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Kyzivat 5 Expires: August 2008 Cisco Systems, Inc. 6 February 25, 2008 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-06.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 25, 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.....................................14 63 5. Content of Offers and Answers................................15 64 5.1. General Principle for Constructing Offers and Answers...15 65 5.2. Choice of Media Types and Formats to Include and Exclude16 66 5.2.1. Sending an Initial INVITE with Offer...............16 67 5.2.2. Responding with an Offer when the Initial INVITE has 68 no Offer..................................................16 69 5.2.3. Answering an Initial INVITE with Offer.............17 70 5.2.4. Answering when the Initial INVITE had no Offer.....18 71 5.2.5. Subsequent Offers and Answers......................18 72 5.3. Hold and Resume of media................................19 73 5.4. Behavior on receiving SDP with c=0.0.0.0................20 74 6. Remaining Issues or Best Practices on Offer/Answer...........20 75 6.1. Rejecting PRACK Offer...................................21 76 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 77 Transaction..................................................22 78 6.3. Offer in a Reliable Response............................23 79 6.4. Requesting Hold while already on Hold...................24 80 7. Add New Offer/Answer Usage in SIP............................24 81 7.1. Explicit Usage..........................................24 82 7.2. Rejection of an Offer...................................24 83 7.3. Backward Compatibility..................................24 84 7.4. Exceptional Case Handling...............................24 85 8. IANA Considerations..........................................25 86 9. Security Considerations......................................25 87 10. References..................................................25 88 10.1. Normative References...................................25 89 10.2. Informative References.................................25 90 Author's Addresses..............................................26 91 Full Copyright Statement........................................26 92 Intellectual Property Statement.................................26 93 Acknowledgment..................................................27 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(as shown in Figure 4), that is, 502 when the PRACK request is acknowledging a reliable provisional 503 response with an answer to an offer in an INVITE request containing 504 a session description, UA A knows it is an offer. To avoid 505 rejecting the PRACK or PRACK offer, UA A is recommended to wait for 506 answer1 before sending a PRACK response with the answer to the 507 offer2. Note that if UA A does not send a new offer until the 508 reliable provisional response with an answer to the offer in the 509 INVITE request is acknowledged with a PRACK request, this case 510 never happens. Therefore, to simplify implementations, a UA acting 511 as a UAS for an INVITE transaction is recommended not to defer 512 sending an UPDATE request with an offer until after the reliable 513 response with an answer to the offer in the INVITE request is 514 acknowledged with a PRACK request. 516 When offer2 is in a reliable provisional response or a successful 517 final response (as shown in Figure 5), UA A knows it is not the 518 answer to the offer1. For a reliable response to an initial INVITE 519 request, this case never happens. For a reliable response to a re- 520 INVITE request, UA A can infer that offer2 is not the answer1. In 521 this case, since UA A can not reject offer2 in a reliable response, 522 it is recommended that it wait for answer1 before sending a PRACK 523 request with the answer to offer2. Note that this case only occurs 524 when UA A, while waiting for an answer, sends an INVITE request 525 without session description. 527 Table 3 summarizes the discussions above. 529 offer2 | How to know it's not answer1 | Actions to take 530 -------+------------------------------+-------------------------- 531 INVITE | Never be an answer | 491 response 532 UPDATE | Glare case for UA A | with Retry-After 533 -------+------------------------------+-------------------------- 534 PRACK | Not a pattern 4. in Table 1. | Delay sending response 535 | 1xx-rel must have an answer, | until answer1 is received 536 | not an offer. | 537 -------+------------------------------+-------------------------- 538 1xx-rel| Only one INVITE transaction | Delay ACK/PRACK 539 2xx | at a time. Then UA can know | until answer1 is received 540 -------+------------------------------+-------------------------- 542 NOTE: PRACK and 1xx-rel/2xx case is extremely rare case and easily 543 avoidable. See Figure 4 and Figure 5. 