idnits 2.17.1 draft-ietf-sipping-sip-offeranswer-09.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** It looks like you're using RFC 3978 boilerplate. You should update this to the boilerplate described in the IETF Trust License Policy document (see https://trustee.ietf.org/license-info), which is required now. -- Found old boilerplate from RFC 3978, Section 5.1 on line 17. -- Found old boilerplate from RFC 3978, Section 5.5, updated by RFC 4748 on line 1168. -- Found old boilerplate from RFC 3979, Section 5, paragraph 1 on line 1179. -- Found old boilerplate from RFC 3979, Section 5, paragraph 2 on line 1187. -- Found old boilerplate from RFC 3979, Section 5, paragraph 3 on line 1193. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust Copyright Line does not match the current year -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (November 3, 2008) is 5653 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: April 2009 Cisco Systems, Inc. 6 November 3, 2008 8 SIP (Session Initiation Protocol) Usage of the Offer/Answer Model 9 draft-ietf-sipping-sip-offeranswer-09.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 3, 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............................8 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.........11 61 4. Exceptional Case Handling...................................11 62 4.1. Message Crossing Case Handling.........................12 63 4.2. Glare Case Handling...................................14 64 5. Content of Offers and Answers...............................15 65 5.1. General Principle for Constructing Offers and Answers...16 66 5.2. Choice of Media Types and Formats to Include and Exclude16 67 5.2.1. Sending an Initial INVITE with Offer..............16 68 5.2.2. Responding with an Offer when the Initial INVITE has 69 no Offer................................................17 70 5.2.3. Answering an Initial INVITE with Offer............17 71 5.2.4. Answering when the Initial INVITE had no Offer.....18 72 5.2.5. Subsequent Offers and Answers.....................18 73 5.3. Hold and Resume of media...............................19 74 5.4. Behavior on receiving SDP with c=0.0.0.0...............20 75 6. Remaining Issues or Best Practices on Offer/Answer..........21 76 6.1. Rejecting PRACK Offer..................................21 77 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 78 Transaction................................................22 79 6.3. Offer in a Reliable Response...........................24 80 6.4. Requesting Hold while already on Hold..................24 81 7. Add New Offer/Answer Usage in SIP...........................25 82 7.1. Explicit Usage........................................25 83 7.2. Rejection of an Offer..................................25 84 7.3. Backward Compatibility.................................25 85 7.4. Exceptional Case Handling..............................25 86 8. IANA Considerations........................................25 87 9. Security Considerations....................................25 88 10. Acknowledgement...........................................25 89 11. References................................................26 90 11.1. Normative References..................................26 91 11.2. Informative References................................26 92 Author's Addresses............................................26 93 Full Copyright Statement......................................27 94 Intellectual Property Statement................................27 95 Acknowledgment................................................27 97 1. Introduction 99 SIP utilizes the offer/answer model to establish and update 100 sessions. The rules to govern the offer/answer behaviors in SIP are 101 described in the several RFCs. (RFC 3261 [2], RFC 3262 [3], RFC 102 3264 [4], and RFC 3311 [5].) 104 The primary purpose of this document is to describe all forms of 105 SIP usage of the offer/answer model in one document to help the 106 readers to fully understand it. Also, this document tries to 107 incorporate the results of the discussions on the controversial 108 issues to avoid repeating the same discussions later. 110 This document is not intended to make normative changes. Rather, it 111 makes the remaining open issues clear and leaves them for further 112 study. 114 1.1. Terminology 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 118 this document are to be interpreted as described in RFC 2119 [1]. 119 This document only uses these key words when referencing normative 120 statements in existing RFCs. 122 2. Summary of SIP usage of the Offer/Answer Model 124 The offer/answer model itself is independent from the higher layer 125 application protocols which utilize it. SIP is one of the 126 applications using the offer/answer model. RFC 3264 [4] defines the 127 offer/answer model, but does not specify which SIP messages should 128 convey an offer or an answer. This should be defined in the SIP 129 core and extensions RFCs. 131 In theory, any SIP message can include a session description in its 132 body. But a session description in a SIP message is not necessarily 133 an offer or an answer. Only certain session description usages that 134 conform to the rules described in standards-track RFCs can be 135 interpreted as an offer or an answer. The rules for how to handle 136 the offer/answer model are currently defined in several RFCs. 138 The offer/answer model defines a mechanism for update of sessions. 139 In SIP, a dialog is used to associate an offer/answer exchange with 140 the session which it is to update. In other words, only the 141 offer/answer exchange in the SIP dialog can update the session 142 which is managed by that dialog. 