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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-18) exists of draft-ietf-sipping-config-framework-15 Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Sipping T. Sawada 3 Internet-Draft KDDI Corporation 4 Intended status: Informational P. Kyzivat 5 Expires: July 5, 2009 Cisco Systems, Inc. 6 January 1, 2009 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-10 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on July 5, 2009. 34 Copyright Notice 36 Copyright (c) 2009 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. 46 Abstract 48 The Session Initiation Protocol (SIP) utilizes the offer/answer model 49 to establish and update multimedia sessions using the Session 50 Description Protocol (SDP). The description of the offer/answer 51 model in SIP is dispersed across multiple RFCs. This document 52 summarizes all the current usages of the offer/answer model in SIP 53 communication. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 59 2. Summary of SIP usage of the Offer/Answer Model . . . . . . . . 4 60 2.1. Offer/Answer Exchange Pairs in SIP Messages . . . . . . . 5 61 2.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 6 62 2.3. Session Description which is not Offer nor Answer . . . . 7 63 3. Detailed Discussion of the Offer/Answer Model for SIP . . . . 7 64 3.1. Offer/Answer for the INVITE method with 100rel 65 extension . . . . . . . . . . . . . . . . . . . . . . . . 7 66 3.1.1. INVITE Request with SDP . . . . . . . . . . . . . . . 8 67 3.1.2. INVITE request without SDP . . . . . . . . . . . . . . 10 68 3.2. Offer/Answer Exchange in Early Dialog . . . . . . . . . . 11 69 3.3. Offer/Answer Exchange in an Established Dialog . . . . . . 11 70 4. Exceptional Case Handling . . . . . . . . . . . . . . . . . . 12 71 4.1. Message Crossing Case Handling . . . . . . . . . . . . . . 12 72 4.2. Glare Case Handling . . . . . . . . . . . . . . . . . . . 14 73 5. Content of Offers and Answers . . . . . . . . . . . . . . . . 15 74 5.1. General Principle for Constructing Offers and Answers . . 16 75 5.2. Choice of Media Types and Formats to Include and 76 Exclude . . . . . . . . . . . . . . . . . . . . . . . . . 16 77 5.2.1. Sending an Initial INVITE with Offer . . . . . . . . . 16 78 5.2.2. Responding with an Offer when the Initial INVITE 79 has no Offer . . . . . . . . . . . . . . . . . . . . . 17 80 5.2.3. Answering an Initial INVITE with Offer . . . . . . . . 17 81 5.2.4. Answering when the Initial INVITE had no Offer . . . . 18 82 5.2.5. Subsequent Offers and Answers . . . . . . . . . . . . 18 83 5.3. Hold and Resume of media . . . . . . . . . . . . . . . . . 19 84 5.4. Behavior on receiving SDP with c=0.0.0.0 . . . . . . . . . 20 85 6. Remaining Issues or Best Practices on Offer/Answer . . . . . . 20 86 6.1. Rejecting PRACK Offer . . . . . . . . . . . . . . . . . . 21 87 6.2. Commit/Rollback of Offer/Answer on Unsuccessful 88 re-INVITE Transaction . . . . . . . . . . . . . . . . . . 21 89 6.3. Offer in a Reliable Response . . . . . . . . . . . . . . . 23 90 6.4. Requesting Hold while already on Hold . . . . . . . . . . 23 91 7. Add New Offer/Answer Usage in SIP . . . . . . . . . . . . . . 24 92 7.1. Explicit Usage . . . . . . . . . . . . . . . . . . . . . . 24 93 7.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 24 94 7.3. Backward Compatibility . . . . . . . . . . . . . . . . . . 24 95 7.4. Exceptional Case Handling . . . . . . . . . . . . . . . . 24 96 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 97 9. Security Considerations . . . . . . . . . . . . . . . . . . . 24 98 10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 25 99 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 100 11.1. Normative References . . . . . . . . . . . . . . . . . . . 25 101 11.2. Informative References . . . . . . . . . . . . . . . . . . 25 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 104 1. Introduction 106 SIP utilizes the offer/answer model to establish and update sessions. 107 The rules to govern the offer/answer behaviors in SIP are described 108 in the several RFCs. ([RFC3261], [RFC3262], [RFC3264], and 109 [RFC3311].) 111 The primary purpose of this document is to describe all forms of SIP 112 usage of the offer/answer model in one document to help the readers 113 to fully understand it. Also, this document tries to incorporate the 114 results of the discussions on the controversial issues to avoid 115 repeating the same discussions later. 117 This document is not intended to make normative changes. Rather, it 118 makes the remaining open issues clear and leaves them for further 119 study. 121 1.1. Terminology 123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 125 document are to be interpreted as described in RFC 2119 [RFC2119]. 126 This document only uses these key words when referencing normative 127 statements in existing RFCs. 129 2. Summary of SIP usage of the Offer/Answer Model 131 The offer/answer model itself is independent from the higher layer 132 application protocols which utilize it. SIP is one of the 133 applications using the offer/answer model. [RFC3264] defines the 134 offer/answer model, but does not specify which SIP messages should 135 convey an offer or an answer. This should be defined in the SIP core 136 and extensions RFCs. 138 In theory, any SIP message can include a session description in its 139 body. But a session description in a SIP message is not necessarily 140 an offer or an answer. Only certain session description usages that 141 conform to the rules described in standards-track RFCs can be 142 interpreted as an offer or an answer. The rules for how to handle 143 the offer/answer model are currently defined in several RFCs. 145 The offer/answer model defines a mechanism for update of sessions. 146 In SIP, a dialog is used to associate an offer/answer exchange with 147 the session which it is to update. In other words, only the offer/ 148 answer exchange in the SIP dialog can update the session which is 149 managed by that dialog. 151 2.1. Offer/Answer Exchange Pairs in SIP Messages 153 Currently, the rules on the offer/answer model are defined in 154 [RFC3261], [RFC3262], [RFC3264], and [RFC3311]. In these RFCs, only 155 the six patterns shown in Table 1 are defined for exchanging an offer 156 and an answer with SIP messages. 