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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (April 25, 2008) is 5844 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIPPING Working Group T. Sawada 3 Internet Draft KDDI Corporation 4 Intended status: Informational P. Kyzivat 5 Expires: October 2008 Cisco Systems, Inc. 6 April 25, 2008 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-08.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 1.1. Terminology.............................................. 3 51 2. Summary of SIP usage of the Offer/Answer Model................ 3 52 2.1. Offer/Answer Exchange Pairs in SIP Messages.............. 4 53 2.2. Rejection of an Offer.................................... 5 54 2.3. Session Description which is not Offer nor Answer........ 6 55 3. Detailed Discussion of the Offer/Answer Model for SIP......... 7 56 3.1. Offer/Answer for the INVITE method with 100rel extension. 7 57 3.1.1. INVITE Request with SDP............................. 7 58 3.1.2. INVITE request without SDP.......................... 9 59 3.2. Offer/Answer Exchange in Early Dialog................... 10 60 3.3. Offer/Answer Exchange in an Established Dialog.......... 10 61 4. Exceptional Case Handling.................................... 11 62 4.1. Message Crossing Case Handling.......................... 11 63 4.2. Glare Case Handling..................................... 13 64 5. Content of Offers and Answers................................ 14 65 5.1. General Principle for Constructing Offers and Answers... 14 66 5.2. Choice of Media Types and Formats to Include and Exclude 15 67 5.2.1. Sending an Initial INVITE with Offer............... 15 68 5.2.2. Responding with an Offer when the Initial INVITE has 69 no Offer.................................................. 15 70 5.2.3. Answering an Initial INVITE with Offer............. 16 71 5.2.4. Answering when the Initial INVITE had no Offer..... 17 72 5.2.5. Subsequent Offers and Answers...................... 17 73 5.3. Hold and Resume of media................................ 18 74 5.4. Behavior on receiving SDP with c=0.0.0.0................ 19 75 6. Remaining Issues or Best Practices on Offer/Answer........... 19 76 6.1. Rejecting PRACK Offer................................... 20 77 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 78 Transaction.................................................. 21 79 6.3. Offer in a Reliable Response............................ 22 80 6.4. Requesting Hold while already on Hold................... 23 81 7. Add New Offer/Answer Usage in SIP............................ 23 82 7.1. Explicit Usage.......................................... 23 83 7.2. Rejection of an Offer................................... 23 84 7.3. Backward Compatibility.................................. 23 85 7.4. Exceptional Case Handling............................... 23 86 8. IANA Considerations.......................................... 24 87 9. Security Considerations...................................... 24 88 10. Acknowledgement............................................. 24 89 11. References.................................................. 24 90 11.1. Normative References................................... 24 91 11.2. Informative References................................. 24 92 Author's Addresses.............................................. 25 93 Full Copyright Statement........................................ 25 94 Intellectual Property Statement................................. 25 95 Acknowledgment.................................................. 26 97 1. Introduction 99 SIP utilizes the offer/answer model to establish and update the 100 session. The rules to govern the offer/answer behaviors in SIP are 101 described in the several RFCs. 103 The primary purpose of this document is to describe all forms of 104 SIP usage of the offer/answer model in one document to help the 105 readers to fully understand it. Also, this document tries to 106 incorporate the results of the discussions on the controversial 107 issues to avoid repeating the same discussions later. 109 This document is not intended to make normative changes. Rather, it 110 makes the remaining open issues clear and leaves them for further 111 study. 113 1.1. Terminology 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 117 this document are to be interpreted as described in RFC 2119 [1]. 118 This document only uses these key words when referencing normative 119 statements in existing RFCs. 121 2. Summary of SIP usage of the Offer/Answer Model 123 The offer/answer model itself is independent from the higher layer 124 application protocols which utilize it. SIP is one of the 125 applications using the offer/answer model. RFC 3264 [4] defines the 126 offer/answer model, but does not specify which SIP messages should 127 convey an offer or an answer. This should be defined in the SIP 128 core and extensions RFCs. 130 In theory, any SIP message can include a session description in its 131 body. But a session description in a SIP message is not necessarily 132 an offer or an answer. Only certain session description usages that 133 conform to the rules described in standards-track RFCs can be 134 interpreted as an offer or an answer. The rules for how to handle 135 the offer/answer model are currently defined in several RFCs. 137 The offer/answer model defines a mechanism for update of sessions. 138 In SIP, a dialog is used to associate an offer/answer exchange with 139 the session which it is to update. In other words, only the 140 offer/answer exchange in the SIP dialog can update the session 141 which is managed by that dialog. 143 2.1. Offer/Answer Exchange Pairs in SIP Messages 145 Currently, the rules on the offer/answer model are defined in RFC 146 3261 [2], RFC 3262 [3] and RFC 3311 [5]. In these RFCs, only the 147 six patterns shown in Table 1 are defined for exchanging an offer 148 and an answer with SIP messages. 150 Note that an offer/answer exchange initiated by an INVITE request 151 must follow exactly one of the patterns 1, 2, 3, 4. When an initial 152 INVITE causes multiple dialogs due to forking, an offer/answer 153 exchange is carried out independently in each distinct dialog. When 154 an INVITE request contains no offer, only pattern 2 or pattern 4 155 apply. 