545 Table 3. UA's action to an offer (offer2) overtaking the previous 546 answer (answer1) 548 A B 549 | | 550 | INV (offer0)| 551 |<------------------------------| 552 | 1xx-rel (answer0) | 553 |------------------------------>| --+ 554 |offer1(e.g. UPD) | | 555 |==============================>| | 556 | answer1 (2xx-UPD)| | Acknowledge 557 |<===========\ /===============| | 558 | \/ | | 559 | /\ offer2(PRA)| | 560 |<===========/ \===============| <-+ 561 | answer2 (2xx-PRA) | 562 |------------------------------>| Wait until answer1 563 | | 565 Figure 4 PRACK as a message with offer2 in message crossing case 567 A B 568 | | 569 |offer1(e.g. UPD) | 570 |==============================>| 571 |re-INV (no offer) | 572 |------------------------------>| --+ 573 | answer1 (2xx-UPD)| | 574 |<===========\ /===============| | The first reliable response 575 | \/ offer2| | 576 | /\ (1xx-rel/2xx)| | 577 |<===========/ \===============| <-+ 578 | answer2 (PRACK/ACK) | 579 |------------------------------>| Wait until answer1 580 | | 582 Figure 5 Reliable response as a message with offer2 in message 583 crossing case 585 4.2. Glare Case Handling 587 When both ends in a dialog send a new offer at nearly the same time, 588 as described in Figure 6, a UA may receive a new offer before it 589 receives the answer to the offer it sent. This case is usually 590 called a 'glare' case. 592 A B 593 |offer1 offer2| 594 |-------\ /-------| 595 | \/ | 596 | /\ | 597 |<------/ \------>| 599 Figure 6 Glare Case 601 When offer2 is in an UPDATE request or (re-)INVITE request, it must 602 be rejected with a 491 response. 604 When offer2 is in a PRACK request (within the current rules, only 605 possible if offer1 is in an UPDATE request), the PRACK may be 606 accepted with 200 or may be rejected with a 491 response. A 491 607 response is valid to satisfy the offer/answer model but it may 608 delay the completion of the reliable response transfer mechanism or, 609 in worst case, may result in the failure to complete the SIP 610 transaction because there is no clear retry rule when a PRACK 611 request is rejected with a 491 response. To avoid this glare 612 condition, UA A should not send an offer if it has already sent a 613 reliable provisional response containing an answer to a previous 614 offer and has not received the corresponding PRACK request. 616 To avoid a glare condition involving an offer in a response, when 617 UA A has sent a (re)INVITE request without session description, it 618 should not send an offer until it has received an offer in a 619 reliable response to the (re)INVITE, and sent an answer to that 620 offer. 622 5. Content of Offers and Answers 624 While RFCs 3264[3] and 3312[5] give some guidance, questions remain 625 about exactly what should be included in an offer or answer. This 626 is especially a problem when the common "hold" feature has been 627 activated, and when there is the potential for a multimedia call. 629 Details of behavior depend on the capabilities and state of the 630 User Agent. The kinds of recommendations that can be made are 631 limited by the model of device capabilities and state that is 632 presumed to exist. 634 This section focuses on a few key aspects of offers and answers 635 that have been identified as troublesome, and will consider other 636 aspects to be out of scope. This section considers: 638 - choice of supported media types and formats to include and 639 exclude 641 - hold and resume of media 643 The following are out of scope for this document: 645 - NAT traversal and ICE 647 - specific codecs and their parameters 649 - the negotiation of secure media streams 651 - grouping of media streams 653 - preconditions 655 5.1. General Principle for Constructing Offers and Answers 657 A UA should send an offer that indicates what it, and its user, are 658 interested in using/doing at that time, without regard for what the 659 other party in the call may have indicated previously. This is the 660 case even when the offer is sent in response to an INVITE or re- 661 INVITE that contains no offer. (However in the case of re-INVITE 662 the constraints of RFCs 3261 and 3264 must be observed.) 664 A UA should send an answer that includes as close an approximation 665 to what the UA and its user are interested in doing at that time, 666 while remaining consistent with the offer/answer rules of RFC 667 3264[3] and other RFCs. 669 NOTE: "at that time" is important. The device may permit the 670 user to configure which supported media are to be used by 671 default. 