144 2.1. Offer/Answer Exchange Pairs in SIP Messages 146 Currently, the rules on the offer/answer model are defined in RFC 147 3261 [1], RFC 3262 [3], RFC 3264 [4], and RFC 3311 [5]. In these 148 RFCs, only the six patterns shown in Table 1 are defined for 149 exchanging an offer and an answer with SIP messages. 151 Note that an offer/answer exchange initiated by an INVITE request 152 must follow exactly one of the patterns 1, 2, 3, 4. When an initial 153 INVITE causes multiple dialogs due to forking, an offer/answer 154 exchange is carried out independently in each distinct dialog. When 155 an INVITE request contains no offer, only pattern 2 or pattern 4 156 apply. 'The first reliable non-failure message' must have an offer 157 if there is no offer in the INVITE request. This means that UA 158 which receives the INVITE request without an offer must include an 159 offer in the first reliable response with 100rel extension. If no 160 reliable provisional response has been sent, the UAS must include 161 an offer when sending 2xx response. 163 In pattern 3, the first reliable provisional response may or may 164 not have an answer. When a reliable provisional response contains a 165 session description, and is the first to do so, then that session 166 description is the answer to the offer in the INVITE request. The 167 answer can not be updated, and a new offer can not be sent in a 168 subsequent reliable response for the same INVITE transaction. 170 In pattern 5, a PRACK request can contain an offer only if the 171 reliable response which it acknowledges contains an answer to the 172 previous offer/answer exchange. 174 NOTE: It is legal to have UPDATE/2xx exchanges without 175 offer/answer exchanges (pattern 6). However when re-INVITEs 176 are sent for non-offer/answer purposes, an offer/answer 177 exchange is required. In that case the prior SDP will 178 typically be repeated. 180 There may be ONLY ONE offer/answer negotiation in progress for a 181 single dialog at any point in time. Section 4 explains how to 182 ensure this. When an INVITE results in multiple dialogs each has a 183 separate offer/answer negotiation. 185 NOTE: This is when using a Content-Disposition of "session". 186 There may be a second offer/answer negotiation in progress 187 using a Content-Disposition of "early-session" [7]. That is 188 not addressed by this draft. 190 Offer Answer RFC Ini Est Early 191 ------------------------------------------------------------------- 192 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 193 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 194 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 195 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 196 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 197 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y 199 Table 1. Summary of SIP Usage of the Offer/Answer Model 201 In Table 1, '1xx-rel' corresponds to the reliable provisional 202 response which contains the 100rel option defined in RFC 3262 [3]. 204 The 'Ini' column shows the ability to exchange the offer/answer to 205 initiate the session. 'Y' indicates that the pattern can be used in 206 the initial offer/answer exchange, while 'N' indicates that it can 207 not. Only the initial INVITE transaction can be used to exchange 208 the offer/answer to establish a multimedia session. 210 The 'Est' column shows the ability to update the established 211 session. 213 The 'Early' column indicates which patterns may be used to modify 214 the established session in an early dialog. There are two ways to 215 exchange a subsequent offer/answer in an early dialog. 217 2.2. Rejection of an Offer 219 It is not entirely clear how to reject an offer when it is 220 unacceptable, and some methods do not allow explicit rejection of 221 an offer. For each of the patterns in Table 1, Table 2 shows how to 222 reject an offer. 224 When a UA receives an INVITE request with an unacceptable offer, it 225 should respond with a 488 response, preferably with Warning header 226 field indicating the reason of the rejection, unless another 227 response code is more appropriate to reject it. (Pattern 1 and 228 Pattern 3) 230 When a UA receives an UPDATE request with an offer which it can not 231 accept, it should respond with a 488 response preferably with 232 Warning header field indicating the reason of the rejection, unless 233 another response code is more appropriate to reject it. (Pattern 6) 234 When a UA receives a PRACK request with an offer which it can not 235 accept, it may respond with a 200 response with a syntactically 236 correct session description. This may optionally be followed by an 237 UPDATE request to rearrange the session parameters if both ends 238 support the UPDATE method. Alternatively the UA may terminate the 239 dialog and send an error response to the INVITE request. The 240 validity and consequences of a 488 response to PRACK is an open 241 issue which is discussed within a subsequent section (Section 242 6.1. ). (Pattern 5) 244 When a UA receives a response with an offer which it can not accept, 245 the UA does not have a way to reject it explicitly. Therefore, a UA 246 should respond to the offer with the correct session description 247 and rearrange the session parameters by initiating a new 248 offer/answer exchange, or alternatively terminate the session. 249 (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA 250 should take care not to cause an infinite offer/answer loop. 252 Offer Rejection 253 ----------------------------------------------------- 254 1. INVITE Req. 488 INVITE Response 255 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 256 OR termination of dialog 257 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 258 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 259 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 260 OR termination of dialog 261 6. UPDATE Req. 488 UPDATE Response 263 Table 2. Rejection of an Offer 265 (*) A UA should only use PRACK to send an offer when it has strong 266 reasons to expect the receiver will accept the offer. 