'The first reliable non-failure message' must have an offer 156 if there is no offer in the INVITE request. This means that UA 157 which receives the INVITE request without an offer must include an 158 offer in the first reliable response with 100rel extension. If no 159 reliable provisional response has been sent, the UAS must include 160 an offer when sending 2xx response. 162 In pattern 3, the first reliable provisional response may or may 163 not have an answer. When a reliable provisional response contains a 164 session description, and is the first to do so, then that session 165 description is the answer to the offer in the INVITE request. The 166 answer can not be updated, and a new offer can not be sent in a 167 subsequent reliable response for the same INVITE transaction. 169 In pattern 5, a PRACK request can contain an offer only if the 170 reliable response which it acknowledges contains an answer to the 171 previous offer/answer exchange. 173 NOTE: It is legal to have UPDATE/2xx exchanges without 174 offer/answer exchanges (pattern 6). However when re-INVITEs 175 are sent for non-offer/answer purposes, an offer/answer 176 exchange is required. In that case the prior SDP will 177 typically be repeated. 179 There may be ONLY ONE offer/answer negotiation in progress for a 180 single dialog at any point in time. Section 4 explains how to 181 ensure this. When an INVITE results in multiple dialogs each has a 182 separate offer/answer negotiation. 184 NOTE: This is when using a Content-Disposition of "session". 185 There may be a second offer/answer negotiation in progress 186 using a Content-Disposition of "early-session" [7]. That is 187 not addressed by this draft. 189 Offer Answer RFC Ini Est Early 190 ------------------------------------------------------------------- 191 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 192 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 193 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 194 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 195 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 196 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y 198 Table 1. Summary of SIP Usage of the Offer/Answer Model 200 In Table 1, '1xx-rel' corresponds to the reliable provisional 201 response which contains the 100rel option defined in RFC 3262 [3]. 203 The 'Ini' column shows the ability to exchange the offer/answer to 204 initiate the session. 'Y' indicates that the pattern can be used in 205 the initial offer/answer exchange, while 'N' indicates that it can 206 not. Only the initial INVITE transaction can be used to exchange 207 the offer/answer to establish a multimedia session. 209 The 'Est' column shows the ability to update the established 210 session. 212 The 'Early' column indicates which patterns may be used to modify 213 the established session in an early dialog. There are two ways to 214 exchange a subsequent offer/answer in an early dialog. 216 2.2. Rejection of an Offer 218 It is not entirely clear how to reject an offer when it is 219 unacceptable, and some methods do not allow explicit rejection of 220 an offer. For each of the patterns in Table 1, Table 2 shows how to 221 reject an offer. 223 When a UA receives an INVITE request with an unacceptable offer, it 224 should respond with a 488 response, preferably with Warning header 225 field indicating the reason of the rejection, unless another 226 response code is more appropriate to reject it. (Pattern 1 and 227 Pattern 3) 229 When a UA receives an UPDATE request with an offer which it can not 230 accept, it should respond with a 488 response preferably with 231 Warning header field indicating the reason of the rejection, unless 232 another response code is more appropriate to reject it. (Pattern 6) 233 When a UA receives a PRACK request with an offer which it can not 234 accept, it may respond with a 200 response with a syntactically 235 correct session description. This may optionally be followed by an 236 UPDATE request to rearrange the session parameters if both ends 237 support the UPDATE method. Alternatively the UA may terminate the 238 dialog and send an error response to the INVITE request. The 239 validity and consequences of a 488 response to PRACK is an open 240 issue which is discussed within a subsequent section (Section 241 6.1. ). (Pattern 5) 243 When a UA receives a response with an offer which it can not accept, 244 the UA does not have a way to reject it explicitly. Therefore, a UA 245 should respond to the offer with the correct session description 246 and rearrange the session parameters by initiating a new 247 offer/answer exchange, or alternatively terminate the session. 248 (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA 249 should take care not to cause an infinite offer/answer loop. 251 Offer Rejection 252 ----------------------------------------------------- 253 1. INVITE Req. 488 INVITE Response 254 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 255 OR termination of dialog 256 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 257 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 258 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 259 OR termination of dialog 260 6. UPDATE Req. 488 UPDATE Response 262 Table 2. Rejection of an Offer 264 (*) A UA should only use PRACK to send an offer when it has strong 265 reasons to expect the receiver will accept the offer. 267 2.3. Session Description which is not Offer nor Answer 269 As previously stated, a session description in a SIP message is not 270 necessarily an offer or an answer. For example, SIP can use a 271 session description to describe capabilities apart from 272 offer/answer exchange. Examples of this are 200 OK responses for 273 OPTIONS and 488 responses for INVITE. 275 3. Detailed Discussion of the Offer/Answer Model for SIP 277 3.1. Offer/Answer for the INVITE method with 100rel extension 279 The INVITE method provides the basic procedure for offer/answer 280 exchange in SIP. Without the 100rel option, the rules are simple as 281 described in RFC 3261 [2]. If an INVITE request includes a session 282 description, pattern 1 is applied and if an INVITE request does not 283 include a session description, pattern 2 is applied. 