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Kyzivat 5 Expires: July 2008 Cisco Systems, Inc. 6 January 13, 2008 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-05.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 13, 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..................................................21 78 6.3. Offer in a Reliable Response............................23 79 6.4. Requesting Hold while already on Hold...................23 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..........................................24 86 9. Security Considerations......................................24 87 10. References..................................................25 88 10.1. Normative References...................................25 89 10.2. Informative References.................................25 90 Author's Addresses..............................................25 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. (Pattern 230 5) 232 When a UA receives a response with an offer which it can not accept, 233 the UA does not have a way to reject it explicitly. Therefore, a UA 234 should respond to the offer with the correct session description 235 and rearrange the session parameters by initiating a new 236 offer/answer exchange, or alternatively terminate the session. 237 (Pattern 2 and Pattern 4.) When initiating a new offer/answer, a UA 238 should take care not to cause an infinite offer/answer loop. 240 Offer Rejection 241 ----------------------------------------------------- 242 1. INVITE Req. 488 INVITE Response 243 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 244 OR termination of dialog 245 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 246 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 247 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 248 OR termination of dialog 249 6. UPDATE Req. 488 UPDATE Response 251 Table 2. Rejection of an Offer 253 (*) A UA should only use PRACK to send an offer when it has strong 254 reasons to expect the receiver will accept the offer. 256 2.3. Session Description which is not Offer nor Answer 258 As previously stated, a session description in a SIP message is not 259 necessarily an offer or an answer. For example, SIP can use a 260 session description to describe capabilities apart from 261 offer/answer exchange. Examples of this are 200 OK responses for 262 OPTIONS and 488 responses for INVITE. 264 3. Detailed Discussion of the Offer/Answer Model for SIP 266 3.1. Offer/Answer for the INVITE method with 100rel extension 268 The INVITE method provides the basic procedure for offer/answer 269 exchange in SIP. Without the 100rel option, the rules are simple as 270 described in RFC 3261 [1]. If an INVITE request includes a session 271 description, pattern 1 is applied and if an INVITE request does not 272 include a session description, pattern 2 is applied. 274 With 100rel, pattern 3 and pattern 4 are added and this complicates 275 the rules. An INVITE request may cause multiple responses. Note 276 that even if both UAs support the 100rel extension, not all the 277 provisional responses may be sent reliably. Note also that a 278 reliable provisional response is allowed without a session 279 description if the UAS does not wish to send the answer yet. An 280 unreliable provisional response may include a session description 281 in the body if the UAS has not sent a reliable response, but its 282 session description is neither an offer nor an answer. All the 283 session descriptions in the unreliable responses to the INVITE 284 request must be identical to the answer which is included in the 285 reliable response. A session description in an unreliable response 286 that precedes a reliable response can be considered a "preview" of 287 the answer that will be coming, and hence may be treated like an 288 answer until the actual one arrives. 290 NOTE: This "preview" session description rule applies to a 291 single offer/answer exchange. In parallel offer/answer 292 exchanges (caused by forking) a UA may obviously receive a 293 different "preview" of an answer in each dialog. UAs are 294 expected to deal with this. 296 Although RFC 3261 says a UA should accept media once an INVITE with 297 an offer has been sent, in many cases, an answer (or, at least a 298 preview of it) is required in order for media to be accepted. 299 Therefore, a UAS should send an SDP answer reliably (if possible) 300 before it starts sending media. And, if neither the UAC nor the UAS 301 support 100rel, the UAS should send a preview of the answer before 302 it starts sending media. 304 3.1.1. INVITE Request with SDP 306 When a UAC includes an SDP body in the INVITE request as an offer, 307 it expects the answer to be received with one of the reliable 308 responses. Other than that, no offer/answer exchanges can occur in 309 the messages within the INVITE transaction. 311 UAC UAS 312 | F1 INVITE (SDP) | <- The offer in the offer/answer model 313 |-------------------->| 314 | F2 1xx (SDP) | <- The offer/answer exchange is not 315 |<--------------------| closed yet, but UAC acts as if it 316 | | ^ receives the answer. 