268 2.3. Session Description which is not Offer nor Answer 270 As previously stated, a session description in a SIP message is not 271 necessarily an offer or an answer. For example, SIP can use a 272 session description to describe capabilities apart from 273 offer/answer exchange. Examples of this are 200 OK responses for 274 OPTIONS and 488 responses for INVITE. 276 3. Detailed Discussion of the Offer/Answer Model for SIP 278 3.1. Offer/Answer for the INVITE method with 100rel extension 280 The INVITE method provides the basic procedure for offer/answer 281 exchange in SIP. Without the 100rel option, the rules are simple as 282 described in RFC 3261 [1]. If an INVITE request includes a session 283 description, pattern 1 is applied and if an INVITE request does not 284 include a session description, pattern 2 is applied. 286 With 100rel, pattern 3 and pattern 4 are added and this complicates 287 the rules. An INVITE request may cause multiple responses. Note 288 that even if both UAs support the 100rel extension, not all the 289 provisional responses may be sent reliably. Note also that a 290 reliable provisional response is allowed without a session 291 description if the UAS does not wish to send the answer yet. An 292 unreliable provisional response may include a session description 293 in the body if the UAS has not sent a reliable response, but its 294 session description is neither an offer nor an answer. All the 295 session descriptions in the unreliable responses to the INVITE 296 request must be identical to the answer which is included in the 297 reliable response. A session description in an unreliable response 298 that precedes a reliable response can be considered a "preview" of 299 the answer that will be coming, and hence may be treated like an 300 answer until the actual one arrives. 302 NOTE: This "preview" session description rule applies to a 303 single offer/answer exchange. In parallel offer/answer 304 exchanges (caused by forking) a UA may obviously receive a 305 different "preview" of an answer in each dialog. UAs are 306 expected to deal with this. 308 Although RFC 3261 says a UA should accept media once an INVITE with 309 an offer has been sent, in many cases, an answer (or, at least a 310 preview of it) is required in order for media to be accepted. Two 311 examples of why this might be required are: 313 o To avoid receiving media from undesired sources, some User 314 Agents assume symmetric RTP will be used, ignore all incoming 315 media packets until an address/port has been received from the 316 other end, and then use that address/port to filter incoming 317 media packets. 319 o In some networks, an intermediate node must authorize a media 320 stream before it can flow and requires a confirming answer to 321 the offer before doing so. 323 Therefore, a UAS should send an SDP answer reliably (if possible) 324 before it starts sending media. And, if neither the UAC nor the UAS 325 support 100rel, the UAS should send a preview of the answer before 326 it starts sending media. 328 3.1.1. INVITE Request with SDP 330 When a UAC includes an SDP body in the INVITE request as an offer, 331 it expects the answer to be received with one of the reliable 332 responses. Other than that, no offer/answer exchanges can occur in 333 the messages within the INVITE transaction. 335 UAC UAS 336 | F1 INVITE (SDP) | <- The offer in the offer/answer model 337 |-------------------->| 338 | F2 1xx (SDP) | <- The offer/answer exchange is not 339 |<--------------------| closed yet, but UAC acts as if it 340 | | ^ receives the answer. 341 | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer 342 |<--------------------| | SDP. 343 | F4 PRACK (no SDP) | | 344 |-------------------->| | UAC must not send a new offer. 345 | F5 2xx PRA (no SDP) | | 346 |<--------------------| v 347 | | 348 | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model 349 |<--------------------| - 350 | F7 PRACK | | UAC can send a new offer in a PRACK 351 |-------------------->| | request to acknowledge F6. 352 | F8 2xx PRA | | After F7 UAC and UAS can send a new 353 |<--------------------| v offer in an UPDATE request. 354 | | 355 | F9 1xx-rel | <- SDP should not be included in the 356 |<--------------------| subsequent 1xx-rel once offer/answer 357 | F10 PRACK | has been completed. 358 |-------------------->| 359 | F11 2xx PRA | 360 |<--------------------| 361 | | 362 | F12 2xx INV | <- SDP should not be included in the 363 |<--------------------| final response once offer/answer has 364 | F13 ACK | been completed. 365 |-------------------->| 367 Figure 1 Example of Offer/Answer with 100rel Extension (1) 369 For example, in Figure 1, only the SDP in F6 is the answer. The SDP 370 in the non-reliable response (F2) is the preview of the answer and 371 must be the same as the answer in F6. Receiving F2, the UAC should 372 act as if it receives the answer. However, offer/answer exchange is 373 not completed yet and the UAC must not send a new offer until it 374 receives the same SDP in the first reliable response, which is the 375 real answer. After sending the SDP in F6, the UAS must prepare to 376 receive a new offer from the UAC with an UPDATE request or a PRACK 377 request. 379 The UAS does not include SDP in responses F9 and F12. However, the 380 UAC should prepare to receive SDP bodies in F9 and/or F12, and just 381 ignore them, to handle a peer that does not conform to the 382 recommended implementation. 384 3.1.2. INVITE request without SDP 386 When a UAC does not include an SDP body in the INVITE request, it 387 expects the offer to be received with the first reliable response. 