285 With 100rel, pattern 3 and pattern 4 are added and this complicates 286 the rules. An INVITE request may cause multiple responses. Note 287 that even if both UAs support the 100rel extension, not all the 288 provisional responses may be sent reliably. Note also that a 289 reliable provisional response is allowed without a session 290 description if the UAS does not wish to send the answer yet. An 291 unreliable provisional response may include a session description 292 in the body if the UAS has not sent a reliable response, but its 293 session description is neither an offer nor an answer. All the 294 session descriptions in the unreliable responses to the INVITE 295 request must be identical to the answer which is included in the 296 reliable response. A session description in an unreliable response 297 that precedes a reliable response can be considered a "preview" of 298 the answer that will be coming, and hence may be treated like an 299 answer until the actual one arrives. 301 NOTE: This "preview" session description rule applies to a 302 single offer/answer exchange. In parallel offer/answer 303 exchanges (caused by forking) a UA may obviously receive a 304 different "preview" of an answer in each dialog. UAs are 305 expected to deal with this. 307 Although RFC 3261 says a UA should accept media once an INVITE with 308 an offer has been sent, in many cases, an answer (or, at least a 309 preview of it) is required in order for media to be accepted. 310 Therefore, a UAS should send an SDP answer reliably (if possible) 311 before it starts sending media. And, if neither the UAC nor the UAS 312 support 100rel, the UAS should send a preview of the answer before 313 it starts sending media. 315 3.1.1. INVITE Request with SDP 317 When a UAC includes an SDP body in the INVITE request as an offer, 318 it expects the answer to be received with one of the reliable 319 responses. Other than that, no offer/answer exchanges can occur in 320 the messages within the INVITE transaction. 322 UAC UAS 323 | F1 INVITE (SDP) | <- The offer in the offer/answer model 324 |-------------------->| 325 | F2 1xx (SDP) | <- The offer/answer exchange is not 326 |<--------------------| closed yet, but UAC acts as if it 327 | | ^ receives the answer. 328 | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer 329 |<--------------------| | SDP. 330 | F4 PRACK (no SDP) | | 331 |-------------------->| | UAC must not send a new offer. 332 | F5 2xx PRA (no SDP) | | 333 |<--------------------| v 334 | | 335 | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model 336 |<--------------------| - 337 | F7 PRACK | | UAC can send a new offer in a PRACK 338 |-------------------->| | request to acknowledge F6. 339 | F8 2xx PRA | | After F7 UAC and UAS can send a new 340 |<--------------------| v offer in an UPDATE request. 341 | | 342 | F9 1xx-rel | <- SDP should not be included in the 343 |<--------------------| subsequent 1xx-rel once offer/answer 344 | F10 PRACK | has been completed. 345 |-------------------->| 346 | F11 2xx PRA | 347 |<--------------------| 348 | | 349 | F12 2xx INV | <- SDP should not be included in the 350 |<--------------------| final response once offer/answer has 351 | F13 ACK | been completed. 352 |-------------------->| 354 Figure 1 Example of Offer/Answer with 100rel Extension (1) 356 For example, in Figure 1, only the SDP in F6 is the answer. The SDP 357 in the non-reliable response (F2) is the preview of the answer and 358 must be the same as the answer in F6. Receiving F2, the UAC should 359 act as if it receives the answer. However, offer/answer exchange is 360 not completed yet and the UAC must not send a new offer until it 361 receives the same SDP in the first reliable response, which is the 362 real answer. After sending the SDP in F6, the UAS must prepare to 363 receive a new offer from the UAC with an UPDATE request or a PRACK 364 request. 366 The UAS does not include SDP in responses F9 and F12. However, the 367 UAC should prepare to receive SDP bodies in F9 and/or F12, and just 368 ignore them, to handle a peer that does not conform to the 369 recommended implementation. 371 3.1.2. INVITE request without SDP 373 When a UAC does not include an SDP body in the INVITE request, it 374 expects the offer to be received with the first reliable response. 375 The UAC will send the answer in the request to acknowledge the 376 response, i.e. PRACK or ACK request of the reliable response. Other 377 than that, no offer/answer exchanges can occur in the messages 378 within the INVITE transaction. 380 NOTE: The UAS should not include SDP in the responses F6 and 381 F9. However, the UAC should prepare to receive SDP bodies in 382 F6 and/or F9, and just ignore them to handle a peer that does 383 not conform to the recommended implementation. 385 UAC UAS 386 | F1 INVITE (no SDP) | 387 |-------------------->| 388 | F2 1xx | 389 |<--------------------| 390 | | 391 | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP 392 |<--------------------| as the offer. 393 | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel 394 |-------------------->| must contain SDP as the answer. 395 | F5 2xx PRA (no SDP) | - 396 |<--------------------| | 397 | | | 398 | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not 399 |<--------------------| | contain SDP. 400 | F7 PRACK | | 401 |-------------------->| | UAC can send a new offer in an UPDATE 402 | F8 2xx PRA | | request after F4. 403 |<--------------------| v 404 | | 405 | F9 2xx INV (no SDP) | <- The final response should not 406 |<--------------------| contain SDP. 407 | F10 ACK | 408 |-------------------->| 410 Figure 2 Example of Offer/Answer with 100rel Extension (2) 412 Note that in the case that the UAC needs to prompt the user to 413 accept or reject the offer, the reliable provisional response with 414 SDP as an offer (pattern 4) can result in the retransmission until 415 the PRACK request can be sent. The UAC should take care to avoid 416 this situation when it sends the INVITE request without SDP. 418 3.2. Offer/Answer Exchange in Early Dialog 420 When both UAs support the 100rel extension, they can update the 421 session in the early dialog once the first offer/answer exchange 422 has been completed. 424 From a UA sending an INVITE request: 426 A UA can send an UPDATE request with a new offer if both ends 427 support the UPDATE method. Note that if the UAS needs to prompt the 428 user to accept or reject the offer, the delay can result in 429 retransmission of the UPDATE request. 