to forking, an offer/answer 143 exchange is carried out independently in each distinct dialog. When 144 an INVITE request contains no offer, only pattern 2 or pattern 4 145 apply. 'The first reliable non-failure message' must have an offer 146 if there is no offer in the INVITE request. This means that UA 147 which receives the INVITE request without an offer must include an 148 offer in the first reliable response with 100rel extension. If no 149 reliable provisional response has been sent, the UAS must include 150 an offer when sending 2xx response. 152 In pattern 3, the first reliable provisional response may or may 153 not have an answer. When a reliable provisional response contains a 154 session description, and is the first to do so, then that session 155 description is the answer to the offer in the INVITE request. The 156 answer can not be updated, and a new offer can not be sent in a 157 subsequent reliable response for the same INVITE transaction. 159 In pattern 5, a PRACK request can contain an offer only if the 160 reliable response which it acknowledges contains an answer to the 161 previous offer/answer exchange. 163 NOTE: It is legal to have UPDATE/2xx exchanges without 164 offer/answer exchanges (pattern 6). However when re-INVITEs 165 are sent for non-offer/answer purposes, an offer/answer 166 exchange is required. In that case the prior SDP will 167 typically be repeated. 169 There may be ONLY ONE offer/answer negotiation in progress for a 170 single dialog at any point in time. Section 4 explains how to 171 ensure this. When an INVITE results in multiple dialogs each has a 172 separate offer/answer negotiation. 174 NOTE: This is when using a Content-Disposition of "session". 175 There may be a second offer/answer negotiation in progress 176 using a Content-Disposition of "early-session" [6]. That is 177 not addressed by this draft. 179 Offer Answer RFC Ini Est Early 180 ------------------------------------------------------------------- 181 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 182 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 183 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 184 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 185 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 186 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y 188 Table 1. Summary of SIP Usage of the Offer/Answer Model 190 In Table 1, '1xx-rel' corresponds to the reliable provisional 191 response which contains the 100rel option defined in RFC 3262 [2]. 193 The 'Ini' column shows the ability to exchange the offer/answer to 194 initiate the session. 'Y' indicates that the pattern can be used in 195 the initial offer/answer exchange, while 'N' indicates that it can 196 not. Only the initial INVITE transaction can be used to exchange 197 the offer/answer to establish a multimedia session. 199 The 'Est' column shows the ability to update the established 200 session. 202 The 'Early' column indicates which patterns may be used to modify 203 the established session in an early dialog. There are two ways to 204 exchange a subsequent offer/answer in an early dialog. 206 2.2. Rejection of an Offer 208 It is not entirely clear how to reject an offer when it is 209 unacceptable, and some methods do not allow explicit rejection of 210 an offer. For each of the patterns in Table 1, Table 2 shows how to 211 reject an offer. 213 When a UA receives an INVITE request with an unacceptable offer, it 214 should respond with a 488 response, preferably with Warning header 215 field indicating the reason of the rejection, unless another 216 response code is more appropriate to reject it. (Pattern 1 and 217 Pattern 3) 219 When a UA receives an UPDATE request with an offer which it can not 220 accept, it should respond with a 488 response preferably with 221 Warning header field indicating the reason of the rejection, unless 222 another response code is more appropriate to reject it. (Pattern 6) 224 When a UA receives a PRACK request with an offer which it can not 225 accept, it may respond with a 200 response with a syntactically 226 correct session description. This may optionally be followed by an 227 UPDATE request to rearrange the session parameters if both ends 228 support the UPDATE method. Alternatively the UA may terminate the 229 dialog and send an error response to the INVITE request. The 230 validity and consequences of a 488 response to PRACK is an open 231 issue which is discussed within a sequent section. (Pattern 5) 233 When a UA receives a response with an offer which it can not accept, 234 the UA does not have a way to reject it explicitly. Therefore, a UA 235 should respond to the offer with the correct session description 236 and rearrange the session parameters by initiating a new 237 offer/answer exchange, or alternatively terminate the session. 238 (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA 239 should take care not to cause an infinite offer/answer loop. 241 Offer Rejection 242 ----------------------------------------------------- 243 1. INVITE Req. 488 INVITE Response 244 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 245 OR termination of dialog 246 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 247 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 248 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 249 OR termination of dialog 250 6. UPDATE Req. 488 UPDATE Response 252 Table 2. Rejection of an Offer 254 (*) A UA should only use PRACK to send an offer when it has strong 255 reasons to expect the receiver will accept the offer. 