388 The UAC will send the answer in the request to acknowledge the 389 response, i.e. PRACK or ACK request of the reliable response. Other 390 than that, no offer/answer exchanges can occur in the messages 391 within the INVITE transaction. 393 NOTE: The UAS should not include SDP in the responses F6 and 394 F9. However, the UAC should prepare to receive SDP bodies in 395 F6 and/or F9, and just ignore them to handle a peer that does 396 not conform to the recommended implementation. 398 UAC UAS 399 | F1 INVITE (no SDP) | 400 |-------------------->| 401 | F2 1xx | 402 |<--------------------| 403 | | 404 | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP 405 |<--------------------| as the offer. 406 | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel 407 |-------------------->| must contain SDP as the answer. 408 | F5 2xx PRA (no SDP) | - 409 |<--------------------| | 410 | | | 411 | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not 412 |<--------------------| | contain SDP. 413 | F7 PRACK | | 414 |-------------------->| | UAC can send a new offer in an UPDATE 415 | F8 2xx PRA | | request after F4. 416 |<--------------------| v 417 | | 418 | F9 2xx INV (no SDP) | <- The final response should not 419 |<--------------------| contain SDP. 420 | F10 ACK | 421 |-------------------->| 423 Figure 2 Example of Offer/Answer with 100rel Extension (2) 425 Note that in the case that the UAC needs to prompt the user to 426 accept or reject the offer, the reliable provisional response with 427 SDP as an offer (pattern 4) can result in the retransmission until 428 the PRACK request can be sent. The UAC should take care to avoid 429 this situation when it sends the INVITE request without SDP. 431 3.2. Offer/Answer Exchange in Early Dialog 433 When both UAs support the 100rel extension, they can update the 434 session in the early dialog once the first offer/answer exchange 435 has been completed. 437 From a UA sending an INVITE request: 439 A UA can send an UPDATE request with a new offer if both ends 440 support the UPDATE method. Note that if the UAS needs to prompt the 441 user to accept or reject the offer, the delay can result in 442 retransmission of the UPDATE request. 444 A UA can send a PRACK request with a new offer only when 445 acknowledging the reliable provisional response carrying the answer 446 to an offer in the INVITE request. Compared to using the UPDATE 447 method, using PRACK can reduce the number of messages exchanged 448 between the UAs. However, to avoid problems or delays caused by 449 PRACK offer rejection, the UA is recommended to send a PRACK 450 request only when it has strong reasons to expect the receiver will 451 accept it. For example, the procedure used in precondition 452 extension [6] is a case where a PRACK request should be used for 453 updating the session status in an early dialog. Note also that if a 454 UAS needs to prompt the user to accept or reject the offer, the 455 delay can result in retransmission of the PRACK request. 457 From a UA receiving an INVITE request: 459 A UA can send an UPDATE request with a new offer if both ends 460 support the UPDATE method. A UAS can not send a new offer in the 461 reliable provisional response, so the UPDATE method is the only 462 method for a UAS to update an early session. 464 3.3. Offer/Answer Exchange in an Established Dialog 466 Both the re-INVITE and UPDATE methods can be used in an established 467 dialog to update the session. 469 The UPDATE method is simpler and can save at least one message 470 compared with the INVITE method. But both ends must support the 471 UPDATE method for it to be used. 473 The INVITE method needs at least three messages to complete but no 474 extensions are needed. Additionally, the INVITE method allows the 475 peer to take time to decide whether it will accept a session update 476 or not by sending provisional responses. That is, re-INVITE allows 477 the UAS to interact with the user at the peer, while UPDATE needs 478 to be answered automatically by the UAS. It is noted that re-INVITE 479 should be answered immediately unless such a user interaction is 480 needed. Otherwise, some 3pcc flows will break. 482 4. Exceptional Case Handling 484 In RFC 3264 [4], the following restrictions are defined with regard 485 to sending a new offer. 487 "At any time, either agent MAY generate a new offer that updates 488 the session. However, it MUST NOT generate a new offer if it 489 has received an offer which it has not yet answered or rejected. 490 It MUST NOT generate a new offer if it has generated a prior 491 offer for which it has not yet received an answer or a 492 rejection." 494 Assuming that the above rules are guaranteed, there seem to be two 495 possible 'exceptional' cases to be considered in SIP offer/answer 496 usage: the 'message crossing' case, and the 'glare' case. One of 497 the reasons why the usage of SIP methods to exchange offer/answer 498 needs to be carefully restricted in the RFCs is to ensure that the 499 UA can detect and handle appropriately the 'exceptional' cases to 500 avoid incompatible behavior. 502 4.1. Message Crossing Case Handling 504 When message packets cross in the transport network, an offer may 505 be received before the answer for the previous offer/answer 506 exchange, as shown in Figure 3. In such a case, UA A must detect 507 that the session description SDP-2 is not the answer to offer1. 509 A B 510 |SDP-1 (offer1)| 511 M1 |----------------->| 512 |SDP-2 (answer1)| 513 M2 |<------\ /-------| 514 | \/ | 515 |SDP-3 /\(offer2)| 516 M3 |<------/ \-------| 518 Figure 3 Message Crossing Case 520 Because of the restrictions on placement of offers and answers 521 (summarized in Table 1) there are a limited number of valid 522 exchanges of messages that may lead to this message crossing case. 523 These are enumerated in Table 3. (This table only shows messages 524 containing offers or answers. There could be other messages, 525 without session descriptions, which are not shown.) 