317 | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer 318 |<--------------------| | SDP. 319 | F4 PRACK (no SDP) | | 320 |-------------------->| | UAC must not send a new offer. 321 | F5 2xx PRA (no SDP) | | 322 |<--------------------| v 323 | | 324 | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model 325 |<--------------------| - 326 | F7 PRACK | | UAC can send a new offer in a PRACK 327 |-------------------->| | request to acknowledge F6. 328 | F8 2xx PRA | | After F7 UAC and UAS can send a new 329 |<--------------------| v offer in an UPDATE request. 330 | | 331 | F9 1xx-rel | <- SDP should not be included in the 332 |<--------------------| subsequent 1xx-rel once offer/answer 333 | F10 PRACK | has been completed. 334 |-------------------->| 335 | F11 2xx PRA | 336 |<--------------------| 337 | | 338 | F12 2xx INV | <- SDP should not be included in the 339 |<--------------------| final response once offer/answer has 340 | F13 ACK | been completed. 341 |-------------------->| 343 Figure 1 Example of Offer/Answer with 100rel Extension (1) 345 For example, in Figure 1, only the SDP in F6 is the answer. The SDP 346 in the non-reliable response (F2) is the preview of the answer and 347 must be the same as the answer in F6. Receiving F2, the UAC should 348 act as if it receives the answer. However, offer/answer exchange is 349 not completed yet and the UAC must not send a new offer until it 350 receives the same SDP in the first reliable response, which is the 351 real answer. After sending the SDP in F6, the UAS must prepare to 352 receive a new offer from the UAC with an UPDATE request or a PRACK 353 request. 355 The UAS does not include SDP in responses F9 and F12. However, the 356 UAC should prepare to receive SDP bodies in F9 and/or F12, and just 357 ignore them, to handle a peer that does not conform to the 358 recommended implementation. 360 3.1.2. INVITE request without SDP 362 When a UAC does not include an SDP body in the INVITE request, it 363 expects the offer to be received with the first reliable response. 364 The UAC will send the answer in the request to acknowledge the 365 response, i.e. PRACK or ACK request of the reliable response. Other 366 than that, no offer/answer exchanges can occur in the messages 367 within the INVITE transaction. 369 NOTE: The UAS should not include SDP in the responses F6 and 370 F9. However, the UAC should prepare to receive SDP bodies in 371 F6 and/or F9, and just ignore them to handle a peer that does 372 not conform to the recommended implementation. 374 UAC UAS 375 | F1 INVITE (no SDP) | 376 |-------------------->| 377 | F2 1xx | 378 |<--------------------| 379 | | 380 | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP 381 |<--------------------| as the offer. 382 | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel 383 |-------------------->| must contain SDP as the answer. 384 | F5 2xx PRA (no SDP) | - 385 |<--------------------| | 386 | | | 387 | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not 388 |<--------------------| | contain SDP. 389 | F7 PRACK | | 390 |-------------------->| | UAC can send a new offer in an UPDATE 391 | F8 2xx PRA | | request after F4. 392 |<--------------------| v 393 | | 394 | F9 2xx INV (no SDP) | <- The final response should not 395 |<--------------------| contain SDP. 396 | F10 ACK | 397 |-------------------->| 399 Figure 2 Example of Offer/Answer with 100rel Extension (2) 401 Note that in the case that the UAC needs to prompt the user to 402 accept or reject the offer, the reliable provisional response with 403 SDP as an offer (pattern 4) can result in the retransmission until 404 the PRACK request can be sent. The UAC should take care to avoid 405 this situation when it sends the INVITE request without SDP. 407 3.2. Offer/Answer Exchange in Early Dialog 409 When both UAs support the 100rel extension, they can update the 410 session in the early dialog once the first offer/answer exchange 411 has been completed. 413 From a UA sending an INVITE request: 415 A UA can send an UPDATE request with a new offer if both ends 416 support the UPDATE method. Note that if the UAS needs to prompt the 417 user to accept or reject the offer, the delay can result in 418 retransmission of the UPDATE request. 420 A UA can send a PRACK request with a new offer only when 421 acknowledging the reliable provisional response carrying the answer 422 to an offer in the INVITE request. Compared to using the UPDATE 423 method, using PRACK can reduce the number of messages exchanged 424 between the UAs. However, as a PRACK request should not be rejected, 425 the UA is recommended to send a PRACK request only when it has 426 strong reasons to expect the receiver will accept it. For example, 427 the procedure used in precondition extension [5] is a case where a 428 PRACK request should be used for updating the session status in an 429 early dialog. Note also that if a UAS needs to prompt the user to 430 accept or reject the offer, the delay can result in retransmission 431 of the PRACK request. 433 From a UA receiving an INVITE request: 435 A UA can send an UPDATE request with a new offer if both ends 436 support the UPDATE method. A UAS can not send a new offer in the 437 reliable provisional response, so the UPDATE method is the only 438 method for a UAS to update an early session. 