257 2.3. Session Description which is not Offer nor Answer 259 As previously stated, a session description in a SIP message is not 260 necessarily an offer or an answer. For example, SIP can use a 261 session description to describe capabilities apart from 262 offer/answer exchange. Examples of this are 200 OK responses for 263 OPTIONS and 488 responses for INVITE. 265 3. Detailed Discussion of the Offer/Answer Model for SIP 267 3.1. Offer/Answer for the INVITE method with 100rel extension 269 The INVITE method provides the basic procedure for offer/answer 270 exchange in SIP. Without the 100rel option, the rules are simple as 271 described in RFC 3261 [1]. If an INVITE request includes a session 272 description, pattern 1 is applied and if an INVITE request does not 273 include a session description, pattern 2 is applied. 275 With 100rel, pattern 3 and pattern 4 are added and this complicates 276 the rules. An INVITE request may cause multiple responses. Note 277 that even if both UAs support the 100rel extension, not all the 278 provisional responses may be sent reliably. Note also that a 279 reliable provisional response is allowed without a session 280 description if the UAS does not wish to send the answer yet. An 281 unreliable provisional response may include a session description 282 in the body if the UAS has not sent a reliable response, but its 283 session description is neither an offer nor an answer. All the 284 session descriptions in the unreliable responses to the INVITE 285 request must be identical to the answer which is included in the 286 reliable response. A session description in an unreliable response 287 that precedes a reliable response can be considered a "preview" of 288 the answer that will be coming, and hence may be treated like an 289 answer until the actual one arrives. 291 NOTE: This "preview" session description rule applies to a 292 single offer/answer exchange. In parallel offer/answer 293 exchanges (caused by forking) a UA may obviously receive a 294 different "preview" of an answer in each dialog. UAs are 295 expected to deal with this. 297 Although RFC 3261 says a UA should accept media once an INVITE with 298 an offer has been sent, in many cases, an answer (or, at least a 299 preview of it) is required in order for media to be accepted. 300 Therefore, a UAS should send an SDP answer reliably (if possible) 301 before it starts sending media. And, if neither the UAC nor the UAS 302 support 100rel, the UAS should send a preview of the answer before 303 it starts sending media. 305 3.1.1. INVITE Request with SDP 307 When a UAC includes an SDP body in the INVITE request as an offer, 308 it expects the answer to be received with one of the reliable 309 responses. Other than that, no offer/answer exchanges can occur in 310 the messages within the INVITE transaction. 312 UAC UAS 313 | F1 INVITE (SDP) | <- The offer in the offer/answer model 314 |-------------------->| 315 | F2 1xx (SDP) | <- The offer/answer exchange is not 316 |<--------------------| closed yet, but UAC acts as if it 317 | | ^ receives the answer. 318 | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer 319 |<--------------------| | SDP. 320 | F4 PRACK (no SDP) | | 321 |-------------------->| | UAC must not send a new offer. 322 | F5 2xx PRA (no SDP) | | 323 |<--------------------| v 324 | | 325 | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model 326 |<--------------------| - 327 | F7 PRACK | | UAC can send a new offer in a PRACK 328 |-------------------->| | request to acknowledge F6. 329 | F8 2xx PRA | | After F7 UAC and UAS can send a new 330 |<--------------------| v offer in an UPDATE request. 331 | | 332 | F9 1xx-rel | <- SDP should not be included in the 333 |<--------------------| subsequent 1xx-rel once offer/answer 334 | F10 PRACK | has been completed. 335 |-------------------->| 336 | F11 2xx PRA | 337 |<--------------------| 338 | | 339 | F12 2xx INV | <- SDP should not be included in the 340 |<--------------------| final response once offer/answer has 341 | F13 ACK | been completed. 342 |-------------------->| 344 Figure 1 Example of Offer/Answer with 100rel Extension (1) 346 For example, in Figure 1, only the SDP in F6 is the answer. The SDP 347 in the non-reliable response (F2) is the preview of the answer and 348 must be the same as the answer in F6. Receiving F2, the UAC should 349 act as if it receives the answer. However, offer/answer exchange is 350 not completed yet and the UAC must not send a new offer until it 351 receives the same SDP in the first reliable response, which is the 352 real answer. After sending the SDP in F6, the UAS must prepare to 353 receive a new offer from the UAC with an UPDATE request or a PRACK 354 request. 356 The UAS does not include SDP in responses F9 and F12. However, the 357 UAC should prepare to receive SDP bodies in F9 and/or F12, and just 358 ignore them, to handle a peer that does not conform to the 359 recommended implementation. 361 3.1.2. INVITE request without SDP 363 When a UAC does not include an SDP body in the INVITE request, it 364 expects the offer to be received with the first reliable response. 365 The UAC will send the answer in the request to acknowledge the 366 response, i.e. PRACK or ACK request of the reliable response. Other 367 than that, no offer/answer exchanges can occur in the messages 368 within the INVITE transaction. 370 NOTE: The UAS should not include SDP in the responses F6 and 371 F9. However, the UAC should prepare to receive SDP bodies in 372 F6 and/or F9, and just ignore them to handle a peer that does 373 not conform to the recommended implementation. 375 UAC UAS 376 | F1 INVITE (no SDP) | 377 |-------------------->| 378 | F2 1xx | 379 |<--------------------| 380 | | 381 | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP 382 |<--------------------| as the offer. 