158 Note that an offer/answer exchange initiated by an INVITE request 159 must follow exactly one of the patterns 1, 2, 3, 4. When an initial 160 INVITE causes multiple dialogs due to forking, an offer/answer 161 exchange is carried out independently in each distinct dialog. When 162 an INVITE request contains no offer, only pattern 2 or pattern 4 163 apply. 'The first reliable non-failure message' must have an offer 164 if there is no offer in the INVITE request. This means that UA which 165 receives the INVITE request without an offer must include an offer in 166 the first reliable response with 100rel extension. If no reliable 167 provisional response has been sent, the UAS must include an offer 168 when sending 2xx response. 170 In pattern 3, the first reliable provisional response may or may not 171 have an answer. When a reliable provisional response contains a 172 session description, and is the first to do so, then that session 173 description is the answer to the offer in the INVITE request. The 174 answer can not be updated, and a new offer can not be sent in a 175 subsequent reliable response for the same INVITE transaction. 177 In pattern 5, a PRACK request can contain an offer only if the 178 reliable response which it acknowledges contains an answer to the 179 previous offer/answer exchange. 181 NOTE: It is legal to have UPDATE/2xx exchanges without offer/ 182 answer exchanges (pattern 6). However when re-INVITEs are sent 183 for non-offer/answer purposes, an offer/answer exchange is 184 required. In that case the prior SDP will typically be repeated. 186 There may be ONLY ONE offer/answer negotiation in progress for a 187 single dialog at any point in time. Section 4 explains how to ensure 188 this. When an INVITE results in multiple dialogs each has a separate 189 offer/answer negotiation. 191 NOTE: This is when using a Content-Disposition of "session". 192 There may be a second offer/answer negotiation in progress using a 193 Content-Disposition of "early-session" [RFC3959]. That is not 194 addressed by this draft. 196 Offer Answer RFC Ini Est Early 197 ------------------------------------------------------------------- 198 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 199 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 200 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 201 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 202 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 203 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y 205 Table 1. Summary of SIP Usage of the Offer/Answer Model 207 In Table 1, '1xx-rel' corresponds to the reliable provisional 208 response which contains the 100rel option defined in [RFC3262]. 210 The 'Ini' column shows the ability to exchange the offer/answer to 211 initiate the session. 'Y' indicates that the pattern can be used in 212 the initial offer/answer exchange, while 'N' indicates that it can 213 not. Only the initial INVITE transaction can be used to exchange the 214 offer/answer to establish a multimedia session. 216 The 'Est' column shows the ability to update the established session. 218 The 'Early' column indicates which patterns may be used to modify the 219 established session in an early dialog. There are two ways to 220 exchange a subsequent offer/answer in an early dialog. 222 2.2. Rejection of an Offer 224 It is not entirely clear how to reject an offer when it is 225 unacceptable, and some methods do not allow explicit rejection of an 226 offer. For each of the patterns in Table 1, Table 2 shows how to 227 reject an offer. 229 When a UA receives an INVITE request with an unacceptable offer, it 230 should respond with a 488 response, preferably with Warning header 231 field indicating the reason of the rejection, unless another response 232 code is more appropriate to reject it. (Pattern 1 and Pattern 3) 234 When a UA receives an UPDATE request with an offer which it can not 235 accept, it should respond with a 488 response preferably with Warning 236 header field indicating the reason of the rejection, unless another 237 response code is more appropriate to reject it. (Pattern 6) 239 When a UA receives a PRACK request with an offer which it can not 240 accept, it may respond with a 200 response with a syntactically 241 correct session description. This may optionally be followed by an 242 UPDATE request to rearrange the session parameters if both ends 243 support the UPDATE method. Alternatively the UA may terminate the 244 dialog and send an error response to the INVITE request. The 245 validity and consequences of a 488 response to PRACK is an open issue 246 which is discussed in Section 6.1. (Pattern 5) 248 When a UA receives a response with an offer which it can not accept, 249 the UA does not have a way to reject it explicitly. Therefore, a UA 250 should respond to the offer with the correct session description and 251 rearrange the session parameters by initiating a new offer/answer 252 exchange, or alternatively terminate the session. (Pattern 2 and 253 Pattern 4) When initiating a new offer/answer, a UA should take care 254 not to cause an infinite offer/answer loop. 256 Offer Rejection 257 ----------------------------------------------------- 258 1. INVITE Req. 488 INVITE Response 259 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 260 OR termination of dialog 261 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 262 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 263 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 264 OR termination of dialog 265 6. UPDATE Req. 488 UPDATE Response 267 (*) A UA should only use PRACK to send an offer when it has strong 268 reasons to expect the receiver will accept the offer. 270 Table 2. Rejection of an Offer 272 2.3. Session Description which is not Offer nor Answer 274 As previously stated, a session description in a SIP message is not 275 necessarily an offer or an answer. For example, SIP can use a 276 session description to describe capabilities apart from offer/answer 277 exchange. Examples of this are 200 OK responses for OPTIONS and 488 278 responses for INVITE. 280 3. Detailed Discussion of the Offer/Answer Model for SIP 282 3.1. Offer/Answer for the INVITE method with 100rel extension 284 The INVITE method provides the basic procedure for offer/answer 285 exchange in SIP. Without the 100rel option, the rules are simple as 286 described in [RFC3261]. If an INVITE request includes a session 287 description, pattern 1 is applied and if an INVITE request does not 288 include a session description, pattern 2 is applied. 