440 3.3. Offer/Answer Exchange in an Established Dialog 442 Both the re-INVITE and UPDATE methods can be used in an established 443 dialog to update the session. 445 The UPDATE method is simpler and can save at least one message 446 compared with the INVITE method. But both ends must support the 447 UPDATE method for it to be used. 449 The INVITE method needs at least three messages to complete but no 450 extensions are needed. Additionally, the INVITE method allows the 451 peer to take time to decide whether it will accept a session update 452 or not by sending provisional responses. That is, re-INVITE allows 453 the UAS to interact with the user at the peer, while UPDATE needs 454 to be answered automatically by the UAS. It is noted that re-INVITE 455 should be answered immediately unless such a user interaction is 456 needed. Otherwise, some 3pcc flows will break. 458 4. Exceptional Case Handling 460 In RFC 3264 [3], the following restrictions are defined with regard 461 to sending a new offer. 463 "It MUST NOT generate a new offer if it has received an offer 464 which it has not yet answered or rejected. It MUST NOT generate 465 a new offer if it has generated a prior offer for which it has 466 not yet received an answer or a rejection." 468 Assuming that the above rules are guaranteed, there seem to be two 469 possible 'exceptional' cases to be considered in SIP offer/answer 470 usage: the 'message crossing' case, and the 'glare' case. One of 471 the reasons why the usage of SIP methods to exchange offer/answer 472 needs to be carefully restricted in the RFCs is to ensure that the 473 UA can detect and handle appropriately the 'exceptional' cases to 474 avoid incompatible behavior. 476 4.1. Message Crossing Case Handling 478 When message packets crossed in the transport network, an offer may 479 be received before the answer for the previous offer/answer 480 exchange, as described in Figure 3. In such a case, UA A must 481 detect the session description of the offer2 is not the answer to 482 offer1. 484 A B 485 |offer1 | 486 |----------------->| 487 | answer1| 488 |<------\ /-------| 489 | \/ | 490 | /\ offer2| 491 |<------/ \-------| 493 Figure 3 Message Crossing Case 495 When offer2 is in an UPDATE request or a re-INVITE request, a 496 session description cannot be the expected answer. Then UA A must 497 reject the message including offer2 with a 491 response with Retry- 498 After header field. 500 When offer2 is in a PRACK request(as shown in Figure 4), that is, 501 when the PRACK request is acknowledging a reliable provisional 502 response with an answer to an offer in an INVITE request containing 503 a session description, UA A knows it is an offer. As a PRACK 504 request should not be rejected, UA A is recommended to wait for 505 answer1 before sending a PRACK response with the answer to the 506 offer2. Note that if UA A does not send a new offer until the 507 reliable provisional response with an answer to the offer in the 508 INVITE request is acknowledged with a PRACK request, this case 509 never happens. Therefore, to simplify implementations, a UA acting 510 as a UAS for an INVITE transaction is recommended not to defer 511 sending an UPDATE request with an offer until after the reliable 512 response with an answer to the offer in the INVITE request is 513 acknowledged with a PRACK request. 515 When offer2 is in a reliable provisional response or a successful 516 final response (as shown in Figure 5), UA A knows it is not the 517 answer to the offer1. For a reliable response to an initial INVITE 518 request, this case never happens. For a reliable response to a re- 519 INVITE request, UA A can infer that offer2 is not the answer1. In 520 this case, since UA A can not reject offer2 in a reliable response, 521 it is recommended that it wait for answer1 before sending a PRACK 522 request with the answer to offer2. Note that this case only occurs 523 when UA A, while waiting for an answer, sends an INVITE request 524 without session description. 526 Table 3 summarizes the discussions above. 528 offer2 | How to know it's not answer1 | Actions to take 529 -------+------------------------------+-------------------------- 530 INVITE | Never be an answer | 491 response 531 UPDATE | Glare case for UA A | with Retry-After 532 -------+------------------------------+-------------------------- 533 PRACK | Not a pattern 4. in Table 1. | Delay sending response 534 | 1xx-rel must have an answer, | until answer1 is received 535 | not an offer. | 536 -------+------------------------------+-------------------------- 537 1xx-rel| Only one INVITE transaction | Delay ACK/PRACK 538 2xx | at a time. Then UA can know | until answer1 is received 539 -------+------------------------------+-------------------------- 541 NOTE: PRACK and 1xx-rel/2xx case is extremely rare case and easily 542 avoidable. See Figure 4 and Figure 5. 