383 | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel 384 |-------------------->| must contain SDP as the answer. 385 | F5 2xx PRA (no SDP) | - 386 |<--------------------| | 387 | | | 388 | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not 389 |<--------------------| | contain SDP. 390 | F7 PRACK | | 391 |-------------------->| | UAC can send a new offer in an UPDATE 392 | F8 2xx PRA | | request after F4. 393 |<--------------------| v 394 | | 395 | F9 2xx INV (no SDP) | <- The final response should not 396 |<--------------------| contain SDP. 397 | F10 ACK | 398 |-------------------->| 400 Figure 2 Example of Offer/Answer with 100rel Extension (2) 402 Note that in the case that the UAC needs to prompt the user to 403 accept or reject the offer, the reliable provisional response with 404 SDP as an offer (pattern 4) can result in the retransmission until 405 the PRACK request can be sent. The UAC should take care to avoid 406 this situation when it sends the INVITE request without SDP. 408 3.2. Offer/Answer Exchange in Early Dialog 410 When both UAs support the 100rel extension, they can update the 411 session in the early dialog once the first offer/answer exchange 412 has been completed. 414 From a UA sending an INVITE request: 416 A UA can send an UPDATE request with a new offer if both ends 417 support the UPDATE method. Note that if the UAS needs to prompt the 418 user to accept or reject the offer, the delay can result in 419 retransmission of the UPDATE request. 421 A UA can send a PRACK request with a new offer only when 422 acknowledging the reliable provisional response carrying the answer 423 to an offer in the INVITE request. Compared to using the UPDATE 424 method, using PRACK can reduce the number of messages exchanged 425 between the UAs. However, to avoid problems or delays caused by 426 PRACK offer rejection, the UA is recommended to send a PRACK 427 request only when it has strong reasons to expect the receiver will 428 accept it. For example, the procedure used in precondition 429 extension [5] is a case where a PRACK request should be used for 430 updating the session status in an early dialog. Note also that if a 431 UAS needs to prompt the user to accept or reject the offer, the 432 delay can result in retransmission of the PRACK request. 434 From a UA receiving an INVITE request: 436 A UA can send an UPDATE request with a new offer if both ends 437 support the UPDATE method. A UAS can not send a new offer in the 438 reliable provisional response, so the UPDATE method is the only 439 method for a UAS to update an early session. 441 3.3. Offer/Answer Exchange in an Established Dialog 443 Both the re-INVITE and UPDATE methods can be used in an established 444 dialog to update the session. 446 The UPDATE method is simpler and can save at least one message 447 compared with the INVITE method. But both ends must support the 448 UPDATE method for it to be used. 450 The INVITE method needs at least three messages to complete but no 451 extensions are needed. Additionally, the INVITE method allows the 452 peer to take time to decide whether it will accept a session update 453 or not by sending provisional responses. That is, re-INVITE allows 454 the UAS to interact with the user at the peer, while UPDATE needs 455 to be answered automatically by the UAS. It is noted that re-INVITE 456 should be answered immediately unless such a user interaction is 457 needed. Otherwise, some 3pcc flows will break. 459 4. Exceptional Case Handling 461 In RFC 3264 [3], the following restrictions are defined with regard 462 to sending a new offer. 464 "It MUST NOT generate a new offer if it has received an offer 465 which it has not yet answered or rejected. It MUST NOT generate 466 a new offer if it has generated a prior offer for which it has 467 not yet received an answer or a rejection." 469 Assuming that the above rules are guaranteed, there seem to be two 470 possible 'exceptional' cases to be considered in SIP offer/answer 471 usage: the 'message crossing' case, and the 'glare' case. One of 472 the reasons why the usage of SIP methods to exchange offer/answer 473 needs to be carefully restricted in the RFCs is to ensure that the 474 UA can detect and handle appropriately the 'exceptional' cases to 475 avoid incompatible behavior. 477 4.1. Message Crossing Case Handling 479 When message packets crossed in the transport network, an offer may 480 be received before the answer for the previous offer/answer 481 exchange, as described in Figure 3. In such a case, UA A must 482 detect the session description of the offer2 is not the answer to 483 offer1. 485 A B 486 |offer1 | 487 |----------------->| 488 | answer1| 489 |<------\ /-------| 490 | \/ | 491 | /\ offer2| 492 |<------/ \-------| 494 Figure 3 Message Crossing Case 496 When offer2 is in an UPDATE request or a re-INVITE request, a 497 session description cannot be the expected answer. Then UA A must 498 reject the message including offer2 with a 491 response with Retry- 499 After header field. 501 When offer2 is in a PRACK request, that is, when the PRACK request 502 is acknowledging a reliable provisional response with an answer to 503 an offer in an INVITE request containing a session description, UA 504 A knows it is an offer. To avoid rejecting the PRACK or PRACK offer, 505 UA A is recommended to wait for answer1 before sending a PRACK 506 response with the answer to the offer2. Note that if UA A does not 507 send a new offer until the reliable provisional response with an 508 answer to the offer in the INVITE request is acknowledged with a 509 PRACK request, this case never happens. Fortunately, under the 510 current rules, UA A can not send a new offer until the reliable 511 response. 