290 With 100rel, pattern 3 and pattern 4 are added and this complicates 291 the rules. An INVITE request may cause multiple responses. Note 292 that even if both UAs support the 100rel extension, not all the 293 provisional responses may be sent reliably. Note also that a 294 reliable provisional response is allowed without a session 295 description if the UAS does not wish to send the answer yet. An 296 unreliable provisional response may include a session description in 297 the body if the UAS has not sent a reliable response, but its session 298 description is neither an offer nor an answer. All the session 299 descriptions in the unreliable responses to the INVITE request must 300 be identical to the answer which is included in the reliable 301 response. A session description in an unreliable response that 302 precedes a reliable response can be considered a "preview" of the 303 answer that will be coming, and hence may be treated like an answer 304 until the actual one arrives. 306 NOTE: This "preview" session description rule applies to a single 307 offer/answer exchange. In parallel offer/answer exchanges (caused 308 by forking) a UA may obviously receive a different "preview" of an 309 answer in each dialog. UAs are expected to deal with this. 311 Although RFC 3261 says a UA should accept media once an INVITE with 312 an offer has been sent, in many cases, an answer (or, at least a 313 preview of it) is required in order for media to be accepted. Two 314 examples of why this might be required are: 316 o To avoid receiving media from undesired sources, some User Agents 317 assume symmetric RTP will be used, ignore all incoming media 318 packets until an address/port has been received from the other 319 end, and then use that address/port to filter incoming media 320 packets. 322 o In some networks, an intermediate node must authorize a media 323 stream before it can flow and requires a confirming answer to the 324 offer before doing so. 326 Therefore, a UAS should send an SDP answer reliably (if possible) 327 before it starts sending media. And, if neither the UAC nor the UAS 328 support 100rel, the UAS should send a preview of the answer before it 329 starts sending media. 331 3.1.1. INVITE Request with SDP 333 When a UAC includes an SDP body in the INVITE request as an offer, it 334 expects the answer to be received with one of the reliable responses. 335 Other than that, no offer/answer exchanges can occur in the messages 336 within the INVITE transaction. 338 UAC UAS 339 | F1 INVITE (SDP) | <- The offer in the offer/answer model 340 |-------------------->| 341 | F2 1xx (SDP) | <- The offer/answer exchange is not 342 |<--------------------| closed yet, but UAC acts as if it 343 | | ^ receives the answer. 344 | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer 345 |<--------------------| | SDP. 346 | F4 PRACK (no SDP) | | 347 |-------------------->| | UAC must not send a new offer. 348 | F5 2xx PRA (no SDP) | | 349 |<--------------------| v 350 | | 351 | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model 352 |<--------------------| - 353 | F7 PRACK | | UAC can send a new offer in a PRACK 354 |-------------------->| | request to acknowledge F6. 355 | F8 2xx PRA | | After F7 UAC and UAS can send a new 356 |<--------------------| v offer in an UPDATE request. 357 | | 358 | F9 1xx-rel | <- SDP should not be included in the 359 |<--------------------| subsequent 1xx-rel once offer/answer 360 | F10 PRACK | has been completed. 361 |-------------------->| 362 | F11 2xx PRA | 363 |<--------------------| 364 | | 365 | F12 2xx INV | <- SDP should not be included in the 366 |<--------------------| final response once offer/answer has 367 | F13 ACK | been completed. 368 |-------------------->| 370 Figure 1 Example of Offer/Answer with 100rel Extension (1) 372 For example, in Figure 1, only the SDP in F6 is the answer. The SDP 373 in the non-reliable response (F2) is the preview of the answer and 374 must be the same as the answer in F6. Receiving F2, the UAC should 375 act as if it receives the answer. However, offer/answer exchange is 376 not completed yet and the UAC must not send a new offer until it 377 receives the same SDP in the first reliable response, which is the 378 real answer. After sending the SDP in F6, the UAS must prepare to 379 receive a new offer from the UAC with an UPDATE request or a PRACK 380 request. 382 The UAS does not include SDP in responses F9 and F12. However, the 383 UAC should prepare to receive SDP bodies in F9 and/or F12, and just 384 ignore them, to handle a peer that does not conform to the 385 recommended implementation. 387 3.1.2. INVITE request without SDP 389 When a UAC does not include an SDP body in the INVITE request, it 390 expects the offer to be received with the first reliable response. 391 The UAC will send the answer in the request to acknowledge the 392 response, i.e. PRACK or ACK request of the reliable response. Other 393 than that, no offer/answer exchanges can occur in the messages within 394 the INVITE transaction. 396 NOTE: The UAS should not include SDP in the responses F6 and F9. 397 However, the UAC should prepare to receive SDP bodies in F6 and/or 398 F9, and just ignore them to handle a peer that does not conform to 399 the recommended implementation. 401 UAC UAS 402 | F1 INVITE (no SDP) | 403 |-------------------->| 404 | F2 1xx | 405 |<--------------------| 406 | | 407 | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP 408 |<--------------------| as the offer. 409 | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel 410 |-------------------->| must contain SDP as the answer. 411 | F5 2xx PRA (no SDP) | - 412 |<--------------------| | 413 | | | 414 | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not 415 |<--------------------| | contain SDP. 416 | F7 PRACK | | 417 |-------------------->| | UAC can send a new offer in an UPDATE 418 | F8 2xx PRA | | request after F4. 419 |<--------------------| v 420 | | 421 | F9 2xx INV (no SDP) | <- The final response should not 422 |<--------------------| contain SDP. 423 | F10 ACK | 424 |-------------------->| 426 Figure 2 Example of Offer/Answer with 100rel Extension (2) 428 Note that in the case that the UAC needs to prompt the user to accept 429 or reject the offer, the reliable provisional response with SDP as an 430 offer (pattern 4) can result in the retransmission until the PRACK 431 request can be sent. The UAC should take care to avoid this 432 situation when it sends the INVITE request without SDP. 434 3.2. Offer/Answer Exchange in Early Dialog 436 When both UAs support the 100rel extension, they can update the 437 session in the early dialog once the first offer/answer exchange has 438 been completed. 