544 Table 3. UA's action to an offer (offer2) overtaking the previous 545 answer (answer1) 547 A B 548 | | 549 | INV (offer0)| 550 |<------------------------------| 551 | 1xx-rel (answer0) | 552 |------------------------------>| --+ 553 |offer1(e.g. UPD) | | 554 |==============================>| | 555 | answer1 (2xx-UPD)| | Acknowledge 556 |<===========\ /===============| | 557 | \/ | | 558 | /\ offer2(PRA)| | 559 |<===========/ \===============| <-+ 560 | answer2 (2xx-PRA) | 561 |------------------------------>| Wait until answer1 562 | | 564 Figure 4 PRACK as a message with offer2 in message crossing case 566 A B 567 | | 568 |offer1(e.g. UPD) | 569 |==============================>| 570 |re-INV (no offer) | 571 |------------------------------>| --+ 572 | answer1 (2xx-UPD)| | 573 |<===========\ /===============| | The first reliable response 574 | \/ offer2| | 575 | /\ (1xx-rel/2xx)| | 576 |<===========/ \===============| <-+ 577 | answer2 (PRACK/ACK) | 578 |------------------------------>| Wait until answer1 579 | | 581 Figure 5 Reliable response as a message with offer2 in message 582 crossing case 584 4.2. Glare Case Handling 586 When both ends in a dialog send a new offer at nearly the same time, 587 as described in Figure 6, a UA may receive a new offer before it 588 receives the answer to the offer it sent. This case is usually 589 called a 'glare' case. 591 A B 592 |offer1 offer2| 593 |-------\ /-------| 594 | \/ | 595 | /\ | 596 |<------/ \------>| 598 Figure 6 Glare Case 600 When offer2 is in an UPDATE request or (re-)INVITE request, it must 601 be rejected with a 491 response. 603 When offer2 is in a PRACK request (within the current rules, only 604 possible if offer1 is in an UPDATE request), the PRACK may be 605 accepted with 200 or may be rejected with a 491 response. A 491 606 response is valid to satisfy the offer/answer model but it may 607 delay the completion of the reliable response transfer mechanism or, 608 in worst case, may result in the failure to complete the SIP 609 transaction because there is no clear retry rule when a PRACK 610 request is rejected with a 491 response. To avoid this glare 611 condition, UA A should not send an offer if it has already sent a 612 reliable provisional response containing an answer to a previous 613 offer and has not received the corresponding PRACK request. 615 To avoid a glare condition involving an offer in a response, when 616 UA A has sent a (re)INVITE request without session description, it 617 should not send an offer until it has received an offer in a 618 reliable response to the (re)INVITE, and sent an answer to that 619 offer. 621 5. Content of Offers and Answers 623 While RFCs 3264[3] and 3312[5] give some guidance, questions remain 624 about exactly what should be included in an offer or answer. This 625 is especially a problem when the common "hold" feature has been 626 activated, and when there is the potential for a multimedia call. 628 Details of behavior depend on the capabilities and state of the 629 User Agent. The kinds of recommendations that can be made are 630 limited by the model of device capabilities and state that is 631 presumed to exist. 633 This section focuses on a few key aspects of offers and answers 634 that have been identified as troublesome, and will consider other 635 aspects to be out of scope. This section considers: 637 - choice of supported media types and formats to include and 638 exclude 640 - hold and resume of media 642 The following are out of scope for this document: 644 - NAT traversal and ICE 646 - specific codecs and their parameters 648 - the negotiation of secure media streams 650 - grouping of media streams 652 - preconditions 654 5.1. General Principle for Constructing Offers and Answers 656 A UA should send an offer that indicates what it, and its user, are 657 interested in using/doing at that time, without regard for what the 658 other party in the call may have indicated previously. This is the 659 case even when the offer is sent in response to an INVITE or re- 660 INVITE that contains no offer. (However in the case of re-INVITE 661 the constraints of RFCs 3261 and 3264 must be observed.) 663 A UA should send an answer that includes as close an approximation 664 to what the UA and its user are interested in doing at that time, 665 while remaining consistent with the offer/answer rules of RFC 666 3264[3] and other RFCs. 668 NOTE: "at that time" is important. The device may permit the 669 user to configure which supported media are to be used by 670 default. 672 In some cases a UA may not have direct knowledge of what it is 673 interested in doing at a particular time. If it is an intermediary 674 it may be able to delegate the decision. In the worst case it may 675 apply a default, such as assuming it wants to use all of its 676 capabilities. 678 5.2. Choice of Media Types and Formats to Include and Exclude 680 5.2.1. Sending an Initial INVITE with Offer 682 When a UAC sends an initial INVITE with an offer, it has complete 683 freedom to choose which media type(s) and media format(s) (payload 684 types in the case of RTP) it should include in the offer. 