513 When offer2 is in a reliable provisional response or a successful 514 final response (as shown in Figure 4), UA A knows it is not the 515 answer to the offer1. For a reliable response to an initial INVITE 516 request, this case never happens. For a reliable response to a re- 517 INVITE request, UA A can infer that offer2 is not the answer1. In 518 this case, since UA A can not reject offer2 in a reliable response, 519 it is recommended that it wait for answer1 before sending a PRACK 520 request with the answer to offer2. Note that this case only occurs 521 when UA A, while waiting for an answer, sends an INVITE request 522 without session description. 524 Table 3 summarizes the discussions above. 526 offer2 | How to know it's not answer1 | Actions to take 527 -------+------------------------------+-------------------------- 528 INVITE | Never be an answer | 491 response 529 UPDATE | Glare case for UA A | with Retry-After 530 -------+------------------------------+-------------------------- 531 PRACK | This case never happens | - 532 | under the current rules. | 533 -------+------------------------------+-------------------------- 534 1xx-rel| Only one INVITE transaction | Delay ACK/PRACK 535 2xx | at a time. Then UA can know | until answer1 is received 536 -------+------------------------------+-------------------------- 538 NOTE: 1xx-rel/2xx case is extremely rare case and easily avoidable. 539 See Figure 4. 541 Table 3. UA's action to an offer (offer2) overtaking the previous 542 answer (answer1) 543 A B 544 | | 545 |offer1(e.g. UPD) | 546 |==============================>| 547 |re-INV (no offer) | 548 |------------------------------>| --+ 549 | answer1 (2xx-UPD)| | 550 |<===========\ /===============| | The first reliable response 551 | \/ offer2| | 552 | /\ (1xx-rel/2xx)| | 553 |<===========/ \===============| <-+ 554 | answer2 (PRACK/ACK) | 555 |------------------------------>| Wait until answer1 556 | | 558 Figure 4 Reliable response as a message with offer2 in message 559 crossing case 561 4.2. Glare Case Handling 563 When both ends in a dialog send a new offer at nearly the same time, 564 as described in Figure 5, a UA may receive a new offer before it 565 receives the answer to the offer it sent. This case is usually 566 called a 'glare' case. 568 A B 569 |offer1 offer2| 570 |-------\ /-------| 571 | \/ | 572 | /\ | 573 |<------/ \------>| 575 Figure 5 Glare Case 577 When offer2 is in an UPDATE request or (re-)INVITE request, it must 578 be rejected with a 491 response. 580 When offer2 is in a PRACK request (within the current rules, only 581 possible if offer1 is in an UPDATE request), the PRACK may be 582 accepted with 200 or may be rejected with a 491 response. A 491 583 response is valid to satisfy the offer/answer model but it may 584 delay the completion of the reliable response transfer mechanism or, 585 in worst case, may result in the failure to complete the SIP 586 transaction because there is no clear retry rule when a PRACK 587 request is rejected with a 491 response. To avoid this glare 588 condition, UA A should not send an offer if it has already sent a 589 reliable provisional response containing an answer to a previous 590 offer and has not received the corresponding PRACK request. 592 To avoid a glare condition involving an offer in a response, when 593 UA A has sent a (re)INVITE request without session description, it 594 should not send an offer until it has received an offer in a 595 reliable response to the (re)INVITE, and sent an answer to that 596 offer. 598 5. Content of Offers and Answers 600 While RFCs 3264[3] and 3312[5] give some guidance, questions remain 601 about exactly what should be included in an offer or answer. This 602 is especially a problem when the common "hold" feature has been 603 activated, and when there is the potential for a multimedia call. 605 Details of behavior depend on the capabilities and state of the 606 User Agent. The kinds of recommendations that can be made are 607 limited by the model of device capabilities and state that is 608 presumed to exist. 610 This section focuses on a few key aspects of offers and answers 611 that have been identified as troublesome, and will consider other 612 aspects to be out of scope. This section considers: 614 - choice of supported media types and formats to include and 615 exclude 617 - hold and resume of media 619 The following are out of scope for this document: 621 - NAT traversal and ICE 623 - specific codecs and their parameters 625 - the negotiation of secure media streams 627 - grouping of media streams 629 - preconditions 631 5.1. General Principle for Constructing Offers and Answers 633 A UA should send an offer that indicates what it, and its user, are 634 interested in using/doing at that time, without regard for what the 635 other party in the call may have indicated previously. This is the 636 case even when the offer is sent in response to an INVITE or re- 637 INVITE that contains no offer. (However in the case of re-INVITE 638 the constraints of RFCs 3261 and 3264 must be observed.) 640 A UA should send an answer that includes as close an approximation 641 to what the UA and its user are interested in doing at that time, 642 while remaining consistent with the offer/answer rules of RFC 643 3264[3] and other RFCs. 645 NOTE: "at that time" is important. The device may permit the 646 user to configure which supported media are to be used by 647 default. 649 In some cases a UA may not have direct knowledge of what it is 650 interested in doing at a particular time. If it is an intermediary 651 it may be able to delegate the decision. In the worst case it may 652 apply a default, such as assuming it wants to use all of its 653 capabilities. 