431 A UA can send a PRACK request with a new offer only when 432 acknowledging the reliable provisional response carrying the answer 433 to an offer in the INVITE request. Compared to using the UPDATE 434 method, using PRACK can reduce the number of messages exchanged 435 between the UAs. However, to avoid problems or delays caused by 436 PRACK offer rejection, the UA is recommended to send a PRACK 437 request only when it has strong reasons to expect the receiver will 438 accept it. For example, the procedure used in precondition 439 extension [6] is a case where a PRACK request should be used for 440 updating the session status in an early dialog. Note also that if a 441 UAS needs to prompt the user to accept or reject the offer, the 442 delay can result in retransmission of the PRACK request. 444 From a UA receiving an INVITE request: 446 A UA can send an UPDATE request with a new offer if both ends 447 support the UPDATE method. A UAS can not send a new offer in the 448 reliable provisional response, so the UPDATE method is the only 449 method for a UAS to update an early session. 451 3.3. Offer/Answer Exchange in an Established Dialog 453 Both the re-INVITE and UPDATE methods can be used in an established 454 dialog to update the session. 456 The UPDATE method is simpler and can save at least one message 457 compared with the INVITE method. But both ends must support the 458 UPDATE method for it to be used. 460 The INVITE method needs at least three messages to complete but no 461 extensions are needed. Additionally, the INVITE method allows the 462 peer to take time to decide whether it will accept a session update 463 or not by sending provisional responses. That is, re-INVITE allows 464 the UAS to interact with the user at the peer, while UPDATE needs 465 to be answered automatically by the UAS. It is noted that re-INVITE 466 should be answered immediately unless such a user interaction is 467 needed. Otherwise, some 3pcc flows will break. 469 4. Exceptional Case Handling 471 In RFC 3264 [4], the following restrictions are defined with regard 472 to sending a new offer. 474 "It MUST NOT generate a new offer if it has received an offer 475 which it has not yet answered or rejected. It MUST NOT generate 476 a new offer if it has generated a prior offer for which it has 477 not yet received an answer or a rejection." 479 Assuming that the above rules are guaranteed, there seem to be two 480 possible 'exceptional' cases to be considered in SIP offer/answer 481 usage: the 'message crossing' case, and the 'glare' case. One of 482 the reasons why the usage of SIP methods to exchange offer/answer 483 needs to be carefully restricted in the RFCs is to ensure that the 484 UA can detect and handle appropriately the 'exceptional' cases to 485 avoid incompatible behavior. 487 4.1. Message Crossing Case Handling 489 When message packets crossed in the transport network, an offer may 490 be received before the answer for the previous offer/answer 491 exchange, as described in Figure 3. In such a case, UA A must 492 detect the session description of the offer2 is not the answer to 493 offer1. 495 A B 496 |offer1 | 497 |----------------->| 498 | answer1| 499 |<------\ /-------| 500 | \/ | 501 | /\ offer2| 502 |<------/ \-------| 504 Figure 3 Message Crossing Case 506 When offer2 is in an UPDATE request or a re-INVITE request, a 507 session description cannot be the expected answer. Then UA A must 508 reject the message including offer2 with a 491 response with Retry- 509 After header field. 511 When offer2 is in a PRACK request, that is, when the PRACK request 512 is acknowledging a reliable provisional response with an answer to 513 an offer in an INVITE request containing a session description, UA 514 A knows it is an offer. To avoid rejecting the PRACK or PRACK offer, 515 UA A is recommended to wait for answer1 before sending a PRACK 516 response with the answer to the offer2. Note that if UA A does not 517 send a new offer until the reliable provisional response with an 518 answer to the offer in the INVITE request is acknowledged with a 519 PRACK request, this case never happens. Fortunately, under the 520 current rules, UA A can not send a new offer until the reliable 521 response. 523 When offer2 is in a reliable provisional response or a successful 524 final response (as shown in Figure 4), UA A knows it is not the 525 answer to the offer1. For a reliable response to an initial INVITE 526 request, this case never happens. For a reliable response to a re- 527 INVITE request, UA A can infer that offer2 is not the answer1. In 528 this case, since UA A can not reject offer2 in a reliable response, 529 it is recommended that it wait for answer1 before sending a PRACK 530 request with the answer to offer2. Note that this case only occurs 531 when UA A, while waiting for an answer, sends an INVITE request 532 without session description. 534 Table 3 summarizes the discussions above. 536 offer2 | How to know it's not answer1 | Actions to take 537 -------+------------------------------+-------------------------- 538 INVITE | Never be an answer | 491 response 539 UPDATE | Glare case for UA A | with Retry-After 540 -------+------------------------------+-------------------------- 541 PRACK | This case never happens | - 542 | under the current rules. | 543 -------+------------------------------+-------------------------- 544 1xx-rel| Only one INVITE transaction | Delay ACK/PRACK 545 2xx | at a time. Then UA can know | until answer1 is received 546 -------+------------------------------+-------------------------- 548 NOTE: 1xx-rel/2xx case is extremely rare case and easily avoidable. 549 See Figure 4. 551 Table 3. UA's action to an offer (offer2) overtaking the previous 552 answer (answer1) 553 A B 554 | | 555 |offer1(e.g. UPD) | 556 |==============================>| 557 |re-INV (no offer) | 558 |------------------------------>| --+ 559 | answer1 (2xx-UPD)| | 560 |<===========\ /===============| | The first reliable response 561 | \/ offer2| | 562 | /\ (1xx-rel/2xx)| | 563 |<===========/ \===============| <-+ 564 | answer2 (PRACK/ACK) | 565 |------------------------------>| Wait until answer1 566 | | 568 Figure 4 Reliable response as a message with offer2 in message 569 crossing case 571 4.2. Glare Case Handling 573 When both ends in a dialog send a new offer at nearly the same time, 574 as described in Figure 5, a UA may receive a new offer before it 575 receives the answer to the offer it sent. This case is usually 576 called a 'glare' case. 578 A B 579 |offer1 offer2| 580 |-------\ /-------| 581 | \/ | 582 | /\ | 583 |<------/ \------>| 585 Figure 5 Glare Case 587 When offer2 is in an UPDATE request or (re-)INVITE request, it must 588 be rejected with a 491 response. 