686 The media types may be all or a subset of the media the UAC is 687 capable of supporting, with the particular subset being determined 688 by the design and configuration [6] of the UAC combined with input 689 from the user interface of the UAC. 691 The media formats may be all or a subset of the media formats the 692 UAC is capable of supporting for the corresponding media type, with 693 the particular subset being determined by the design and 694 configuration [6] of the UAC combined with input from the user 695 interface of the UAC. 697 Including all supported media formats will maximize the possibility 698 that the other party will have a supported format in common. But 699 including many can result in an unacceptably large SDP body. 701 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 703 When a UAS has received an initial INVITE without an offer, it must 704 include an offer in the first reliable response to the INVITE. It 705 has largely the same options as when sending an initial INVITE with 706 an offer, but there are some differences. The choice may be 707 governed by both static (default) selections of media types as well 708 as dynamic selections made by a user via interaction with the 709 device while it is alerting. 711 NOTE: The offer may be sent in a provisional response, before 712 the user of the device has been alerted and had an opportunity 713 to select media options for the call. In this case the UAS 714 cannot include any call-specific options from the user of the 715 device. If there is a possibility that the user of the device 716 will wish to change what is offered before answering the call, 717 then special care should be taken. If PRACK and UPDATE are 718 supported by caller and callee then an initial offer can be 719 sent reliably, and changed with an UPDATE if the user desires 720 a change. If PRACK and UPDATE are not supported then the 721 initial offer cannot be changed until the call is fully 722 established. In that case either the offer should be delayed 723 until the 200 is sent, or else the offer should include the 724 minimum set of media the user is able to select. 726 5.2.3. Answering an Initial INVITE with Offer 728 When a UAS receives an initial INVITE with an offer, what media 729 lines the answer may contain is constrained by RFC 3264.[3] The 730 answer must contain the same number of m-lines as the offer, and 731 they must contain the same media types. Each media line may be 732 accepted, by including a non-zero port number, or rejected by 733 including a zero port number in the answer. The media lines that 734 are accepted should typically be those that would have been offered 735 had the INVITE not contained an offer, excluding those not offered. 737 The media formats the answer may contain are constrained by RFC 738 3264 [3]. For each accepted m-line in the answer, there must be at 739 least one media format in common with the corresponding m-line of 740 the offer. The UAS may also include other media formats it is able 741 to support at this time. However there is little benefit to 742 including added types. 744 If the UAS does not wish to indicate support for any of the media 745 types in a particular media line of the offer it must reject the 746 corresponding media line, by setting the port number to zero. 748 5.2.4. Answering when the Initial INVITE had no Offer 750 When a UAC has sent an initial INVITE without an offer, and then 751 receives a response with the first offer, it should answer in the 752 same way as a UAS receiving an initial INVITE with an offer. 754 5.2.5. Subsequent Offers and Answers 756 The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) 757 apply, but constraints in RFC 3264 [3] must also be followed. The 758 following are of particular note because they have proven 759 troublesome: 761 o The number of m-lines may not be reduced in a subsequent offer. 762 Previously rejected media streams must remain, or be reused to 763 offer the same or a different stream. (RFC 3264[3] section 6.) 765 o In the o-line, only the version number may change, and if it 766 changes it must increment by one from the one previously sent as 767 an offer or answer. (RFC 3264[3] section 8.) If it doesn't 768 change then the entire SDP body must be identical to what was 769 previously sent as an offer or answer. Changing the o-line, 770 except version number value, during the session is an error case. 771 The behavior when receiving such a non-compliant offer/answer 772 SDP body is implementation dependent. If a UA needs to negotiate 773 a 'new' SDP session, it should use the INVITE/Replaces method. 775 o In the case of RTP, the mapping from a particular dynamic 776 payload type number to a particular codec within that media 777 stream (m-line) must not change for the duration of the session. 778 (RFC 3264[3] section 8.3.2.) 780 NOTE: This may be impossible for a B2BUA to follow in some 781 cases (e.g. 3pcc transfer) if it does not terminate media. 783 When the new offer is sent in response to an offerless (re)INVITE, 784 all codecs supported by the UA are to be included, not just the 785 ones that were negotiated by previous offer/answer exchanges. The 786 same is true for media types - so if UA A initially offered audio 787 and video to UA B, and they end up with only audio, and UA B sends 788 an offerless (re)INVITE to UA A, A's resulting offer should re- 789 attempt video, by reusing the zeroed m-line used previously. 