655 5.2. Choice of Media Types and Formats to Include and Exclude 657 5.2.1. Sending an Initial INVITE with Offer 659 When a UAC sends an initial INVITE with an offer, it has complete 660 freedom to choose which media type(s) and media format(s) (payload 661 types in the case of RTP) it should include in the offer. 663 The media types may be all or a subset of the media the UAC is 664 capable of supporting, with the particular subset being determined 665 by the design and configuration [6] of the UAC combined with input 666 from the user interface of the UAC. 668 The media formats may be all or a subset of the media formats the 669 UAC is capable of supporting for the corresponding media type, with 670 the particular subset being determined by the design and 671 configuration [6] of the UAC combined with input from the user 672 interface of the UAC. 674 Including all supported media formats will maximize the possibility 675 that the other party will have a supported format in common. But 676 including many can result in an unacceptably large SDP body. 678 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 680 When a UAS has received an initial INVITE without an offer, it must 681 include an offer in the first reliable response to the INVITE. It 682 has largely the same options as when sending an initial INVITE with 683 an offer, but there are some differences. The choice may be 684 governed by both static (default) selections of media types as well 685 as dynamic selections made by a user via interaction with the 686 device while it is alerting. 688 NOTE: The offer may be sent in a provisional response, before 689 the user of the device has been alerted and had an opportunity 690 to select media options for the call. In this case the UAS 691 cannot include any call-specific options from the user of the 692 device. If there is a possibility that the user of the device 693 will wish to change what is offered before answering the call, 694 then special care should be taken. If PRACK and UPDATE are 695 supported by caller and callee then an initial offer can be 696 sent reliably, and changed with an UPDATE if the user desires 697 a change. If PRACK and UPDATE are not supported then the 698 initial offer cannot be changed until the call is fully 699 established. In that case either the offer should be delayed 700 until the 200 is sent, or else the offer should include the 701 minimum set of media the user is able to select. 703 5.2.3. Answering an Initial INVITE with Offer 705 When a UAS receives an initial INVITE with an offer, what media 706 lines the answer may contain is constrained by RFC 3264.[3] The 707 answer must contain the same number of m-lines as the offer, and 708 they must contain the same media types. Each media line may be 709 accepted, by including a non-zero port number, or rejected by 710 including a zero port number in the answer. The media lines that 711 are accepted should typically be those that would have been offered 712 had the INVITE not contained an offer, excluding those not offered. 714 The media formats the answer may contain are constrained by RFC 715 3264 [3]. For each accepted m-line in the answer, there must be at 716 least one media format in common with the corresponding m-line of 717 the offer. The UAS may also include other media formats it is able 718 to support at this time. However there is little benefit to 719 including added types. 721 If the UAS does not wish to indicate support for any of the media 722 types in a particular media line of the offer it must reject the 723 corresponding media line, by setting the port number to zero. 725 5.2.4. Answering when the Initial INVITE had no Offer 727 When a UAC has sent an initial INVITE without an offer, and then 728 receives a response with the first offer, it should answer in the 729 same way as a UAS receiving an initial INVITE with an offer. 731 5.2.5. Subsequent Offers and Answers 733 The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) 734 apply, but constraints in RFC 3264 [3] must also be followed. The 735 following are of particular note because they have proven 736 troublesome: 738 o The number of m-lines may not be reduced in a subsequent offer. 739 Previously rejected media streams must remain, or be reused to 740 offer the same or a different stream. (RFC 3264[3] section 6.) 742 o In the o-line, only the version number may change, and if it 743 changes it must increment by one from the one previously sent as 744 an offer or answer. (RFC 3264[3] section 8.) If it doesn't 745 change then the entire SDP body must be identical to what was 746 previously sent as an offer or answer. Changing the o-line, 747 except version number value, during the session is an error case. 748 The behavior when receiving such a non-compliant offer/answer 749 SDP body is implementation dependent. If a UA needs to negotiate 750 a 'new' SDP session, it should use the INVITE/Replaces method. 752 o In the case of RTP, the mapping from a particular dynamic 753 payload type number to a particular codec within that media 754 stream (m-line) must not change for the duration of the session. 755 (RFC 3264[3] section 8.3.2.) 757 NOTE: This may be impossible for a B2BUA to follow in some 758 cases (e.g. 3pcc transfer) if it does not terminate media. 760 When the new offer is sent in response to an offerless (re)INVITE, 761 all codecs supported by the UA are to be included, not just the 762 ones that were negotiated by previous offer/answer exchanges. The 763 same is true for media types - so if UA A initially offered audio 764 and video to UA B, and they end up with only audio, and UA B sends 765 an offerless (re)INVITE to UA A, A's resulting offer should re- 766 attempt video, by reusing the zeroed m-line used previously. 