673 In some cases a UA may not have direct knowledge of what it is 674 interested in doing at a particular time. If it is an intermediary 675 it may be able to delegate the decision. In the worst case it may 676 apply a default, such as assuming it wants to use all of its 677 capabilities. 679 5.2. Choice of Media Types and Formats to Include and Exclude 681 5.2.1. Sending an Initial INVITE with Offer 683 When a UAC sends an initial INVITE with an offer, it has complete 684 freedom to choose which media type(s) and media format(s) (payload 685 types in the case of RTP) it should include in the offer. 687 The media types may be all or a subset of the media the UAC is 688 capable of supporting, with the particular subset being determined 689 by the design and configuration [6] of the UAC combined with input 690 from the user interface of the UAC. 692 The media formats may be all or a subset of the media formats the 693 UAC is capable of supporting for the corresponding media type, with 694 the particular subset being determined by the design and 695 configuration [6] of the UAC combined with input from the user 696 interface of the UAC. 698 Including all supported media formats will maximize the possibility 699 that the other party will have a supported format in common. But 700 including many can result in an unacceptably large SDP body. 702 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 704 When a UAS has received an initial INVITE without an offer, it must 705 include an offer in the first reliable response to the INVITE. It 706 has largely the same options as when sending an initial INVITE with 707 an offer, but there are some differences. The choice may be 708 governed by both static (default) selections of media types as well 709 as dynamic selections made by a user via interaction with the 710 device while it is alerting. 712 NOTE: The offer may be sent in a provisional response, before 713 the user of the device has been alerted and had an opportunity 714 to select media options for the call. In this case the UAS 715 cannot include any call-specific options from the user of the 716 device. If there is a possibility that the user of the device 717 will wish to change what is offered before answering the call, 718 then special care should be taken. If PRACK and UPDATE are 719 supported by caller and callee then an initial offer can be 720 sent reliably, and changed with an UPDATE if the user desires 721 a change. If PRACK and UPDATE are not supported then the 722 initial offer cannot be changed until the call is fully 723 established. In that case either the offer should be delayed 724 until the 200 is sent, or else the offer should include the 725 minimum set of media the user is able to select. 727 5.2.3. Answering an Initial INVITE with Offer 729 When a UAS receives an initial INVITE with an offer, what media 730 lines the answer may contain is constrained by RFC 3264.[3] The 731 answer must contain the same number of m-lines as the offer, and 732 they must contain the same media types. Each media line may be 733 accepted, by including a non-zero port number, or rejected by 734 including a zero port number in the answer. The media lines that 735 are accepted should typically be those that would have been offered 736 had the INVITE not contained an offer, excluding those not offered. 738 The media formats the answer may contain are constrained by RFC 739 3264 [3]. For each accepted m-line in the answer, there must be at 740 least one media format in common with the corresponding m-line of 741 the offer. The UAS may also include other media formats it is able 742 to support at this time. However there is little benefit to 743 including added types. 745 If the UAS does not wish to indicate support for any of the media 746 types in a particular media line of the offer it must reject the 747 corresponding media line, by setting the port number to zero. 749 5.2.4. Answering when the Initial INVITE had no Offer 751 When a UAC has sent an initial INVITE without an offer, and then 752 receives a response with the first offer, it should answer in the 753 same way as a UAS receiving an initial INVITE with an offer. 755 5.2.5. Subsequent Offers and Answers 757 The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) 758 apply, but constraints in RFC 3264 [3] must also be followed. The 759 following are of particular note because they have proven 760 troublesome: 762 o The number of m-lines may not be reduced in a subsequent offer. 