590 When offer2 is in a PRACK request (within the current rules, only 591 possible if offer1 is in an UPDATE request), the PRACK may be 592 accepted with 200 or may be rejected with a 491 response. A 491 593 response is valid to satisfy the offer/answer model but it may 594 delay the completion of the reliable response transfer mechanism or, 595 in worst case, may result in the failure to complete the SIP 596 transaction because there is no clear retry rule when a PRACK 597 request is rejected with a 491 response. To avoid this glare 598 condition, UA A should not send an offer if it has already sent a 599 reliable provisional response containing an answer to a previous 600 offer and has not received the corresponding PRACK request. 602 To avoid a glare condition involving an offer in a response, when 603 UA A has sent a (re)INVITE request without session description, it 604 should not send an offer until it has received an offer in a 605 reliable response to the (re)INVITE, and sent an answer to that 606 offer. 608 5. Content of Offers and Answers 610 While RFCs 3264 [4] and 3312[6] give some guidance, questions remain 611 about exactly what should be included in an offer or answer. This 612 is especially a problem when the common "hold" feature has been 613 activated, and when there is the potential for a multimedia call. 615 Details of behavior depend on the capabilities and state of the 616 User Agent. The kinds of recommendations that can be made are 617 limited by the model of device capabilities and state that is 618 presumed to exist. 620 This section focuses on a few key aspects of offers and answers 621 that have been identified as troublesome, and will consider other 622 aspects to be out of scope. This section considers: 624 - choice of supported media types and formats to include and 625 exclude 627 - hold and resume of media 629 The following are out of scope for this document: 631 - NAT traversal and ICE 633 - specific codecs and their parameters 635 - the negotiation of secure media streams 637 - grouping of media streams 639 - preconditions 641 5.1. General Principle for Constructing Offers and Answers 643 A UA should send an offer that indicates what it, and its user, are 644 interested in using/doing at that time, without regard for what the 645 other party in the call may have indicated previously. This is the 646 case even when the offer is sent in response to an INVITE or re- 647 INVITE that contains no offer. (However in the case of re-INVITE 648 the constraints of RFCs 3261 and 3264 must be observed.) 650 A UA should send an answer that includes as close an approximation 651 to what the UA and its user are interested in doing at that time, 652 while remaining consistent with the offer/answer rules of RFC 653 3264 [4] and other RFCs. 655 NOTE: "at that time" is important. The device may permit the 656 user to configure which supported media are to be used by 657 default. 659 In some cases a UA may not have direct knowledge of what it is 660 interested in doing at a particular time. If it is an intermediary 661 it may be able to delegate the decision. In the worst case it may 662 apply a default, such as assuming it wants to use all of its 663 capabilities. 665 5.2. Choice of Media Types and Formats to Include and Exclude 667 5.2.1. Sending an Initial INVITE with Offer 669 When a UAC sends an initial INVITE with an offer, it has complete 670 freedom to choose which media type(s) and media format(s) (payload 671 types in the case of RTP) it should include in the offer. 673 The media types may be all or a subset of the media the UAC is 674 capable of supporting, with the particular subset being determined 675 by the design and configuration [7] of the UAC combined with input 676 from the user interface of the UAC. 678 The media formats may be all or a subset of the media formats the 679 UAC is capable of supporting for the corresponding media type, with 680 the particular subset being determined by the design and 681 configuration [7] of the UAC combined with input from the user 682 interface of the UAC. 684 Including all supported media formats will maximize the possibility 685 that the other party will have a supported format in common. But 686 including many can result in an unacceptably large SDP body. 688 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 690 When a UAS has received an initial INVITE without an offer, it must 691 include an offer in the first reliable response to the INVITE. It 692 has largely the same options as when sending an initial INVITE with 693 an offer, but there are some differences. The choice may be 694 governed by both static (default) selections of media types as well 695 as dynamic selections made by a user via interaction with the 696 device while it is alerting. 698 NOTE: The offer may be sent in a provisional response, before 699 the user of the device has been alerted and had an opportunity 700 to select media options for the call. In this case the UAS 701 cannot include any call-specific options from the user of the 702 device. If there is a possibility that the user of the device 703 will wish to change what is offered before answering the call, 704 then special care should be taken. If PRACK and UPDATE are 705 supported by caller and callee then an initial offer can be 706 sent reliably, and changed with an UPDATE if the user desires 707 a change. If PRACK and UPDATE are not supported then the 708 initial offer cannot be changed until the call is fully 709 established. In that case either the offer should be delayed 710 until the 200 is sent, or else the offer should include the 711 minimum set of media the user is able to select. 713 5.2.3. Answering an Initial INVITE with Offer 715 When a UAS receives an initial INVITE with an offer, what media 716 lines the answer may contain is constrained by RFC 3264 [4]. The 717 answer must contain the same number of m-lines as the offer, and 718 they must contain the same media types. Each media line may be 719 accepted, by including a non-zero port number, or rejected by 720 including a zero port number in the answer. The media lines that 721 are accepted should typically be those that would have been offered 722 had the INVITE not contained an offer, excluding those not offered. 724 The media formats the answer may contain are constrained by RFC 725 3264 [4]. For each accepted m-line in the answer, there must be at 726 least one media format in common with the corresponding m-line of 727 the offer. The UAS may also include other media formats it is able 728 to support at this time. However there is little benefit to 729 including added types. 