768 NOTE: The behavior above is recommended, but it is not always 769 achievable - for example in some interworking scenarios. Or, 770 the offerer may simply not have enough resources to offer 771 "everything" at that point. Even if the UAS is not able to 772 offer any other SDP that the one currently being used, it 773 should not reject the re-INVITE. Instead, it should generate 774 an offer with the currently used SDP with o- line unchanged. 776 5.3. Hold and Resume of media 778 RFC 3264 [3] specifies (non-normatively) that "hold" should be 779 indicated in an established session by sending a new offer 780 containing "a=sendonly" for each media stream to be held. An 781 answerer is then to respond with "a=recvonly" to acknowledge that 782 the hold request has been understood. 784 Note that the use of sendonly/recvonly is not limited to hold. 785 These may be used for other reasons, such as devices that are only 786 capable of sending or receiving. So receiving an offer with 787 "a=sendonly" must not be treated as a certain indication that the 788 offerer has placed the media stream on hold. 790 This model is based on an assumption that the UA initiating the 791 hold will want to play Music on Hold, which is not always the case. 792 A UA may, if desired, initiate hold by offering "a=inactive" if it 793 does not intend to transmit any media while in hold status. 795 The rules of RFC 3264 [3] constrain what may be in an answer when 796 the offer contains "sendonly", "recvonly", or "inactive" in an a= 797 line. But they do not constrain what must be in a subsequent offer. 798 The General Principle for Constructing Offers and Answers (section 799 5.1. ) is important here. The initiation of "hold" is a local 800 action. It should reflect the desired state of the UA. It then 801 affects what the UA includes in offers and answers until the local 802 state is reset. 804 The receipt of an offer containing "a=sendonly" or "a=inactive" and 805 the sending of a compatible answer should not change the desired 806 state of the recipient. However, a UA that has been "placed on 807 hold" may itself desire to initiate its own hold status, based on 808 local input. 810 If UA2 has previously been "placed on hold" by UA1, via receipt of 811 "a=sendonly", then it may initiate its own hold by sending a new 812 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 813 will answer with "a=inactive" because that is the only valid answer 814 that reflects its desire not to receive media. 816 Once in this state, to resume a two way exchange of media each side 817 must reset its local hold status. If UA1 is first to go off hold it 818 will then send an offer with "a=sendrecv". The UA2 will respond 819 with its desired state of "a=sendonly" because that is a permitted 820 response. When UA2 desires to also resume, it will send an offer 821 with "a=sendrecv". In this case, because UA1 has the same desire it 822 will respond with "a=sendrecv". In the same case, when UA2 receives 823 the offer with "a=sendrecv", if it has decided it wants to reset 824 its local hold but has not yet signaled the intent, it may send 825 "a=sendrecv" in the answer. 827 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 828 and subsequently wants to initiate its own hold, also using 829 "a=inactive", it need not send a new offer, since the only valid 830 response is "a=inactive" and that is already in effect. However, 831 its local desired state will now be either "inactive" or 832 "a=sendonly". This affects what it will send in future offers and 833 answers. 835 If a UA has occasion to send another offer in the session, without 836 any desire to change the hold status (e.g. in response to a re- 837 INVITE without an offer, or when sending a re-INVITE to refresh the 838 session timer) it should follow the General Principle for 839 Constructing Offers and Answers (section 5.1. ). If it previously 840 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 841 should offer that again. If it had not previously initiated "hold" 842 then it should offer "a=sendrecv", even if it had previously been 843 forced to answer something else. Without this behavior it is 844 possible to get "stuck on hold" in some cases, especially when a 845 third-party call controller is involved. 847 5.4. Behavior on receiving SDP with c=0.0.0.0 849 RFC 3264[3] specifies that An agent MUST be capable of receiving 850 SDP with a connection address of 0.0.0.0, in which case it means 851 that neither RTP nor RTCP should be sent to the peer. 853 If a UA generates an answer to the offer received with c=0.0.0.0, 854 the direction attribute of the accepted media stream in the answer 855 must be based on direction attribute of the offered stream and 856 rules specified in RFC 3264 to form the a-line in the answer. 857 c=0.0.0.0 has no special meaning for the direction attribute of the 858 accepted stream in the answer. 860 6. Remaining Issues or Best Practices on Offer/Answer 862 This document clarifies the offer/answer usage in SIP and 863 summarizes the correct or recommended behaviors along with the 864 existing RFCs. To create any new normative behaviors beyond these 865 RFCs is not the intent of this document. 867 However, through the scrutiny of the offer/answer model in SIP, 868 some issues are found to be unresolved within the current set of 869 RFCs. Those remaining issues are described in this section mainly 870 for further study. 