440 From a UA sending an INVITE request: 442 A UA can send an UPDATE request with a new offer if both ends support 443 the UPDATE method. Note that if the UAS needs to prompt the user to 444 accept or reject the offer, the delay can result in retransmission of 445 the UPDATE request. 447 A UA can send a PRACK request with a new offer only when 448 acknowledging the reliable provisional response carrying the answer 449 to an offer in the INVITE request. Compared to using the UPDATE 450 method, using PRACK can reduce the number of messages exchanged 451 between the UAs. However, to avoid problems or delays caused by 452 PRACK offer rejection, the UA is recommended to send a PRACK request 453 only when it has strong reasons to expect the receiver will accept 454 it. For example, the procedure used in precondition extension 455 [RFC3312] is a case where a PRACK request should be used for updating 456 the session status in an early dialog. Note also that if a UAS needs 457 to prompt the user to accept or reject the offer, the delay can 458 result in retransmission of the PRACK request. 460 From a UA receiving an INVITE request: 462 A UA can send an UPDATE request with a new offer if both ends support 463 the UPDATE method. A UAS can not send a new offer in the reliable 464 provisional response, so the UPDATE method is the only method for a 465 UAS to update an early session. 467 3.3. Offer/Answer Exchange in an Established Dialog 469 Both the re-INVITE and UPDATE methods can be used in an established 470 dialog to update the session. 472 The UPDATE method is simpler and can save at least one message 473 compared with the INVITE method. But both ends must support the 474 UPDATE method for it to be used. 476 The INVITE method needs at least three messages to complete but no 477 extensions are needed. Additionally, the INVITE method allows the 478 peer to take time to decide whether it will accept a session update 479 or not by sending provisional responses. That is, re-INVITE allows 480 the UAS to interact with the user at the peer, while UPDATE needs to 481 be answered automatically by the UAS. It is noted that re-INVITE 482 should be answered immediately unless such a user interaction is 483 needed. Otherwise, some 3pcc flows will break. 485 4. Exceptional Case Handling 487 In [RFC3264], the following restrictions are defined with regard to 488 sending a new offer. 490 "At any time, either agent MAY generate a new offer that updates 491 the session. However, it MUST NOT generate a new offer if it has 492 received an offer which it has not yet answered or rejected. It 493 MUST NOT generate a new offer if it has generated a prior offer 494 for which it has not yet received an answer or a rejection." 496 Assuming that the above rules are guaranteed, there seem to be two 497 possible 'exceptional' cases to be considered in SIP offer/answer 498 usage: the 'message crossing' case, and the 'glare' case. One of the 499 reasons why the usage of SIP methods to exchange offer/answer needs 500 to be carefully restricted in the RFCs is to ensure that the UA can 501 detect and handle appropriately the 'exceptional' cases to avoid 502 incompatible behavior. 504 4.1. Message Crossing Case Handling 506 When message packets cross in the transport network, an offer may be 507 received before the answer for the previous offer/answer exchange, as 508 shown in Figure 3. In such a case, UA A must detect that the session 509 description SDP-2 is not the answer to offer1. 511 A B 512 |SDP-1 (offer1)| 513 M1 |----------------->| 514 |SDP-2 (answer1)| 515 M2 |<------\ /-------| 516 | \/ | 517 |SDP-3 /\(offer2)| 518 M3 |<------/ \-------| 520 Figure 3 Message Crossing Case 522 Because of the restrictions on placement of offers and answers 523 (summarized in Table 1) there are a limited number of valid exchanges 524 of messages that may lead to this message crossing case. These are 525 enumerated in Table 3. (This table only shows messages containing 526 offers or answers. There could be other messages, without session 527 descriptions, which are not shown.) 528 There is a variant, shown in Figure 4, which is dependent on an 529 INVITE (Mx) that contains no offer. This case should be extremely 530 rare - it is easily avoided by delaying Mx until answer1 is received. 531 It adds another possibility to Table 3. 533 A B 534 | | 535 |SDP-1 offer1(UPD) | 536 M1 |==============================>| 537 |re-INV (no offer) | 538 Mx |------------------------------>| --+ 539 |SDP-2 answer1 (2xx-UPD)| | 540 M2 |<===========\ /===============| | first reliable 541 | \/ offer2| | response 542 |SDP-3 /\ (1xx-rel/2xx)| | 543 M3 |<===========/ \===============| <-+ 544 |SDP-4 answer2 (PRACK/ACK)| 545 My |------------------------------>| Wait until answer1 546 | | 548 Figure 4 Reliable response as a message with offer2 in message 549 crossing case 551 | M1 | M3 | M2 | 552 |--------+----------+---------| 553 | INVITE | 1xx-rel | UPDATE | 554 |--------+----------+---------| 555 | PRACK | 200-PRA | UPDATE | 556 |--------+----------+---------| 557 | UPDATE | 200-UPD | UPDATE | 558 | | |---------| 559 | | | INVITE | (no INV in progress) 560 | | |---------| 561 | | | 2xx-INV | (INV in progress) 562 | | |---------| 563 | | | 1xx-rel | (from Figure 4) 564 |-----------------------------| 566 Table 3. Offer / Answer Crossing Message Sequences 568 Table 3 shows that there are only two ambiguous cases when an answer 569 is expected and an arriving message M2 containing SDP could be either 570 the expected answer or an offer. These are a reliable 1xx response 571 to an INVITE, or an UPDATE. 573 When message M2 is an UPDATE request or a (re)INVITE request, then 574 message M1 must also have been an UPDATE or INVITE. There may have 575 been message crossing, or not. If not then it is a glare case. 576 Either way, the remedy is for UA A to reject message M2 with a 491 577 response with Retry-After header field. 579 When M2 is a reliable provisional response or a successful final 580 response, and M1 was an UPDATE, then SDP-2 cannot be the expected 581 answer1. In this case, since UA A can not reject offer2 in reliable 582 response M2, it is recommended that it wait for answer1 before 583 sending a PRACK request with the answer to offer2. Note that this 584 case only occurs when UA A, while waiting for an answer, sends an 585 INVITE request without session description. 