731 If the UAS does not wish to indicate support for any of the media 732 types in a particular media line of the offer it must reject the 733 corresponding media line, by setting the port number to zero. 735 5.2.4. Answering when the Initial INVITE had no Offer 737 When a UAC has sent an initial INVITE without an offer, and then 738 receives a response with the first offer, it should answer in the 739 same way as a UAS receiving an initial INVITE with an offer. 741 5.2.5. Subsequent Offers and Answers 743 The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) 744 apply, but constraints in RFC 3264 [4] must also be followed. The 745 following are of particular note because they have proven 746 troublesome: 748 o The number of m-lines may not be reduced in a subsequent offer. 749 Previously rejected media streams must remain, or be reused to 750 offer the same or a different stream. (RFC 3264 [4] section 6.) 752 o In the o-line, only the version number may change, and if it 753 changes it must increment by one from the one previously sent as 754 an offer or answer. (RFC 3264 [4] section 8.) If it doesn't 755 change then the entire SDP body must be identical to what was 756 previously sent as an offer or answer. Changing the o-line, 757 except version number value, during the session is an error case. 758 The behavior when receiving such a non-compliant offer/answer 759 SDP body is implementation dependent. If a UA needs to negotiate 760 a 'new' SDP session, it should use the INVITE/Replaces method. 762 o In the case of RTP, the mapping from a particular dynamic 763 payload type number to a particular codec within that media 764 stream (m-line) must not change for the duration of the session. 765 (RFC 3264 [4] section 8.3.2.) 767 NOTE: This may be impossible for a B2BUA to follow in some 768 cases (e.g. 3pcc transfer) if it does not terminate media. 770 When the new offer is sent in response to an offerless (re)INVITE, 771 all codecs supported by the UA are to be included, not just the 772 ones that were negotiated by previous offer/answer exchanges. The 773 same is true for media types - so if UA A initially offered audio 774 and video to UA B, and they end up with only audio, and UA B sends 775 an offerless (re)INVITE to UA A, A's resulting offer should re- 776 attempt video, by reusing the zeroed m-line used previously. 778 NOTE: The behavior above is recommended, but it is not always 779 achievable - for example in some interworking scenarios. Or, 780 the offerer may simply not have enough resources to offer 781 "everything" at that point. Even if the UAS is not able to 782 offer any other SDP that the one currently being used, it 783 should not reject the re-INVITE. Instead, it should generate 784 an offer with the currently used SDP with o- line unchanged. 786 5.3. Hold and Resume of media 788 RFC 3264 [4] specifies (non-normatively) that "hold" should be 789 indicated in an established session by sending a new offer 790 containing "a=sendonly" for each media stream to be held. An 791 answerer is then to respond with "a=recvonly" to acknowledge that 792 the hold request has been understood. 794 Note that the use of sendonly/recvonly is not limited to hold. 795 These may be used for other reasons, such as devices that are only 796 capable of sending or receiving. So receiving an offer with 797 "a=sendonly" must not be treated as a certain indication that the 798 offerer has placed the media stream on hold. 800 This model is based on an assumption that the UA initiating the 801 hold will want to play Music on Hold, which is not always the case. 802 A UA may, if desired, initiate hold by offering "a=inactive" if it 803 does not intend to transmit any media while in hold status. 805 The rules of RFC 3264 [4] constrain what may be in an answer when 806 the offer contains "sendonly", "recvonly", or "inactive" in an a= 807 line. But they do not constrain what must be in a subsequent offer. 808 The General Principle for Constructing Offers and Answers (section 809 5.1. ) is important here. The initiation of "hold" is a local 810 action. It should reflect the desired state of the UA. It then 811 affects what the UA includes in offers and answers until the local 812 state is reset. 814 The receipt of an offer containing "a=sendonly" or "a=inactive" and 815 the sending of a compatible answer should not change the desired 816 state of the recipient. However, a UA that has been "placed on 817 hold" may itself desire to initiate its own hold status, based on 818 local input. 820 If UA2 has previously been "placed on hold" by UA1, via receipt of 821 "a=sendonly", then it may initiate its own hold by sending a new 822 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 823 will answer with "a=inactive" because that is the only valid answer 824 that reflects its desire not to receive media. 826 Once in this state, to resume a two way exchange of media each side 827 must reset its local hold status. If UA1 is first to go off hold it 828 will then send an offer with "a=sendrecv". The UA2 will respond 829 with its desired state of "a=sendonly" because that is a permitted 830 response. When UA2 desires to also resume, it will send an offer 831 with "a=sendrecv". In this case, because UA1 has the same desire it 832 will respond with "a=sendrecv". In the same case, when UA2 receives 833 the offer with "a=sendrecv", if it has decided it wants to reset 834 its local hold but has not yet signaled the intent, it may send 835 "a=sendrecv" in the answer. 837 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 838 and subsequently wants to initiate its own hold, also using 839 "a=inactive", it need not send a new offer, since the only valid 840 response is "a=inactive" and that is already in effect. However, 841 its local desired state will now be either "inactive" or 842 "a=sendonly". This affects what it will send in future offers and 843 answers. 845 If a UA has occasion to send another offer in the session, without 846 any desire to change the hold status (e.g. in response to a re- 847 INVITE without an offer, or when sending a re-INVITE to refresh the 848 session timer) it should follow the General Principle for 849 Constructing Offers and Answers (section 5.1. ). If it previously 850 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 851 should offer that again. If it had not previously initiated "hold" 852 then it should offer "a=sendrecv", even if it had previously been 853 forced to answer something else. Without this behavior it is 854 possible to get "stuck on hold" in some cases, especially when a 855 third-party call controller is involved. 