791 NOTE: The behavior above is recommended, but it is not always 792 achievable - for example in some interworking scenarios. Or, 793 the offerer may simply not have enough resources to offer 794 "everything" at that point. Even if the UAS is not able to 795 offer any other SDP that the one currently being used, it 796 should not reject the re-INVITE. Instead, it should generate 797 an offer with the currently used SDP with o- line unchanged. 799 5.3. Hold and Resume of media 801 RFC 3264 [3] specifies (non-normatively) that "hold" should be 802 indicated in an established session by sending a new offer 803 containing "a=sendonly" for each media stream to be held. An 804 answerer is then to respond with "a=recvonly" to acknowledge that 805 the hold request has been understood. 807 Note that the use of sendonly/recvonly is not limited to hold. 808 These may be used for other reasons, such as devices that are only 809 capable of sending or receiving. So receiving an offer with 810 "a=sendonly" must not be treated as a certain indication that the 811 offerer has placed the media stream on hold. 813 This model is based on an assumption that the UA initiating the 814 hold will want to play Music on Hold, which is not always the case. 815 A UA may, if desired, initiate hold by offering "a=inactive" if it 816 does not intend to transmit any media while in hold status. 818 The rules of RFC 3264 [3] constrain what may be in an answer when 819 the offer contains "sendonly", "recvonly", or "inactive" in an a= 820 line. But they do not constrain what must be in a subsequent offer. 821 The General Principle for Constructing Offers and Answers (section 822 5.1. ) is important here. The initiation of "hold" is a local 823 action. It should reflect the desired state of the UA. It then 824 affects what the UA includes in offers and answers until the local 825 state is reset. 827 The receipt of an offer containing "a=sendonly" or "a=inactive" and 828 the sending of a compatible answer should not change the desired 829 state of the recipient. However, a UA that has been "placed on 830 hold" may itself desire to initiate its own hold status, based on 831 local input. 833 If UA2 has previously been "placed on hold" by UA1, via receipt of 834 "a=sendonly", then it may initiate its own hold by sending a new 835 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 836 will answer with "a=inactive" because that is the only valid answer 837 that reflects its desire not to receive media. 839 Once in this state, to resume a two way exchange of media each side 840 must reset its local hold status. If UA1 is first to go off hold it 841 will then send an offer with "a=sendrecv". The UA2 will respond 842 with its desired state of "a=sendonly" because that is a permitted 843 response. When UA2 desires to also resume, it will send an offer 844 with "a=sendrecv". In this case, because UA1 has the same desire it 845 will respond with "a=sendrecv". In the same case, when UA2 receives 846 the offer with "a=sendrecv", if it has decided it wants to reset 847 its local hold but has not yet signaled the intent, it may send 848 "a=sendrecv" in the answer. 850 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 851 and subsequently wants to initiate its own hold, also using 852 "a=inactive", it need not send a new offer, since the only valid 853 response is "a=inactive" and that is already in effect. However, 854 its local desired state will now be either "inactive" or 855 "a=sendonly". This affects what it will send in future offers and 856 answers. 858 If a UA has occasion to send another offer in the session, without 859 any desire to change the hold status (e.g. in response to a re- 860 INVITE without an offer, or when sending a re-INVITE to refresh the 861 session timer) it should follow the General Principle for 862 Constructing Offers and Answers (section 5.1. ). If it previously 863 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 864 should offer that again. If it had not previously initiated "hold" 865 then it should offer "a=sendrecv", even if it had previously been 866 forced to answer something else. Without this behavior it is 867 possible to get "stuck on hold" in some cases, especially when a 868 third-party call controller is involved. 870 5.4. Behavior on receiving SDP with c=0.0.0.0 872 RFC 3264[3] specifies that An agent MUST be capable of receiving 873 SDP with a connection address of 0.0.0.0, in which case it means 874 that neither RTP nor RTCP should be sent to the peer. 876 If a UA generates an answer to the offer received with c=0.0.0.0, 877 the direction attribute of the accepted media stream in the answer 878 must be based on direction attribute of the offered stream and 879 rules specified in RFC 3264 to form the a-line in the answer. 880 c=0.0.0.0 has no special meaning for the direction attribute of the 881 accepted stream in the answer. 883 6. Remaining Issues or Best Practices on Offer/Answer 885 This document clarifies the offer/answer usage in SIP and 886 summarizes the correct or recommended behaviors along with the 887 existing RFCs. To create any new normative behaviors beyond these 888 RFCs is not the intent of this document. 890 However, through the scrutiny of the offer/answer model in SIP, 891 some issues are found to be unresolved within the current set of 892 RFCs. Those remaining issues are described in this section mainly 893 for further study. 