872 6.1. Rejecting PRACK Offer 874 As stated in section 2.2. and 3.2. , it is recommended that an 875 offer not be sent in a PRACK request unless UAC has strong reasons 876 to assume the receiver will accept it. Even so, there may be cases 877 when the UAS has to reject the offer for some reason. The current 878 RFCs do not provide a way to reject the offer and at the same time 879 to indicate that the PRACK adequately acknowledged the reliable 880 response. It is unclear whether a non-200 response can still 881 indicate an acknowledgement of the reliable response. 883 Several ideas were presented to resolve this issue, such as sending 884 2xx PRACK response without SDP to reject the offer, or sending SDP 885 with a decreased version value in the o-line. Some of the 886 candidates may also be adapted as a way to reject an unacceptable 887 offer in a response. Anyway, those proposals violate the current 888 rules and lose backward compatibility to some extent (e.g. section 889 5 of RFC 3262). It is beyond the scope of this document and remains 890 for further study. 892 The 488 response is another proposed solution; however the validity 893 and consequences of a 488 response to PRACK is an open issue. 894 Because the 488 response may be sent by a proxy, the UAC cannot 895 assume the reliable transaction has been adequately acknowledged. 896 If a 488 response is received, the UAC should ensure acknowledgment 897 of the reliable response by sending a new PRACK with the offer 898 removed or modified based upon the received 488 response. If the 899 488 response is sent by UAS (open issue), it cannot assume that the 900 UAC thinks that the reliable transaction has been adequately 901 acknowledged even though the UAS may treat otherwise (open issue). 902 If a 488 response is sent by UAS, the UAC should accommodate 903 receiving the altered PRACK with higher CSeq without expecting it 904 to trigger a 481 response (open issue). 906 NOTE: Deprecation of the usage of offer in PRACK may be 907 another solution. As the precondition mechanism specification 908 [2] explicitly shows a usage of sending offer in PRACK, its 909 deprecation could cause backward compatibility issues. 911 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 912 Transaction 914 When a re-INVITE transaction fails, the dialog remains with the 915 session bound to it. The issue here is: what is the session status 916 if an offer/answer exchange has been completed (if a session 917 description has been sent in a reliable provisional response to the 918 re-INVITE request), or if subsequent offer/answer exchanges have 919 taken place (using UPDATE or PRACK transactions), before the re- 920 INVITE transaction is terminated with a final error response 921 (Figure 6). One option is to take those offer/answer exchanges not 922 committed yet and to make the session status rollback to the one 923 before re-INVITE transaction was initiated. Another option is to 924 take those exchanges committed and to keep the session status as it 925 is even after re-INVITE fails. There is no clear consensus on which 926 one is the correct behavior. 928 There are some cases where it is useful to exchange 929 offer(s)/answer(s) even before re-INVITE completes. The case of 930 adding a new media (like adding video to audio only session) which 931 requires permission from the peer through some user interaction is 932 one example. Precondition procedures can be another case which may 933 require several offer/answer exchanges in one re-INVITE transaction. 935 UAC UAS 936 | session established | 937 |<===================>| 938 | | 939 | F1 re-INVITE (SDP) | 940 |-------------------->| 941 | F2 1xx-rel (SDP) | 942 |<--------------------| 943 | F3 PRACK | <- PRACK request may include new offer 944 |-------------------->| and can complete the offer/answer with 945 | F4 2xx PRA | the answer in 2xx PRACK response. 946 |<--------------------| 947 | | <- UPDATE method can update the session 948 | | status before receiving the final 949 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 950 |<--------------------| 951 | F6 ACK | 952 |-------------------->| Issue: What is the correct session 953 | | status after re-INVITE transaction. 955 Figure 6 Commit/Rollback Issue with re-INVITE transaction 957 To make bad things worse, if a new offer from UAC and the final 958 response to re-INVITE are sent at nearly the same time, the UAS can 959 not know whether this new offer was sent before or after UAC 960 received the final failure response (Figure 7). Note that the ACK 961 request to the failure response is sent hop-by-hop basis, therefore 962 even after receiving the ACK request, UAS can not make sure that 963 UPDATE request was sent after the final response had been reached 964 to the other end. 966 Sending a new UPDATE request from UAC to synchronize the status 967 anytime after the re-INVITE fails may be a good option. This 968 solution, however, requires that the UPDATE method be supported by 969 both ends and needs care to avoid flapping when each end tries to 970 advertise their different views of the session status. 972 The proper handling of this issue is undefined by existing 973 standards. Resolution is beyond the scope of this document, and 974 will require a new normative document. 976 UAC UAS 977 | session established | 978 |<===================>| 979 | | 980 | F1 re-INVITE (SDP) | 981 |-------------------->| 982 | F2 1xx-rel (SDP) | 983 |<--------------------| 984 | F3 PRACK | 985 |-------------------->| 986 | F4 2xx PRA | 987 |<--------------------| 988 | | 989 |UPDATE(SDP) 4xx INV | 990 |---------\ /--------| 991 | \/ | 992 | /\ | 993 |<--------/ \------->| 994 | | 996 Figure 7 Commit/Rollback Issue with Race Condition 998 6.3. Offer in a Reliable Response 1000 In RFC 3261, it is stated that when an INVITE is sent without an 1001 offer, the first reliable response MUST contain an offer. There was 1002 discussion on whether this rule can be loosened up. There is no 1003 clear explanation why this restriction is defined. However, this 1004 rule will be left as it is, unless the strong necessity to loosen 1005 it up is raised in the future. 