587 When M2 is a PRACK request Table 3 shows that it cannot be an offer 588 out of order, so UA A may infer SDP-2 is an answer. 590 Table 4 summarizes the discussions above. 592 SDP-2 | How to know it's not answer1 | Actions to take 593 -------+------------------------------+-------------------------- 594 INVITE | Never be an answer | 491 response 595 UPDATE | Glare case for UA A | with Retry-After 596 -------+------------------------------+-------------------------- 597 1xx-rel| If M1 was UPDATE then SDP-2 | Delay ACK/PRACK 598 2xx-INV| is not answer1 | until answer1 is received 599 -------+------------------------------+-------------------------- 600 PRACK | This case never happens | Not a message cross case 601 | under the current rules. | Treat SDP-2 as answer2 602 -------+------------------------------+-------------------------- 604 Table 4. Message Crossing Resolution 606 4.2. Glare Case Handling 608 When both ends in a dialog send a new offer at nearly the same time, 609 as described in Figure 5, a UA may receive a new offer before it 610 receives the answer to the offer it sent. This case is usually 611 called a 'glare' case. 613 A B 614 |offer1 offer2| 615 |-------\ /-------| 616 | \/ | 617 | /\ | 618 |<------/ \------>| 620 Figure 5 Glare Case 622 When offer2 is in an UPDATE request or (re-)INVITE request, it must 623 be rejected with a 491 response. 625 When offer2 is in a PRACK request (within the current rules, only 626 possible if offer1 is in an UPDATE request), the PRACK may be 627 accepted with 200 or may be rejected with a 491 response. A 491 628 response is valid to satisfy the offer/answer model but it may delay 629 the completion of the reliable response transfer mechanism or, in 630 worst case, may result in the failure to complete the SIP transaction 631 because there is no clear retry rule when a PRACK request is rejected 632 with a 491 response. To avoid this glare condition, UA A should not 633 send an offer if it has already sent a reliable provisional response 634 containing an answer to a previous offer and has not received the 635 corresponding PRACK request. 637 To avoid a glare condition involving an offer in a response, when UA 638 A has sent a (re)INVITE request without session description, it 639 should not send an offer until it has received an offer in a reliable 640 response to the (re)INVITE, and sent an answer to that offer. 642 5. Content of Offers and Answers 644 While [RFC3264] and [RFC3312] give some guidance, questions remain 645 about exactly what should be included in an offer or answer. This is 646 especially a problem when the common "hold" feature has been 647 activated, and when there is the potential for a multimedia call. 649 Details of behavior depend on the capabilities and state of the User 650 Agent. The kinds of recommendations that can be made are limited by 651 the model of device capabilities and state that is presumed to exist. 653 This section focuses on a few key aspects of offers and answers that 654 have been identified as troublesome, and will consider other aspects 655 to be out of scope. This section considers: 656 o choice of supported media types and formats to include and exclude 657 o hold and resume of media 659 The following are out of scope for this document: 660 o NAT traversal and ICE 661 o specific codecs and their parameters 662 o the negotiation of secure media streams 663 o grouping of media streams 664 o preconditions 666 5.1. General Principle for Constructing Offers and Answers 668 A UA should send an offer that indicates what it, and its user, are 669 interested in using/doing at that time, without regard for what the 670 other party in the call may have indicated previously. This is the 671 case even when the offer is sent in response to an INVITE or re- 672 INVITE that contains no offer. (However in the case of re-INVITE the 673 constraints of RFCs 3261 and 3264 must be observed.) 675 A UA should send an answer that includes as close an approximation to 676 what the UA and its user are interested in doing at that time, while 677 remaining consistent with the offer/answer rules of [RFC3264] and 678 other RFCs. 680 NOTE: "at that time" is important. The device may permit the user 681 to configure which supported media are to be used by default. 683 In some cases a UA may not have direct knowledge of what it is 684 interested in doing at a particular time. If it is an intermediary 685 it may be able to delegate the decision. In the worst case it may 686 apply a default, such as assuming it wants to use all of its 687 capabilities. 689 5.2. Choice of Media Types and Formats to Include and Exclude 691 5.2.1. Sending an Initial INVITE with Offer 693 When a UAC sends an initial INVITE with an offer, it has complete 694 freedom to choose which media type(s) and media format(s) (payload 695 types in the case of RTP) it should include in the offer. 697 The media types may be all or a subset of the media the UAC is 698 capable of supporting, with the particular subset being determined by 699 the design and configuration (e.g., via 700 [I-D.ietf-sipping-config-framework]) of the UAC combined with input 701 from the user interface of the UAC. 703 The media formats may be all or a subset of the media formats the UAC 704 is capable of supporting for the corresponding media type, with the 705 particular subset being determined by the design and configuration of 706 the UAC combined with input from the user interface of the UAC. 708 Including all supported media formats will maximize the possibility 709 that the other party will have a supported format in common. But 710 including many can result in an unacceptably large SDP body. 712 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 714 When a UAS has received an initial INVITE without an offer, it must 715 include an offer in the first reliable response to the INVITE. It 716 has largely the same options as when sending an initial INVITE with 717 an offer, but there are some differences. The choice may be governed 718 by both static (default) selections of media types as well as dynamic 719 selections made by a user via interaction with the device while it is 720 alerting. 