527 There is a variant, shown in Figure 4, which is dependent on an 528 INVITE (Mx) that contains no offer. This case should be extremely 529 rare - it is easily avoided by delaying Mx until answer1 is 530 received. It adds another possibility to Table 3. 532 A B 533 | | 534 |SDP-1 offer1(UPD) | 535 M1 |==============================>| 536 |re-INV (no offer) | 537 Mx |------------------------------>| --+ 538 |SDP-2 answer1 (2xx-UPD)| | 539 M2 |<===========\ /===============| | first reliable 540 | \/ offer2| | response 541 |SDP-3 /\ (1xx-rel/2xx)| | 542 M3 |<===========/ \===============| <-+ 543 |SDP-4 answer2 (PRACK/ACK)| 544 My |------------------------------>| Wait until answer1 545 | | 547 Figure 4 Reliable response as a message with offer2 in message 548 crossing case 550 | M1 | M3 | M2 | 551 |--------+----------+---------| 552 | INVITE | 1xx-rel | UPDATE | 553 |--------+----------+---------| 554 | PRACK | 200-PRA | UPDATE | 555 |--------+----------+---------| 556 | UPDATE | 200-UPD | UPDATE | 557 | | |---------| 558 | | | INVITE | (no INV in progress) 559 | | |---------| 560 | | | 2xx-INV | (INV in progress) 561 | | |---------| 562 | | | 1xx-rel | (from Figure 4) 563 |-----------------------------| 564 Table 3. Offer / Answer Crossing Message Sequences 566 Table 3 shows that there are only two ambiguous cases when an 567 answer is expected and an arriving message M2 containing SDP could 568 be either the expected answer or an offer. These are a reliable 1xx 569 response to an INVITE, or an UPDATE. 571 When message M2 is an UPDATE request or a (re)INVITE request, then 572 message M1 must also have been an UPDATE or INVITE. There may have 573 been message crossing, or not. If not then it is a glare case. 574 Either way, the remedy is for UA A to reject message M2 with a 491 575 response with Retry-After header field. 577 When M2 is a reliable provisional response or a successful final 578 response, and M1 was an UPDATE, then SDP-2 cannot be the expected 579 answer1. In this case, since UA A can not reject offer2 in reliable 580 response M2, it is recommended that it wait for answer1 before 581 sending a PRACK request with the answer to offer2. Note that this 582 case only occurs when UA A, while waiting for an answer, sends an 583 INVITE request without session description. 585 When M2 is a PRACK request Table 3 shows that it cannot be an offer 586 out of order, so UA A may infer SDP-2 is an answer. 588 Table 4 summarizes the discussions above. 590 SDP-2 | How to know it's not answer1 | Actions to take 591 -------+------------------------------+-------------------------- 592 INVITE | Never be an answer | 491 response 593 UPDATE | Glare case for UA A | with Retry-After 594 -------+------------------------------+-------------------------- 595 1xx-rel| If M1 was UPDATE then SDP-2 | Delay ACK/PRACK 596 2xx-INV| is not answer1 | until answer1 is received 597 -------+------------------------------+-------------------------- 598 PRACK | This case never happens | Not a message cross case 599 | under the current rules. | Treat SDP-2 as answer2 600 -------+------------------------------+-------------------------- 601 Table 4. Message Crossing Resolution 603 4.2. Glare Case Handling 605 When both ends in a dialog send a new offer at nearly the same time, 606 as described in Figure 5, a UA may receive a new offer before it 607 receives the answer to the offer it sent. This case is usually 608 called a 'glare' case. 610 A B 611 |offer1 offer2| 612 |-------\ /-------| 613 | \/ | 614 | /\ | 615 |<------/ \------>| 617 Figure 5 Glare Case 619 When offer2 is in an UPDATE request or (re-)INVITE request, it must 620 be rejected with a 491 response. 622 When offer2 is in a PRACK request (within the current rules, only 623 possible if offer1 is in an UPDATE request), the PRACK may be 624 accepted with 200 or may be rejected with a 491 response. A 491 625 response is valid to satisfy the offer/answer model but it may 626 delay the completion of the reliable response transfer mechanism or, 627 in worst case, may result in the failure to complete the SIP 628 transaction because there is no clear retry rule when a PRACK 629 request is rejected with a 491 response. To avoid this glare 630 condition, UA A should not send an offer if it has already sent a 631 reliable provisional response containing an answer to a previous 632 offer and has not received the corresponding PRACK request. 634 To avoid a glare condition involving an offer in a response, when 635 UA A has sent a (re)INVITE request without session description, it 636 should not send an offer until it has received an offer in a 637 reliable response to the (re)INVITE, and sent an answer to that 638 offer. 640 5. Content of Offers and Answers 642 While RFCs 3264[4] and 3312[6] give some guidance, questions remain 643 about exactly what should be included in an offer or answer. This 644 is especially a problem when the common "hold" feature has been 645 activated, and when there is the potential for a multimedia call. 647 Details of behavior depend on the capabilities and state of the 648 User Agent. The kinds of recommendations that can be made are 649 limited by the model of device capabilities and state that is 650 presumed to exist. 652 This section focuses on a few key aspects of offers and answers 653 that have been identified as troublesome, and will consider other 654 aspects to be out of scope. This section considers: 656 - choice of supported media types and formats to include and 657 exclude 659 - hold and resume of media 661 The following are out of scope for this document: 663 - NAT traversal and ICE 665 - specific codecs and their parameters 667 - the negotiation of secure media streams 669 - grouping of media streams 670 - preconditions 672 5.1. General Principle for Constructing Offers and Answers 674 A UA should send an offer that indicates what it, and its user, are 675 interested in using/doing at that time, without regard for what the 676 other party in the call may have indicated previously. This is the 677 case even when the offer is sent in response to an INVITE or re- 678 INVITE that contains no offer. (However in the case of re-INVITE 679 the constraints of RFCs 3261 and 3264 must be observed.) 