763 Previously rejected media streams must remain, or be reused to 764 offer the same or a different stream. (RFC 3264[3] section 6.) 766 o In the o-line, only the version number may change, and if it 767 changes it must increment by one from the one previously sent as 768 an offer or answer. (RFC 3264[3] section 8.) If it doesn't 769 change then the entire SDP body must be identical to what was 770 previously sent as an offer or answer. Changing the o-line, 771 except version number value, during the session is an error case. 772 The behavior when receiving such a non-compliant offer/answer 773 SDP body is implementation dependent. If a UA needs to negotiate 774 a 'new' SDP session, it should use the INVITE/Replaces method. 776 o In the case of RTP, the mapping from a particular dynamic 777 payload type number to a particular codec within that media 778 stream (m-line) must not change for the duration of the session. 779 (RFC 3264[3] section 8.3.2.) 781 NOTE: This may be impossible for a B2BUA to follow in some 782 cases (e.g. 3pcc transfer) if it does not terminate media. 784 When the new offer is sent in response to an offerless (re)INVITE, 785 all codecs supported by the UA are to be included, not just the 786 ones that were negotiated by previous offer/answer exchanges. The 787 same is true for media types - so if UA A initially offered audio 788 and video to UA B, and they end up with only audio, and UA B sends 789 an offerless (re)INVITE to UA A, A's resulting offer should re- 790 attempt video, by reusing the zeroed m-line used previously. 792 NOTE: The behavior above is recommended, but it is not always 793 achievable - for example in some interworking scenarios. Or, 794 the offerer may simply not have enough resources to offer 795 "everything" at that point. Even if the UAS is not able to 796 offer any other SDP that the one currently being used, it 797 should not reject the re-INVITE. Instead, it should generate 798 an offer with the currently used SDP with o- line unchanged. 800 5.3. Hold and Resume of media 802 RFC 3264 [3] specifies (non-normatively) that "hold" should be 803 indicated in an established session by sending a new offer 804 containing "a=sendonly" for each media stream to be held. An 805 answerer is then to respond with "a=recvonly" to acknowledge that 806 the hold request has been understood. 808 Note that the use of sendonly/recvonly is not limited to hold. 809 These may be used for other reasons, such as devices that are only 810 capable of sending or receiving. So receiving an offer with 811 "a=sendonly" must not be treated as a certain indication that the 812 offerer has placed the media stream on hold. 814 This model is based on an assumption that the UA initiating the 815 hold will want to play Music on Hold, which is not always the case. 816 A UA may, if desired, initiate hold by offering "a=inactive" if it 817 does not intend to transmit any media while in hold status. 819 The rules of RFC 3264 [3] constrain what may be in an answer when 820 the offer contains "sendonly", "recvonly", or "inactive" in an a= 821 line. But they do not constrain what must be in a subsequent offer. 822 The General Principle for Constructing Offers and Answers (section 823 5.1. ) is important here. The initiation of "hold" is a local 824 action. It should reflect the desired state of the UA. It then 825 affects what the UA includes in offers and answers until the local 826 state is reset. 828 The receipt of an offer containing "a=sendonly" or "a=inactive" and 829 the sending of a compatible answer should not change the desired 830 state of the recipient. However, a UA that has been "placed on 831 hold" may itself desire to initiate its own hold status, based on 832 local input. 834 If UA2 has previously been "placed on hold" by UA1, via receipt of 835 "a=sendonly", then it may initiate its own hold by sending a new 836 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 837 will answer with "a=inactive" because that is the only valid answer 838 that reflects its desire not to receive media. 840 Once in this state, to resume a two way exchange of media each side 841 must reset its local hold status. If UA1 is first to go off hold it 842 will then send an offer with "a=sendrecv". The UA2 will respond 843 with its desired state of "a=sendonly" because that is a permitted 844 response. When UA2 desires to also resume, it will send an offer 845 with "a=sendrecv". In this case, because UA1 has the same desire it 846 will respond with "a=sendrecv". In the same case, when UA2 receives 847 the offer with "a=sendrecv", if it has decided it wants to reset 848 its local hold but has not yet signaled the intent, it may send 849 "a=sendrecv" in the answer. 