722 NOTE: The offer may be sent in a provisional response, before the 723 user of the device has been alerted and had an opportunity to 724 select media options for the call. In this case the UAS cannot 725 include any call-specific options from the user of the device. If 726 there is a possibility that the user of the device will wish to 727 change what is offered before answering the call, then special 728 care should be taken. If PRACK and UPDATE are supported by caller 729 and callee then an initial offer can be sent reliably, and changed 730 with an UPDATE if the user desires a change. If PRACK and UPDATE 731 are not supported then the initial offer cannot be changed until 732 the call is fully established. In that case either the offer 733 should be delayed until the 200 is sent, or else the offer should 734 include the minimum set of media the user is able to select. 736 5.2.3. Answering an Initial INVITE with Offer 738 When a UAS receives an initial INVITE with an offer, what media lines 739 the answer may contain is constrained by [RFC3264]. The answer must 740 contain the same number of m-lines as the offer, and they must 741 contain the same media types. Each media line may be accepted, by 742 including a non-zero port number, or rejected by including a zero 743 port number in the answer. The media lines that are accepted should 744 typically be those that would have been offered had the INVITE not 745 contained an offer, excluding those not offered. 747 The media formats the answer may contain are constrained by 748 [RFC3264]. For each accepted m-line in the answer, there must be at 749 least one media format in common with the corresponding m-line of the 750 offer. The UAS may also include other media formats it is able to 751 support at this time. However there is little benefit to including 752 added types. 754 If the UAS does not wish to indicate support for any of the media 755 types in a particular media line of the offer it must reject the 756 corresponding media line, by setting the port number to zero. 758 5.2.4. Answering when the Initial INVITE had no Offer 760 When a UAC has sent an initial INVITE without an offer, and then 761 receives a response with the first offer, it should answer in the 762 same way as a UAS receiving an initial INVITE with an offer. 764 5.2.5. Subsequent Offers and Answers 766 The guidelines above (Section 5.1 and Section 5.2.1 through 767 Section 5.2.4) apply, but constraints in [RFC3264] must also be 768 followed. The following are of particular note because they have 769 proven troublesome: 770 o The number of m-lines may not be reduced in a subsequent offer. 771 Previously rejected media streams must remain, or be reused to 772 offer the same or a different stream. (Section 6 of [RFC3264].) 773 o In the o-line, only the version number may change, and if it 774 changes it must increment by one from the one previously sent as 775 an offer or answer. (Section 8 of [RFC3264].) If it doesn't 776 change then the entire SDP body must be identical to what was 777 previously sent as an offer or answer. Changing the o-line, 778 except version number value, during the session is an error case. 779 The behavior when receiving such a non-compliant offer/answer SDP 780 body is implementation dependent. If a UA needs to negotiate a 781 'new' SDP session, it should use the INVITE/Replaces method. 782 o In the case of RTP, the mapping from a particular dynamic payload 783 type number to a particular codec within that media stream 784 (m-line) must not change for the duration of the session. 785 (Section 8.3.2 of [RFC3264].) 787 NOTE: This may be impossible for a B2BUA to follow in some 788 cases (e.g. 3pcc transfer) if it does not terminate media. 790 When the new offer is sent in response to an offerless (re)INVITE, 791 all codecs supported by the UA are to be included, not just the ones 792 that were negotiated by previous offer/answer exchanges. The same is 793 true for media types - so if UA A initially offered audio and video 794 to UA B, and they end up with only audio, and UA B sends an offerless 795 (re)INVITE to UA A, A's resulting offer should re- attempt video, by 796 reusing the zeroed m-line used previously. 798 NOTE: The behavior above is recommended, but it is not always 799 achievable - for example in some interworking scenarios. Or, the 800 offerer may simply not have enough resources to offer "everything" 801 at that point. Even if the UAS is not able to offer any other SDP 802 that the one currently being used, it should not reject the re- 803 INVITE. Instead, it should generate an offer with the currently 804 used SDP with o- line unchanged. 806 5.3. Hold and Resume of media 808 [RFC3264] specifies (non-normatively) that "hold" should be indicated 809 in an established session by sending a new offer containing 810 "a=sendonly" for each media stream to be held. An answerer is then 811 to respond with "a=recvonly" to acknowledge that the hold request has 812 been understood. 814 Note that the use of sendonly/recvonly is not limited to hold. These 815 may be used for other reasons, such as devices that are only capable 816 of sending or receiving. So receiving an offer with "a=sendonly" 817 must not be treated as a certain indication that the offerer has 818 placed the media stream on hold. 820 This model is based on an assumption that the UA initiating the hold 821 will want to play Music on Hold, which is not always the case. A UA 822 may, if desired, initiate hold by offering "a=inactive" if it does 823 not intend to transmit any media while in hold status. 825 The rules of [RFC3264] constrain what may be in an answer when the 826 offer contains "sendonly", "recvonly", or "inactive" in an a= line. 827 But they do not constrain what must be in a subsequent offer. The 828 General Principle for Constructing Offers and Answers (Section 5.1) 829 is important here. The initiation of "hold" is a local action. It 830 should reflect the desired state of the UA. It then affects what the 831 UA includes in offers and answers until the local state is reset. 833 The receipt of an offer containing "a=sendonly" or "a=inactive" and 834 the sending of a compatible answer should not change the desired 835 state of the recipient. However, a UA that has been "placed on hold" 836 may itself desire to initiate its own hold status, based on local 837 input. 