681 A UA should send an answer that includes as close an approximation 682 to what the UA and its user are interested in doing at that time, 683 while remaining consistent with the offer/answer rules of RFC 684 3264[4] and other RFCs. 686 NOTE: "at that time" is important. The device may permit the 687 user to configure which supported media are to be used by 688 default. 690 In some cases a UA may not have direct knowledge of what it is 691 interested in doing at a particular time. If it is an intermediary 692 it may be able to delegate the decision. In the worst case it may 693 apply a default, such as assuming it wants to use all of its 694 capabilities. 696 5.2. Choice of Media Types and Formats to Include and Exclude 698 5.2.1. Sending an Initial INVITE with Offer 700 When a UAC sends an initial INVITE with an offer, it has complete 701 freedom to choose which media type(s) and media format(s) (payload 702 types in the case of RTP) it should include in the offer. 704 The media types may be all or a subset of the media the UAC is 705 capable of supporting, with the particular subset being determined 706 by the design and configuration [7] of the UAC combined with input 707 from the user interface of the UAC. 709 The media formats may be all or a subset of the media formats the 710 UAC is capable of supporting for the corresponding media type, with 711 the particular subset being determined by the design and 712 configuration [7] of the UAC combined with input from the user 713 interface of the UAC. 715 Including all supported media formats will maximize the possibility 716 that the other party will have a supported format in common. But 717 including many can result in an unacceptably large SDP body. 719 5.2.2. Responding with an Offer when the Initial INVITE has no Offer 721 When a UAS has received an initial INVITE without an offer, it must 722 include an offer in the first reliable response to the INVITE. It 723 has largely the same options as when sending an initial INVITE with 724 an offer, but there are some differences. The choice may be 725 governed by both static (default) selections of media types as well 726 as dynamic selections made by a user via interaction with the 727 device while it is alerting. 729 NOTE: The offer may be sent in a provisional response, before 730 the user of the device has been alerted and had an opportunity 731 to select media options for the call. In this case the UAS 732 cannot include any call-specific options from the user of the 733 device. If there is a possibility that the user of the device 734 will wish to change what is offered before answering the call, 735 then special care should be taken. If PRACK and UPDATE are 736 supported by caller and callee then an initial offer can be 737 sent reliably, and changed with an UPDATE if the user desires 738 a change. If PRACK and UPDATE are not supported then the 739 initial offer cannot be changed until the call is fully 740 established. In that case either the offer should be delayed 741 until the 200 is sent, or else the offer should include the 742 minimum set of media the user is able to select. 744 5.2.3. Answering an Initial INVITE with Offer 746 When a UAS receives an initial INVITE with an offer, what media 747 lines the answer may contain is constrained by RFC 3264.[4] The 748 answer must contain the same number of m-lines as the offer, and 749 they must contain the same media types. Each media line may be 750 accepted, by including a non-zero port number, or rejected by 751 including a zero port number in the answer. The media lines that 752 are accepted should typically be those that would have been offered 753 had the INVITE not contained an offer, excluding those not offered. 755 The media formats the answer may contain are constrained by RFC 756 3264 [4]. For each accepted m-line in the answer, there must be at 757 least one media format in common with the corresponding m-line of 758 the offer. The UAS may also include other media formats it is able 759 to support at this time. However there is little benefit to 760 including added types. 762 If the UAS does not wish to indicate support for any of the media 763 types in a particular media line of the offer it must reject the 764 corresponding media line, by setting the port number to zero. 766 5.2.4. Answering when the Initial INVITE had no Offer 768 When a UAC has sent an initial INVITE without an offer, and then 769 receives a response with the first offer, it should answer in the 770 same way as a UAS receiving an initial INVITE with an offer. 772 5.2.5. Subsequent Offers and Answers 774 The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) 775 apply, but constraints in RFC 3264 [4] must also be followed. The 776 following are of particular note because they have proven 777 troublesome: 779 o The number of m-lines may not be reduced in a subsequent offer. 780 Previously rejected media streams must remain, or be reused to 781 offer the same or a different stream. (RFC 3264[4] section 6.) 783 o In the o-line, only the version number may change, and if it 784 changes it must increment by one from the one previously sent as 785 an offer or answer. (RFC 3264[4] section 8.) If it doesn't 786 change then the entire SDP body must be identical to what was 787 previously sent as an offer or answer. Changing the o-line, 788 except version number value, during the session is an error case. 789 The behavior when receiving such a non-compliant offer/answer 790 SDP body is implementation dependent. If a UA needs to negotiate 791 a 'new' SDP session, it should use the INVITE/Replaces method. 