851 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 852 and subsequently wants to initiate its own hold, also using 853 "a=inactive", it need not send a new offer, since the only valid 854 response is "a=inactive" and that is already in effect. However, 855 its local desired state will now be either "inactive" or 856 "a=sendonly". This affects what it will send in future offers and 857 answers. 859 If a UA has occasion to send another offer in the session, without 860 any desire to change the hold status (e.g. in response to a re- 861 INVITE without an offer, or when sending a re-INVITE to refresh the 862 session timer) it should follow the General Principle for 863 Constructing Offers and Answers (section 5.1. ). If it previously 864 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 865 should offer that again. If it had not previously initiated "hold" 866 then it should offer "a=sendrecv", even if it had previously been 867 forced to answer something else. Without this behavior it is 868 possible to get "stuck on hold" in some cases, especially when a 869 third-party call controller is involved. 871 5.4. Behavior on receiving SDP with c=0.0.0.0 873 RFC 3264[3] specifies that An agent MUST be capable of receiving 874 SDP with a connection address of 0.0.0.0, in which case it means 875 that neither RTP nor RTCP should be sent to the peer. 877 If a UA generates an answer to the offer received with c=0.0.0.0, 878 the direction attribute of the accepted media stream in the answer 879 must be based on direction attribute of the offered stream and 880 rules specified in RFC 3264 to form the a-line in the answer. 881 c=0.0.0.0 has no special meaning for the direction attribute of the 882 accepted stream in the answer. 884 6. Remaining Issues or Best Practices on Offer/Answer 886 This document clarifies the offer/answer usage in SIP and 887 summarizes the correct or recommended behaviors along with the 888 existing RFCs. To create any new normative behaviors beyond these 889 RFCs is not the intent of this document. 891 However, through the scrutiny of the offer/answer model in SIP, 892 some issues are found to be unresolved within the current set of 893 RFCs. Those remaining issues are described in this section mainly 894 for further study. 896 6.1. Rejecting PRACK Offer 898 As stated in section 2.2. and 3.2. , it is recommended that an 899 offer not be sent in a PRACK request unless UAC has strong reasons 900 to assume the receiver will accept it. Even so, there may be cases 901 when the UAS has to reject the offer for some reason. The current 902 RFCs do not provide a way to reject the offer and at the same time 903 to indicate that the PRACK adequately acknowledged the reliable 904 response. It is unclear whether a non-200 response can still 905 indicate an acknowledgement of the reliable response. 907 Several ideas were presented to resolve this issue, such as sending 908 2xx PRACK response without SDP to reject the offer, or sending SDP 909 with a decreased version value in the o-line. Some of the 910 candidates may also be adapted as a way to reject an unacceptable 911 offer in a response. Anyway, those proposals violate the current 912 rules and lose backward compatibility to some extent (e.g. section 913 5 of RFC 3262). It is beyond the scope of this document and remains 914 for further study. 916 The 488 response is another proposed solution; however the validity 917 and consequences of a 488 response to PRACK is an open issue. 918 Because the 488 response may be sent by a proxy, the UAC cannot 919 assume the reliable transaction has been adequately acknowledged. 920 If a 488 response is received, the UAC should ensure acknowledgment 921 of the reliable response by sending a new PRACK with the offer 922 removed or modified based upon the received 488 response. If the 923 488 response is sent by UAS (open issue), it cannot assume that the 924 UAC thinks that the reliable transaction has been adequately 925 acknowledged even though the UAS may treat otherwise (open issue). 926 If a 488 response is sent by UAS, the UAC should accommodate 927 receiving the altered PRACK with higher CSeq without expecting it 928 to trigger a 481 response (open issue). 930 NOTE: Deprecation of the usage of offer in PRACK may be 931 another solution. As the precondition mechanism specification 932 [2] explicitly shows a usage of sending offer in PRACK, its 933 deprecation could cause backward compatibility issues. 