839 If UA2 has previously been "placed on hold" by UA1, via receipt of 840 "a=sendonly", then it may initiate its own hold by sending a new 841 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will 842 answer with "a=inactive" because that is the only valid answer that 843 reflects its desire not to receive media. 845 Once in this state, to resume a two way exchange of media each side 846 must reset its local hold status. If UA1 is first to go off hold it 847 will then send an offer with "a=sendrecv". The UA2 will respond with 848 its desired state of "a=sendonly" because that is a permitted 849 response. When UA2 desires to also resume, it will send an offer 850 with "a=sendrecv". In this case, because UA1 has the same desire it 851 will respond with "a=sendrecv". In the same case, when UA2 receives 852 the offer with "a=sendrecv", if it has decided it wants to reset its 853 local hold but has not yet signaled the intent, it may send 854 "a=sendrecv" in the answer. 856 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 857 and subsequently wants to initiate its own hold, also using 858 "a=inactive", it need not send a new offer, since the only valid 859 response is "a=inactive" and that is already in effect. However, its 860 local desired state will now be either "inactive" or "a=sendonly". 861 This affects what it will send in future offers and answers. 863 If a UA has occasion to send another offer in the session, without 864 any desire to change the hold status (e.g. in response to a re- 865 INVITE without an offer, or when sending a re-INVITE to refresh the 866 session timer) it should follow the General Principle for 867 Constructing Offers and Answers (Section 5.1). If it previously 868 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 869 should offer that again. If it had not previously initiated "hold" 870 then it should offer "a=sendrecv", even if it had previously been 871 forced to answer something else. Without this behavior it is 872 possible to get "stuck on hold" in some cases, especially when a 873 third-party call controller is involved. 875 5.4. Behavior on receiving SDP with c=0.0.0.0 877 [RFC3264] specifies that an agent MUST be capable of receiving SDP 878 with a connection address of 0.0.0.0, in which case it means that 879 neither RTP nor RTCP should be sent to the peer. 881 If a UA generates an answer to the offer received with c=0.0.0.0, the 882 direction attribute of the accepted media stream in the answer must 883 be based on direction attribute of the offered stream and rules 884 specified in RFC 3264 to form the a-line in the answer. c=0.0.0.0 has 885 no special meaning for the direction attribute of the accepted stream 886 in the answer. 888 6. Remaining Issues or Best Practices on Offer/Answer 890 This document clarifies the offer/answer usage in SIP and summarizes 891 the correct or recommended behaviors along with the existing RFCs. 892 To create any new normative behaviors beyond these RFCs is not the 893 intent of this document. 895 However, through the scrutiny of the offer/answer model in SIP, some 896 issues are found to be unresolved within the current set of RFCs. 897 Those remaining issues are described in this section mainly for 898 further study. 900 6.1. Rejecting PRACK Offer 902 As stated in Section 2.2 and Section 3.2, it is recommended that an 903 offer not be sent in a PRACK request unless UAC has strong reasons to 904 assume the receiver will accept it. Even so, there may be cases when 905 the UAS has to reject the offer for some reason. The current RFCs do 906 not provide a way to reject the offer and at the same time to 907 indicate that the PRACK adequately acknowledged the reliable 908 response. It is unclear whether a non-200 response can still 909 indicate an acknowledgement of the reliable response. 911 Several ideas were presented to resolve this issue, such as sending 912 2xx PRACK response without SDP to reject the offer, or sending SDP 913 with a decreased version value in the o-line. Some of the candidates 914 may also be adapted as a way to reject an unacceptable offer in a 915 response. Anyway, those proposals violate the current rules and lose 916 backward compatibility to some extent (e.g. section 5 of [RFC3262]). 917 It is beyond the scope of this document and remains for further 918 study. 920 The 488 response is another proposed solution; however the validity 921 and consequences of a 488 response to PRACK is an open issue. 922 Because the 488 response may be sent by a proxy, the UAC cannot 923 assume the reliable transaction has been adequately acknowledged. If 924 a 488 response is received, the UAC should ensure acknowledgment of 925 the reliable response by sending a new PRACK with the offer removed 926 or modified based upon the received 488 response. If the 488 927 response is sent by UAS (open issue), it cannot assume that the UAC 928 thinks that the reliable transaction has been adequately acknowledged 929 even though the UAS may treat otherwise (open issue). If a 488 930 response is sent by UAS, the UAC should accommodate receiving the 931 altered PRACK with higher CSeq without expecting it to trigger a 481 932 response (open issue). 934 NOTE: Deprecation of the usage of offer in PRACK may be another 935 solution. As the precondition mechanism specification [RFC3262] 936 explicitly shows a usage of sending offer in PRACK, its 937 deprecation could cause backward compatibility issues. 939 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 940 Transaction 942 When a re-INVITE transaction fails, the dialog remains with the 943 session bound to it. The issue here is: what is the session status 944 if an offer/answer exchange has been completed (if a session 945 description has been sent in a reliable provisional response to the 946 re-INVITE request), or if subsequent offer/answer exchanges have 947 taken place (using UPDATE or PRACK transactions), before the re- 948 INVITE transaction is terminated with a final error response (Figure 949 6). One option is to take those offer/answer exchanges not committed 950 yet and to make the session status rollback to the one before re- 951 INVITE transaction was initiated. Another option is to take those 952 exchanges committed and to keep the session status as it is even 953 after re-INVITE fails. There is no clear consensus on which one is 954 the correct behavior. 956 There are some cases where it is useful to exchange offer(s)/ 957 answer(s) even before re-INVITE completes. The case of adding a new 958 media (like adding video to audio only session) which requires 959 permission from the peer through some user interaction is one 960 example. Precondition procedures can be another case which may 961 require several offer/answer exchanges in one re-INVITE transaction. 