793 o In the case of RTP, the mapping from a particular dynamic 794 payload type number to a particular codec within that media 795 stream (m-line) must not change for the duration of the session. 796 (RFC 3264[4] section 8.3.2.) 798 NOTE: This may be impossible for a B2BUA to follow in some 799 cases (e.g. 3pcc transfer) if it does not terminate media. 801 When the new offer is sent in response to an offerless (re)INVITE, 802 all codecs supported by the UA are to be included, not just the 803 ones that were negotiated by previous offer/answer exchanges. The 804 same is true for media types - so if UA A initially offered audio 805 and video to UA B, and they end up with only audio, and UA B sends 806 an offerless (re)INVITE to UA A, A's resulting offer should re- 807 attempt video, by reusing the zeroed m-line used previously. 809 NOTE: The behavior above is recommended, but it is not always 810 achievable - for example in some interworking scenarios. Or, 811 the offerer may simply not have enough resources to offer 812 "everything" at that point. Even if the UAS is not able to 813 offer any other SDP that the one currently being used, it 814 should not reject the re-INVITE. Instead, it should generate 815 an offer with the currently used SDP with o- line unchanged. 817 5.3. Hold and Resume of media 819 RFC 3264 [4] specifies (non-normatively) that "hold" should be 820 indicated in an established session by sending a new offer 821 containing "a=sendonly" for each media stream to be held. An 822 answerer is then to respond with "a=recvonly" to acknowledge that 823 the hold request has been understood. 825 Note that the use of sendonly/recvonly is not limited to hold. 826 These may be used for other reasons, such as devices that are only 827 capable of sending or receiving. So receiving an offer with 828 "a=sendonly" must not be treated as a certain indication that the 829 offerer has placed the media stream on hold. 831 This model is based on an assumption that the UA initiating the 832 hold will want to play Music on Hold, which is not always the case. 833 A UA may, if desired, initiate hold by offering "a=inactive" if it 834 does not intend to transmit any media while in hold status. 836 The rules of RFC 3264 [4] constrain what may be in an answer when 837 the offer contains "sendonly", "recvonly", or "inactive" in an a= 838 line. But they do not constrain what must be in a subsequent offer. 839 The General Principle for Constructing Offers and Answers (section 840 5.1. ) is important here. The initiation of "hold" is a local 841 action. It should reflect the desired state of the UA. It then 842 affects what the UA includes in offers and answers until the local 843 state is reset. 845 The receipt of an offer containing "a=sendonly" or "a=inactive" and 846 the sending of a compatible answer should not change the desired 847 state of the recipient. However, a UA that has been "placed on 848 hold" may itself desire to initiate its own hold status, based on 849 local input. 851 If UA2 has previously been "placed on hold" by UA1, via receipt of 852 "a=sendonly", then it may initiate its own hold by sending a new 853 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 854 will answer with "a=inactive" because that is the only valid answer 855 that reflects its desire not to receive media. 857 Once in this state, to resume a two way exchange of media each side 858 must reset its local hold status. If UA1 is first to go off hold it 859 will then send an offer with "a=sendrecv". The UA2 will respond 860 with its desired state of "a=sendonly" because that is a permitted 861 response. When UA2 desires to also resume, it will send an offer 862 with "a=sendrecv". In this case, because UA1 has the same desire it 863 will respond with "a=sendrecv". In the same case, when UA2 receives 864 the offer with "a=sendrecv", if it has decided it wants to reset 865 its local hold but has not yet signaled the intent, it may send 866 "a=sendrecv" in the answer. 868 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", 869 and subsequently wants to initiate its own hold, also using 870 "a=inactive", it need not send a new offer, since the only valid 871 response is "a=inactive" and that is already in effect. However, 872 its local desired state will now be either "inactive" or 873 "a=sendonly". This affects what it will send in future offers and 874 answers. 876 If a UA has occasion to send another offer in the session, without 877 any desire to change the hold status (e.g. in response to a re- 878 INVITE without an offer, or when sending a re-INVITE to refresh the 879 session timer) it should follow the General Principle for 880 Constructing Offers and Answers (section 5.1. ). If it previously 881 initiated a "hold" by sending "a=sendonly" or "a=inactive" then it 882 should offer that again. If it had not previously initiated "hold" 883 then it should offer "a=sendrecv", even if it had previously been 884 forced to answer something else. Without this behavior it is 885 possible to get "stuck on hold" in some cases, especially when a 886 third-party call controller is involved. 888 5.4. Behavior on receiving SDP with c=0.0.0.0 890 RFC 3264[4] specifies that An agent MUST be capable of receiving 891 SDP with a connection address of 0.0.0.0, in which case it means 892 that neither RTP nor RTCP should be sent to the peer. 894 If a UA generates an answer to the offer received with c=0.0.0.0, 895 the direction attribute of the accepted media stream in the answer 896 must be based on direction attribute of the offered stream and 897 rules specified in RFC 3264 to form the a-line in the answer. 