895 6.1. Rejecting PRACK Offer 897 As stated in section 2.2. and 3.2. , it is recommended that an 898 offer not be sent in a PRACK request unless UAC has strong reasons 899 to assume the receiver will accept it. Even so, there may be cases 900 when the UAS has to reject the offer for some reason. The current 901 RFCs do not provide a way to reject the offer and at the same time 902 to acknowledge the reliable response. 904 Several ideas were presented to resolve this issue, such as sending 905 2xx PRACK response without SDP to reject the offer, or sending SDP 906 with a decreased version value in the o-line. Some of the 907 candidates may also be adapted as a way to reject an unacceptable 908 offer in a response. Anyway, those proposals violate the current 909 rules and lose backward compatibility to some extent (e.g. section 910 5 of RFC 3262). It is beyond the scope of this document and remains 911 for further study. 913 NOTE: Deprecation of the usage of offer in PRACK may be 914 another solution. As the precondition mechanism specification 915 [2] explicitly shows a usage of sending offer in PRACK, its 916 deprecation could cause backward compatibility issues. 918 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 919 Transaction 921 When a re-INVITE transaction fails, the dialog remains with the 922 session bound to it. The issue here is: what is the session status 923 if an offer/answer exchange has been completed (if a session 924 description has been sent in a reliable provisional response to the 925 re-INVITE request), or if subsequent offer/answer exchanges have 926 taken place (using UPDATE or PRACK transactions), before the re- 927 INVITE transaction is terminated with a final error response 928 (Figure 7). One option is to take those offer/answer exchanges not 929 committed yet and to make the session status rollback to the one 930 before re-INVITE transaction was initiated. Another option is to 931 take those exchanges committed and to keep the session status as it 932 is even after re-INVITE fails. There is no clear consensus on which 933 one is the correct behavior. 935 There are some cases where it is useful to exchange 936 offer(s)/answer(s) even before re-INVITE completes. The case of 937 adding a new media (like adding video to audio only session) which 938 requires permission from the peer through some user interaction is 939 one example. Precondition procedures can be another case which may 940 require several offer/answer exchanges in one re-INVITE transaction. 942 UAC UAS 943 | session established | 944 |<===================>| 945 | | 946 | F1 re-INVITE (SDP) | 947 |-------------------->| 948 | F2 1xx-rel (SDP) | 949 |<--------------------| 950 | F3 PRACK | <- PRACK request may include new offer 951 |-------------------->| and can complete the offer/answer with 952 | F4 2xx PRA | the answer in 2xx PRACK response. 953 |<--------------------| 954 | | <- UPDATE method can update the session 955 | | status before receiving the final 956 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 957 |<--------------------| 958 | F6 ACK | 959 |-------------------->| Issue: What is the correct session 960 | | status after re-INVITE transaction. 962 Figure 7 Commit/Rollback Issue with re-INVITE transaction 964 To make bad things worse, if a new offer from UAC and the final 965 response to re-INVITE are sent at nearly the same time, the UAS can 966 not know whether this new offer was sent before or after UAC 967 received the final failure response (Figure 8). Note that the ACK 968 request to the failure response is sent hop-by-hop basis, therefore 969 even after receiving the ACK request, UAS can not make sure that 970 UPDATE request was sent after the final response had been reached 971 to the other end. 973 Sending a new UPDATE request from UAC to synchronize the status 974 anytime after the re-INVITE fails may be a good option. This 975 solution, however, requires that the UPDATE method be supported by 976 both ends and needs care to avoid flapping when each end tries to 977 advertise their different views of the session status. 979 The proper handling of this issue is undefined by existing 980 standards. Resolution is beyond the scope of this document, and 981 will require a new normative document. 983 UAC UAS 984 | session established | 985 |<===================>| 986 | | 987 | F1 re-INVITE (SDP) | 988 |-------------------->| 989 | F2 1xx-rel (SDP) | 990 |<--------------------| 991 | F3 PRACK | 992 |-------------------->| 993 | F4 2xx PRA | 994 |<--------------------| 995 | | 996 |UPDATE(SDP) 4xx INV | 997 |---------\ /--------| 998 | \/ | 999 | /\ | 1000 |<--------/ \------->| 1001 | | 1003 Figure 8 Commit/Rollback Issue with Race Condition 1005 6.3. Offer in a Reliable Response 1007 In RFC 3261, it is stated that when an INVITE is sent without an 1008 offer, the first reliable response MUST contain an offer. There was 1009 discussion on whether this rule can be loosened up. There is no 1010 clear explanation why this restriction is defined. However, this 1011 rule will be left as it is, unless the strong necessity to loosen 1012 it up is raised in the future. 