935 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 936 Transaction 938 When a re-INVITE transaction fails, the dialog remains with the 939 session bound to it. The issue here is: what is the session status 940 if an offer/answer exchange has been completed (if a session 941 description has been sent in a reliable provisional response to the 942 re-INVITE request), or if subsequent offer/answer exchanges have 943 taken place (using UPDATE or PRACK transactions), before the re- 944 INVITE transaction is terminated with a final error response 945 (Figure 7). One option is to take those offer/answer exchanges not 946 committed yet and to make the session status rollback to the one 947 before re-INVITE transaction was initiated. Another option is to 948 take those exchanges committed and to keep the session status as it 949 is even after re-INVITE fails. There is no clear consensus on which 950 one is the correct behavior. 952 There are some cases where it is useful to exchange 953 offer(s)/answer(s) even before re-INVITE completes. The case of 954 adding a new media (like adding video to audio only session) which 955 requires permission from the peer through some user interaction is 956 one example. Precondition procedures can be another case which may 957 require several offer/answer exchanges in one re-INVITE transaction. 959 UAC UAS 960 | session established | 961 |<===================>| 962 | | 963 | F1 re-INVITE (SDP) | 964 |-------------------->| 965 | F2 1xx-rel (SDP) | 966 |<--------------------| 967 | F3 PRACK | <- PRACK request may include new offer 968 |-------------------->| and can complete the offer/answer with 969 | F4 2xx PRA | the answer in 2xx PRACK response. 970 |<--------------------| 971 | | <- UPDATE method can update the session 972 | | status before receiving the final 973 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 974 |<--------------------| 975 | F6 ACK | 976 |-------------------->| Issue: What is the correct session 977 | | status after re-INVITE transaction. 979 Figure 7 Commit/Rollback Issue with re-INVITE transaction 981 To make bad things worse, if a new offer from UAC and the final 982 response to re-INVITE are sent at nearly the same time, the UAS can 983 not know whether this new offer was sent before or after UAC 984 received the final failure response (Figure 8). Note that the ACK 985 request to the failure response is sent hop-by-hop basis, therefore 986 even after receiving the ACK request, UAS can not make sure that 987 UPDATE request was sent after the final response had been reached 988 to the other end. 990 Sending a new UPDATE request from UAC to synchronize the status 991 anytime after the re-INVITE fails may be a good option. This 992 solution, however, requires that the UPDATE method be supported by 993 both ends and needs care to avoid flapping when each end tries to 994 advertise their different views of the session status. 996 The proper handling of this issue is undefined by existing 997 standards. Resolution is beyond the scope of this document, and 998 will require a new normative document. 1000 UAC UAS 1001 | session established | 1002 |<===================>| 1003 | | 1004 | F1 re-INVITE (SDP) | 1005 |-------------------->| 1006 | F2 1xx-rel (SDP) | 1007 |<--------------------| 1008 | F3 PRACK | 1009 |-------------------->| 1010 | F4 2xx PRA | 1011 |<--------------------| 1012 | | 1013 |UPDATE(SDP) 4xx INV | 1014 |---------\ /--------| 1015 | \/ | 1016 | /\ | 1017 |<--------/ \------->| 1018 | | 1020 Figure 8 Commit/Rollback Issue with Race Condition 1022 6.3. Offer in a Reliable Response 1024 In RFC 3261, it is stated that when an INVITE is sent without an 1025 offer, the first reliable response MUST contain an offer. There was 1026 discussion on whether this rule can be loosened up. There is no 1027 clear explanation why this restriction is defined. However, this 1028 rule will be left as it is, unless the strong necessity to loosen 1029 it up is raised in the future. 1031 6.4. Requesting Hold while already on Hold 1033 RFC 3264, section 8.4, contains procedures for putting a unicast 1034 media stream on hold. Of particular note, it states: 1036 "If the stream to be placed on hold was previously a recvonly 1037 media stream, it is placed on hold by marking it inactive." 1039 Section 5.3. of the current document makes a best practice 1040 recommentation for this case which conflicts with that, and 1041 explains why. Some concerns have been raised that such a 1042 recommendation is invalid because RFC 3264 is normative on this 1043 subject. 1045 This document takes the position that Section 8.4 of RFC 3264 is 1046 non-normative, and so may be overridden. It is further recommended 1047 that RFC 3264 be revised to avoid the confusion. 