1007 6.4. Requesting Hold while already on Hold 1009 RFC 3264, section 8.4, contains procedures for putting a unicast 1010 media stream on hold. Of particular note, it states: 1012 "If the stream to be placed on hold was previously a recvonly 1013 media stream, it is placed on hold by marking it inactive." 1015 Section 5.3. of the current document makes a best practice 1016 recommentation for this case which conflicts with that, and 1017 explains why. Some concerns have been raised that such a 1018 recommendation is invalid because RFC 3264 is normative on this 1019 subject. 1021 This document takes the position that Section 8.4 of RFC 3264 is 1022 non-normative, and so may be overridden. It is further recommended 1023 that RFC 3264 be revised to avoid the confusion. 1025 7. Add New Offer/Answer Usage in SIP 1027 This document recommends against the addition of new offer/answer 1028 methods using SIP. However, it may be necessary to define new 1029 offer/answer exchange methods as SIP extensions evolve. This 1030 section recommends some things that should be taken into 1031 considerations in that case. 1033 7.1. Explicit Usage 1035 New method definitions should define offer/answer usage explicitly 1036 without any ambiguity. 1038 7.2. Rejection of an Offer 1040 New method definitions should define how to reject an offer where 1041 possible. 1043 7.3. Backward Compatibility 1045 New methods must keep backward compatibility. 1047 7.4. Exceptional Case Handling 1049 New methods should take care of how to handle exceptional cases, 1050 message crossing case and glare case. 1052 8. IANA Considerations 1054 This document has no actions for IANA. 1056 9. Security Considerations 1058 There are not any security issues beyond the referenced RFCs. 1060 10. Acknowledgement 1062 The authors would like to thank Christer Holmberg, Rajeev Seth, 1063 Nataraju A B, Byron Campen and Jonathan Rosenberg for their 1064 thorough reviews and comments. Many of their suggestions and ideas 1065 are incorporated to complete this document. 1067 11. References 1069 11.1. Normative References 1071 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1072 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1073 Session Initiation Protocol", RFC 3261, June 2002. 1075 [2] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional 1076 Responses in the Session Initiation Protocol (SIP)", RFC 3262, 1077 June 2002. 1079 [3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1080 SDP", RFC 3264, June 2002. 1082 [4] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE 1083 Method", RFC 3311, September 2002. 1085 [5] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration 1086 of Resource Management and Session Initiation Protocol (SIP)", 1087 RFC 3312, October 2002. 1089 11.2. Informative References 1091 [6] G. Camarillo, "The Early Session Disposition Type for the 1092 Session Initiation Protocol (SIP)", RFC 3959, December 2004. 1094 [7] Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent 1095 Profile Data Set for Media Policy", draft-ietf-sipping-media- 1096 policy-dataset-05 (work in progress), November 2007. 1098 Author's Addresses 1100 Takuya Sawada 1101 KDDI Corporation 1102 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 1104 Email: tu-sawada@kddi.com 1106 Paul H. Kyzivat 1107 Cisco Systems, Inc. 1108 1414 Massachusetts Avenue 1109 Boxborough, MA 01719 1110 USA 1112 Email: pkyzivat@cisco.com 1114 Full Copyright Statement 1116 Copyright (C) The IETF Trust (2008). 1118 This document is subject to the rights, licenses and restrictions 1119 contained in BCP 78, and except as set forth therein, the authors 1120 retain all their rights. 1122 This document and the information contained herein are provided on 1123 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 1124 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 1125 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 1126 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 1127 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 1128 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1129 FOR A PARTICULAR PURPOSE. 1131 Intellectual Property Statement 1133 The IETF takes no position regarding the validity or scope of any 1134 Intellectual Property Rights or other rights that might be claimed 1135 to pertain to the implementation or use of the technology described 1136 in this document or the extent to which any license under such 1137 rights might or might not be available; nor does it represent that 1138 it has made any independent effort to identify any such rights. 1139 Information on the procedures with respect to rights in RFC 1140 documents can be found in BCP 78 and BCP 79. 1142 Copies of IPR disclosures made to the IETF Secretariat and any 1143 assurances of licenses to be made available, or the result of an 1144 attempt made to obtain a general license or permission for the use 1145 of such proprietary rights by implementers or users of this 1146 specification can be obtained from the IETF on-line IPR repository 1147 at 1148 http://www.ietf.org/ipr. 1150 The IETF invites any interested party to bring to its attention any 1151 copyrights, patents or patent applications, or other proprietary 1152 rights that may cover technology that may be required to implement 1153 this standard. 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