1014 6.4. Requesting Hold while already on Hold 1016 RFC 3264, section 8.4, contains procedures for putting a unicast 1017 media stream on hold. Of particular note, it states: 1019 "If the stream to be placed on hold was previously a recvonly 1020 media stream, it is placed on hold by marking it inactive." 1022 Section 5.3. of the current document makes a best practice 1023 recommentation for this case which conflicts with that, and 1024 explains why. Some concerns have been raised that such a 1025 recommendation is invalid because RFC 3264 is normative on this 1026 subject. 1028 This document takes the position that Section 8.4 of RFC 3264 is 1029 non-normative, and so may be overridden. It is further recommended 1030 that RFC 3264 be revised to avoid the confusion. 1032 7. Add New Offer/Answer Usage in SIP 1034 This document recommends against the addition of new offer/answer 1035 methods using SIP. However, it may be necessary to define new 1036 offer/answer exchange methods as SIP extensions evolve. This 1037 section recommends some things that should be taken into 1038 considerations in that case. 1040 7.1. Explicit Usage 1042 New method definitions should define offer/answer usage explicitly 1043 without any ambiguity. 1045 7.2. Rejection of an Offer 1047 New method definitions should define how to reject an offer where 1048 possible. 1050 7.3. Backward Compatibility 1052 New methods must keep backward compatibility. 1054 7.4. Exceptional Case Handling 1056 New methods should take care of how to handle exceptional cases, 1057 message crossing case and glare case. 1059 8. IANA Considerations 1061 This document has no actions for IANA. 1063 9. Security Considerations 1065 There are not any security issues beyond the referenced RFCs. 1067 10. Acknowledgement 1069 The authors would like to thank Christer Holmberg, Rajeev Seth, 1070 Nataraju A B, Byron Campen and Jonathan Rosenberg for their 1071 thorough reviews and comments. Many of their suggestions and ideas 1072 are incorporated to complete this document. 1074 11. References 1076 11.1. Normative References 1078 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1079 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1080 Session Initiation Protocol", RFC 3261, June 2002. 1082 [2] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional 1083 Responses in the Session Initiation Protocol (SIP)", RFC 3262, 1084 June 2002. 1086 [3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1087 SDP", RFC 3264, June 2002. 1089 [4] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE 1090 Method", RFC 3311, September 2002. 1092 [5] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration 1093 of Resource Management and Session Initiation Protocol (SIP)", 1094 RFC 3312, October 2002. 1096 11.2. Informative References 1098 [6] G. Camarillo, "The Early Session Disposition Type for the 1099 Session Initiation Protocol (SIP)", RFC 3959, December 2004. 1101 [7] Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent 1102 Profile Data Set for Media Policy", draft-ietf-sipping-media- 1103 policy-dataset-05 (work in progress), November 2007. 1105 Author's Addresses 1107 Takuya Sawada 1108 KDDI Corporation 1109 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 1111 Email: tu-sawada@kddi.com 1112 Paul H. Kyzivat 1113 Cisco Systems, Inc. 1114 1414 Massachusetts Avenue 1115 Boxborough, MA 01719 1116 USA 1118 Email: pkyzivat@cisco.com 1120 Full Copyright Statement 1122 Copyright (C) The IETF Trust (2008). 1124 This document is subject to the rights, licenses and restrictions 1125 contained in BCP 78, and except as set forth therein, the authors 1126 retain all their rights. 1128 This document and the information contained herein are provided on 1129 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 1130 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 1131 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 1132 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 1133 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 1134 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1135 FOR A PARTICULAR PURPOSE. 1137 Intellectual Property Statement 1139 The IETF takes no position regarding the validity or scope of any 1140 Intellectual Property Rights or other rights that might be claimed 1141 to pertain to the implementation or use of the technology described 1142 in this document or the extent to which any license under such 1143 rights might or might not be available; nor does it represent that 1144 it has made any independent effort to identify any such rights. 1145 Information on the procedures with respect to rights in RFC 1146 documents can be found in BCP 78 and BCP 79. 1148 Copies of IPR disclosures made to the IETF Secretariat and any 1149 assurances of licenses to be made available, or the result of an 1150 attempt made to obtain a general license or permission for the use 1151 of such proprietary rights by implementers or users of this 1152 specification can be obtained from the IETF on-line IPR repository 1153 at 1154 http://www.ietf.org/ipr. 1156 The IETF invites any interested party to bring to its attention any 1157 copyrights, patents or patent applications, or other proprietary 1158 rights that may cover technology that may be required to implement 1159 this standard. 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