963 UAC UAS 964 | session established | 965 |<===================>| 966 | | 967 | F1 re-INVITE (SDP) | 968 |-------------------->| 969 | F2 1xx-rel (SDP) | 970 |<--------------------| 971 | F3 PRACK | <- PRACK request may include new offer 972 |-------------------->| and can complete the offer/answer with 973 | F4 2xx PRA | the answer in 2xx PRACK response. 974 |<--------------------| 975 | | <- UPDATE method can update the session 976 | | status before receiving the final 977 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 978 |<--------------------| 979 | F6 ACK | 980 |-------------------->| Issue: What is the correct session 981 | | status after re-INVITE transaction. 983 Figure 6 Commit/Rollback Issue with re-INVITE transaction 985 To make bad things worse, if a new offer from UAC and the final 986 response to re-INVITE are sent at nearly the same time, the UAS can 987 not know whether this new offer was sent before or after UAC received 988 the final failure response (Figure 7). Note that the ACK request to 989 the failure response is sent hop-by-hop basis, therefore even after 990 receiving the ACK request, UAS can not make sure that UPDATE request 991 was sent after the final response had been reached to the other end. 993 Sending a new UPDATE request from UAC to synchronize the status 994 anytime after the re-INVITE fails may be a good option. This 995 solution, however, requires that the UPDATE method be supported by 996 both ends and needs care to avoid flapping when each end tries to 997 advertise their different views of the session status. 999 The proper handling of this issue is undefined by existing standards. 1000 Resolution is beyond the scope of this document, and will require a 1001 new normative document. 1003 UAC UAS 1004 | session established | 1005 |<===================>| 1006 | | 1007 | F1 re-INVITE (SDP) | 1008 |-------------------->| 1009 | F2 1xx-rel (SDP) | 1010 |<--------------------| 1011 | F3 PRACK | 1012 |-------------------->| 1013 | F4 2xx PRA | 1014 |<--------------------| 1015 | | 1016 |UPDATE(SDP) 4xx INV | 1017 |---------\ /--------| 1018 | \/ | 1019 | /\ | 1020 |<--------/ \------->| 1021 | | 1023 Figure 7 Commit/Rollback Issue with Race Condition 1025 6.3. Offer in a Reliable Response 1027 In RFC 3261, it is stated that when an INVITE is sent without an 1028 offer, the first reliable response MUST contain an offer. There was 1029 discussion on whether this rule can be loosened up. There is no 1030 clear explanation why this restriction is defined. However, this 1031 rule will be left as it is, unless the strong necessity to loosen it 1032 up is raised in the future. 1034 6.4. Requesting Hold while already on Hold 1036 RFC 3264, section 8.4, contains procedures for putting a unicast 1037 media stream on hold. Of particular note, it states: 1039 "If the stream to be placed on hold was previously a recvonly 1040 media stream, it is placed on hold by marking it inactive." 1042 Section 5.3 of the current document makes a recommendation for this 1043 case which conflicts with that, and explains why. Some concerns have 1044 been raised that such a recommendation is invalid because RFC 3264 is 1045 normative on this subject. 1047 This document takes the position that Section 8.4 of RFC 3264 is non- 1048 normative, and so may be overridden. It is further recommended that 1049 RFC 3264 be revised to avoid the confusion. 1051 7. Add New Offer/Answer Usage in SIP 1053 This document recommends against the addition of new offer/answer 1054 methods using SIP. However, it may be necessary to define new offer/ 1055 answer exchange methods as SIP extensions evolve. This section 1056 recommends some things that should be taken into considerations in 1057 that case. 1059 7.1. Explicit Usage 1061 New method definitions should define offer/answer usage explicitly 1062 without any ambiguity. 1064 7.2. Rejection of an Offer 1066 New method definitions should define how to reject an offer where 1067 possible. 1069 7.3. Backward Compatibility 1071 New methods must keep backward compatibility. 1073 7.4. Exceptional Case Handling 1075 New methods should take care of how to handle exceptional cases, 1076 message crossing case and glare case. 1078 8. IANA Considerations 1080 This document has no actions for IANA. 1082 9. Security Considerations 1084 There are not any security issues beyond the referenced RFCs. 1086 10. Acknowledgement 1088 The authors would like to thank Christer Holmberg, Rajeev Seth, 1089 Nataraju A B, Byron Campen and Jonathan Rosenberg for their thorough 1090 reviews and comments. Many of their suggestions and ideas are 1091 incorporated to complete this document. 1093 11. References 1095 11.1. Normative References 1097 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1098 Requirement Levels", BCP 14, RFC 2119, March 1997. 1100 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1101 A., Peterson, J., Sparks, R., Handley, M., and E. 1102 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1103 June 2002. 1105 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1106 Provisional Responses in Session Initiation Protocol 1107 (SIP)", RFC 3262, June 2002. 1109 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1110 with Session Description Protocol (SDP)", RFC 3264, 1111 June 2002. 1113 [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP) 1114 UPDATE Method", RFC 3311, October 2002. 1116 [RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg, 1117 "Integration of Resource Management and Session Initiation 1118 Protocol (SIP)", RFC 3312, October 2002. 1120 11.2. Informative References 1122 [RFC3959] Camarillo, G., "The Early Session Disposition Type for the 1123 Session Initiation Protocol (SIP)", RFC 3959, 1124 December 2004. 1126 [I-D.ietf-sipping-config-framework] 1127 Channabasappa, S., "A Framework for Session Initiation 1128 Protocol User Agent Profile Delivery", 1129 draft-ietf-sipping-config-framework-15 (work in progress), 1130 February 2008. 1132 Authors' Addresses 1134 Takuya Sawada 1135 KDDI Corporation 1136 3-10-10, Iidabashi, Chiyoda-ku 1137 Tokyo 1138 Japan 1140 Email: tu-sawada@kddi.com 1142 Paul H. Kyzivat 1143 Cisco Systems, Inc. 1144 1414 Massachusetts Avenue 1145 Boxborough, MA 01719 1146 USA 1148 Email: pkyzivat@cisco.com