857 5.4. Behavior on receiving SDP with c=0.0.0.0 859 RFC 3264 [4] specifies that An agent MUST be capable of receiving 860 SDP with a connection address of 0.0.0.0, in which case it means 861 that neither RTP nor RTCP should be sent to the peer. 863 If a UA generates an answer to the offer received with c=0.0.0.0, 864 the direction attribute of the accepted media stream in the answer 865 must be based on direction attribute of the offered stream and 866 rules specified in RFC 3264 to form the a-line in the answer. 867 c=0.0.0.0 has no special meaning for the direction attribute of the 868 accepted stream in the answer. 870 6. Remaining Issues or Best Practices on Offer/Answer 872 This document clarifies the offer/answer usage in SIP and 873 summarizes the correct or recommended behaviors along with the 874 existing RFCs. To create any new normative behaviors beyond these 875 RFCs is not the intent of this document. 877 However, through the scrutiny of the offer/answer model in SIP, 878 some issues are found to be unresolved within the current set of 879 RFCs. Those remaining issues are described in this section mainly 880 for further study. 882 6.1. Rejecting PRACK Offer 884 As stated in section 2.2. and 3.2. , it is recommended that an 885 offer not be sent in a PRACK request unless UAC has strong reasons 886 to assume the receiver will accept it. Even so, there may be cases 887 when the UAS has to reject the offer for some reason. The current 888 RFCs do not provide a way to reject the offer and at the same time 889 to indicate that the PRACK adequately acknowledged the reliable 890 response. It is unclear whether a non-200 response can still 891 indicate an acknowledgement of the reliable response. 893 Several ideas were presented to resolve this issue, such as sending 894 2xx PRACK response without SDP to reject the offer, or sending SDP 895 with a decreased version value in the o-line. Some of the 896 candidates may also be adapted as a way to reject an unacceptable 897 offer in a response. Anyway, those proposals violate the current 898 rules and lose backward compatibility to some extent (e.g. section 899 5 of RFC 3262). It is beyond the scope of this document and remains 900 for further study. 902 The 488 response is another proposed solution; however the validity 903 and consequences of a 488 response to PRACK is an open issue. 904 Because the 488 response may be sent by a proxy, the UAC cannot 905 assume the reliable transaction has been adequately acknowledged. 906 If a 488 response is received, the UAC should ensure acknowledgment 907 of the reliable response by sending a new PRACK with the offer 908 removed or modified based upon the received 488 response. If the 909 488 response is sent by UAS (open issue), it cannot assume that the 910 UAC thinks that the reliable transaction has been adequately 911 acknowledged even though the UAS may treat otherwise (open issue). 912 If a 488 response is sent by UAS, the UAC should accommodate 913 receiving the altered PRACK with higher CSeq without expecting it 914 to trigger a 481 response (open issue). 916 NOTE: Deprecation of the usage of offer in PRACK may be 917 another solution. As the precondition mechanism specification 918 [3] explicitly shows a usage of sending offer in PRACK, its 919 deprecation could cause backward compatibility issues. 921 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 922 Transaction 924 When a re-INVITE transaction fails, the dialog remains with the 925 session bound to it. The issue here is: what is the session status 926 if an offer/answer exchange has been completed (if a session 927 description has been sent in a reliable provisional response to the 928 re-INVITE request), or if subsequent offer/answer exchanges have 929 taken place (using UPDATE or PRACK transactions), before the re- 930 INVITE transaction is terminated with a final error response 931 (Figure 6). One option is to take those offer/answer exchanges not 932 committed yet and to make the session status rollback to the one 933 before re-INVITE transaction was initiated. Another option is to 934 take those exchanges committed and to keep the session status as it 935 is even after re-INVITE fails. There is no clear consensus on which 936 one is the correct behavior. 938 There are some cases where it is useful to exchange 939 offer(s)/answer(s) even before re-INVITE completes. The case of 940 adding a new media (like adding video to audio only session) which 941 requires permission from the peer through some user interaction is 942 one example. Precondition procedures can be another case which may 943 require several offer/answer exchanges in one re-INVITE transaction. 945 UAC UAS 946 | session established | 947 |<===================>| 948 | | 949 | F1 re-INVITE (SDP) | 950 |-------------------->| 951 | F2 1xx-rel (SDP) | 952 |<--------------------| 953 | F3 PRACK | <- PRACK request may include new offer 954 |-------------------->| and can complete the offer/answer with 955 | F4 2xx PRA | the answer in 2xx PRACK response. 956 |<--------------------| 957 | | <- UPDATE method can update the session 958 | | status before receiving the final 959 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 960 |<--------------------| 961 | F6 ACK | 962 |-------------------->| Issue: What is the correct session 963 | | status after re-INVITE transaction. 965 Figure 6 Commit/Rollback Issue with re-INVITE transaction 967 To make bad things worse, if a new offer from UAC and the final 968 response to re-INVITE are sent at nearly the same time, the UAS can 969 not know whether this new offer was sent before or after UAC 970 received the final failure response (Figure 7). Note that the ACK 971 request to the failure response is sent hop-by-hop basis, therefore 972 even after receiving the ACK request, UAS can not make sure that 973 UPDATE request was sent after the final response had been reached 974 to the other end. 976 Sending a new UPDATE request from UAC to synchronize the status 977 anytime after the re-INVITE fails may be a good option. This 978 solution, however, requires that the UPDATE method be supported by 979 both ends and needs care to avoid flapping when each end tries to 980 advertise their different views of the session status. 