1049 7. Add New Offer/Answer Usage in SIP 1051 This document recommends against the addition of new offer/answer 1052 methods using SIP. However, it may be necessary to define new 1053 offer/answer exchange methods as SIP extensions evolve. This 1054 section recommends some things that should be taken into 1055 considerations in that case. 1057 7.1. Explicit Usage 1059 New method definitions should define offer/answer usage explicitly 1060 without any ambiguity. 1062 7.2. Rejection of an Offer 1064 New method definitions should define how to reject an offer where 1065 possible. 1067 7.3. Backward Compatibility 1069 New methods must keep backward compatibility. 1071 7.4. Exceptional Case Handling 1073 New methods should take care of how to handle exceptional cases, 1074 message crossing case and glare case. 1076 8. IANA Considerations 1078 This document has no actions for IANA. 1080 9. Security Considerations 1082 There are not any security issues beyond the referenced RFCs. 1084 10. Acknowledgement 1086 The authors would like to thank Christer Holmberg, Rajeev Seth, 1087 Nataraju A B, Byron Campen and Jonathan Rosenberg for their 1088 thorough reviews and comments. Many of their suggestions and ideas 1089 are incorporated to complete this document. 1091 11. References 1093 11.1. Normative References 1095 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1096 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1097 Session Initiation Protocol", RFC 3261, June 2002. 1099 [2] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional 1100 Responses in the Session Initiation Protocol (SIP)", RFC 3262, 1101 June 2002. 1103 [3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1104 SDP", RFC 3264, June 2002. 1106 [4] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE 1107 Method", RFC 3311, September 2002. 1109 [5] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration 1110 of Resource Management and Session Initiation Protocol (SIP)", 1111 RFC 3312, October 2002. 1113 11.2. Informative References 1115 [6] G. Camarillo, "The Early Session Disposition Type for the 1116 Session Initiation Protocol (SIP)", RFC 3959, December 2004. 1118 [7] Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent 1119 Profile Data Set for Media Policy", draft-ietf-sipping-media- 1120 policy-dataset-05 (work in progress), November 2007. 1122 Author's Addresses 1124 Takuya Sawada 1125 KDDI Corporation 1126 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 1128 Email: tu-sawada@kddi.com 1130 Paul H. Kyzivat 1131 Cisco Systems, Inc. 1132 1414 Massachusetts Avenue 1133 Boxborough, MA 01719 1134 USA 1136 Email: pkyzivat@cisco.com 1138 Full Copyright Statement 1140 Copyright (C) The IETF Trust (2008). 1142 This document is subject to the rights, licenses and restrictions 1143 contained in BCP 78, and except as set forth therein, the authors 1144 retain all their rights. 1146 This document and the information contained herein are provided on 1147 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 1148 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 1149 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 1150 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 1151 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 1152 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1153 FOR A PARTICULAR PURPOSE. 1155 Intellectual Property Statement 1157 The IETF takes no position regarding the validity or scope of any 1158 Intellectual Property Rights or other rights that might be claimed 1159 to pertain to the implementation or use of the technology described 1160 in this document or the extent to which any license under such 1161 rights might or might not be available; nor does it represent that 1162 it has made any independent effort to identify any such rights. 1163 Information on the procedures with respect to rights in RFC 1164 documents can be found in BCP 78 and BCP 79. 1166 Copies of IPR disclosures made to the IETF Secretariat and any 1167 assurances of licenses to be made available, or the result of an 1168 attempt made to obtain a general license or permission for the use 1169 of such proprietary rights by implementers or users of this 1170 specification can be obtained from the IETF on-line IPR repository 1171 at 1172 http://www.ietf.org/ipr. 1174 The IETF invites any interested party to bring to its attention any 1175 copyrights, patents or patent applications, or other proprietary 1176 rights that may cover technology that may be required to implement 1177 this standard. 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