982 The proper handling of this issue is undefined by existing 983 standards. Resolution is beyond the scope of this document, and 984 will require a new normative document. 986 UAC UAS 987 | session established | 988 |<===================>| 989 | | 990 | F1 re-INVITE (SDP) | 991 |-------------------->| 992 | F2 1xx-rel (SDP) | 993 |<--------------------| 994 | F3 PRACK | 995 |-------------------->| 996 | F4 2xx PRA | 997 |<--------------------| 998 | | 999 |UPDATE(SDP) 4xx INV | 1000 |---------\ /--------| 1001 | \/ | 1002 | /\ | 1003 |<--------/ \------->| 1004 | | 1006 Figure 7 Commit/Rollback Issue with Race Condition 1008 6.3. Offer in a Reliable Response 1010 In RFC 3261, it is stated that when an INVITE is sent without an 1011 offer, the first reliable response MUST contain an offer. There was 1012 discussion on whether this rule can be loosened up. There is no 1013 clear explanation why this restriction is defined. However, this 1014 rule will be left as it is, unless the strong necessity to loosen 1015 it up is raised in the future. 1017 6.4. Requesting Hold while already on Hold 1019 RFC 3264, section 8.4, contains procedures for putting a unicast 1020 media stream on hold. Of particular note, it states: 1022 "If the stream to be placed on hold was previously a recvonly 1023 media stream, it is placed on hold by marking it inactive." 1025 Section 5.3. of the current document makes a recommendation for 1026 this case which conflicts with that, and explains why. Some 1027 concerns have been raised that such a recommendation is invalid 1028 because RFC 3264 is normative on this subject. 1030 This document takes the position that Section 8.4 of RFC 3264 is 1031 non-normative, and so may be overridden. It is further recommended 1032 that RFC 3264 be revised to avoid the confusion. 1034 7. Add New Offer/Answer Usage in SIP 1036 This document recommends against the addition of new offer/answer 1037 methods using SIP. However, it may be necessary to define new 1038 offer/answer exchange methods as SIP extensions evolve. This 1039 section recommends some things that should be taken into 1040 considerations in that case. 1042 7.1. Explicit Usage 1044 New method definitions should define offer/answer usage explicitly 1045 without any ambiguity. 1047 7.2. Rejection of an Offer 1049 New method definitions should define how to reject an offer where 1050 possible. 1052 7.3. Backward Compatibility 1054 New methods must keep backward compatibility. 1056 7.4. Exceptional Case Handling 1058 New methods should take care of how to handle exceptional cases, 1059 message crossing case and glare case. 1061 8. IANA Considerations 1063 This document has no actions for IANA. 1065 9. Security Considerations 1067 There are not any security issues beyond the referenced RFCs. 1069 10. Acknowledgement 1071 The authors would like to thank Christer Holmberg, Rajeev Seth, 1072 Nataraju A B, Byron Campen and Jonathan Rosenberg for their 1073 thorough reviews and comments. Many of their suggestions and ideas 1074 are incorporated to complete this document. 1076 11. References 1078 11.1. Normative References 1080 [1] Bradner, S., "Key words for use in RFCs to Indicate 1081 Requirement Levels", BCP 14, RFC 2119, March 1997. 1083 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1084 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1085 Session Initiation Protocol", RFC 3261, June 2002. 1087 [3] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional 1088 Responses in the Session Initiation Protocol (SIP)", RFC 3262, 1089 June 2002. 1091 [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1092 SDP", RFC 3264, June 2002. 1094 [5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE 1095 Method", RFC 3311, September 2002. 1097 [6] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration 1098 of Resource Management and Session Initiation Protocol (SIP)", 1099 RFC 3312, October 2002. 1101 11.2. Informative References 1103 [7] G. Camarillo, "The Early Session Disposition Type for the 1104 Session Initiation Protocol (SIP)", RFC 3959, December 2004. 1106 Author's Addresses 1108 Takuya Sawada 1109 KDDI Corporation 1110 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 1112 Email: tu-sawada@kddi.com 1114 Paul H. Kyzivat 1115 Cisco Systems, Inc. 1116 1414 Massachusetts Avenue 1117 Boxborough, MA 01719 1118 USA 1120 Email: pkyzivat@cisco.com 1122 Full Copyright Statement 1124 Copyright (C) The IETF Trust (2008). 1126 This document is subject to the rights, licenses and restrictions 1127 contained in BCP 78, and except as set forth therein, the authors 1128 retain all their rights. 1130 This document and the information contained herein are provided on 1131 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 1132 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 1133 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 1134 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 1135 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 1136 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1137 FOR A PARTICULAR PURPOSE. 1139 Intellectual Property Statement 1141 The IETF takes no position regarding the validity or scope of any 1142 Intellectual Property Rights or other rights that might be claimed 1143 to pertain to the implementation or use of the technology described 1144 in this document or the extent to which any license under such 1145 rights might or might not be available; nor does it represent that 1146 it has made any independent effort to identify any such rights. 1147 Information on the procedures with respect to rights in RFC 1148 documents can be found in BCP 78 and BCP 79. 1150 Copies of IPR disclosures made to the IETF Secretariat and any 1151 assurances of licenses to be made available, or the result of an 1152 attempt made to obtain a general license or permission for the use 1153 of such proprietary rights by implementers or users of this 1154 specification can be obtained from the IETF on-line IPR repository 1155 at 1156 http://www.ietf.org/ipr. 1158 The IETF invites any interested party to bring to its attention any 1159 copyrights, patents or patent applications, or other proprietary 1160 rights that may cover technology that may be required to implement 1161 this standard. 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