898 c=0.0.0.0 has no special meaning for the direction attribute of the 899 accepted stream in the answer. 901 6. Remaining Issues or Best Practices on Offer/Answer 903 This document clarifies the offer/answer usage in SIP and 904 summarizes the correct or recommended behaviors along with the 905 existing RFCs. To create any new normative behaviors beyond these 906 RFCs is not the intent of this document. 908 However, through the scrutiny of the offer/answer model in SIP, 909 some issues are found to be unresolved within the current set of 910 RFCs. Those remaining issues are described in this section mainly 911 for further study. 913 6.1. Rejecting PRACK Offer 915 As stated in section 2.2. and 3.2. , it is recommended that an 916 offer not be sent in a PRACK request unless UAC has strong reasons 917 to assume the receiver will accept it. Even so, there may be cases 918 when the UAS has to reject the offer for some reason. The current 919 RFCs do not provide a way to reject the offer and at the same time 920 to indicate that the PRACK adequately acknowledged the reliable 921 response. It is unclear whether a non-200 response can still 922 indicate an acknowledgement of the reliable response. 924 Several ideas were presented to resolve this issue, such as sending 925 2xx PRACK response without SDP to reject the offer, or sending SDP 926 with a decreased version value in the o-line. Some of the 927 candidates may also be adapted as a way to reject an unacceptable 928 offer in a response. Anyway, those proposals violate the current 929 rules and lose backward compatibility to some extent (e.g. section 930 5 of RFC 3262). It is beyond the scope of this document and remains 931 for further study. 933 The 488 response is another proposed solution; however the validity 934 and consequences of a 488 response to PRACK is an open issue. 935 Because the 488 response may be sent by a proxy, the UAC cannot 936 assume the reliable transaction has been adequately acknowledged. 937 If a 488 response is received, the UAC should ensure acknowledgment 938 of the reliable response by sending a new PRACK with the offer 939 removed or modified based upon the received 488 response. If the 940 488 response is sent by UAS (open issue), it cannot assume that the 941 UAC thinks that the reliable transaction has been adequately 942 acknowledged even though the UAS may treat otherwise (open issue). 943 If a 488 response is sent by UAS, the UAC should accommodate 944 receiving the altered PRACK with higher CSeq without expecting it 945 to trigger a 481 response (open issue). 947 NOTE: Deprecation of the usage of offer in PRACK may be 948 another solution. As the precondition mechanism specification 949 [3] explicitly shows a usage of sending offer in PRACK, its 950 deprecation could cause backward compatibility issues. 952 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 953 Transaction 955 When a re-INVITE transaction fails, the dialog remains with the 956 session bound to it. The issue here is: what is the session status 957 if an offer/answer exchange has been completed (if a session 958 description has been sent in a reliable provisional response to the 959 re-INVITE request), or if subsequent offer/answer exchanges have 960 taken place (using UPDATE or PRACK transactions), before the re- 961 INVITE transaction is terminated with a final error response 962 (Figure 6). One option is to take those offer/answer exchanges not 963 committed yet and to make the session status rollback to the one 964 before re-INVITE transaction was initiated. Another option is to 965 take those exchanges committed and to keep the session status as it 966 is even after re-INVITE fails. There is no clear consensus on which 967 one is the correct behavior. 969 There are some cases where it is useful to exchange 970 offer(s)/answer(s) even before re-INVITE completes. The case of 971 adding a new media (like adding video to audio only session) which 972 requires permission from the peer through some user interaction is 973 one example. Precondition procedures can be another case which may 974 require several offer/answer exchanges in one re-INVITE transaction. 976 UAC UAS 977 | session established | 978 |<===================>| 979 | | 980 | F1 re-INVITE (SDP) | 981 |-------------------->| 982 | F2 1xx-rel (SDP) | 983 |<--------------------| 984 | F3 PRACK | <- PRACK request may include new offer 985 |-------------------->| and can complete the offer/answer with 986 | F4 2xx PRA | the answer in 2xx PRACK response. 987 |<--------------------| 988 | | <- UPDATE method can update the session 989 | | status before receiving the final 990 | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). 991 |<--------------------| 992 | F6 ACK | 993 |-------------------->| Issue: What is the correct session 994 | | status after re-INVITE transaction. 996 Figure 6 Commit/Rollback Issue with re-INVITE transaction 998 To make bad things worse, if a new offer from UAC and the final 999 response to re-INVITE are sent at nearly the same time, the UAS can 1000 not know whether this new offer was sent before or after UAC 1001 received the final failure response (Figure 7). Note that the ACK 1002 request to the failure response is sent hop-by-hop basis, therefore 1003 even after receiving the ACK request, UAS can not make sure that 1004 UPDATE request was sent after the final response had been reached 1005 to the other end. 1007 Sending a new UPDATE request from UAC to synchronize the status 1008 anytime after the re-INVITE fails may be a good option. This 1009 solution, however, requires that the UPDATE method be supported by 1010 both ends and needs care to avoid flapping when each end tries to 1011 advertise their different views of the session status. 1013 The proper handling of this issue is undefined by existing 1014 standards. Resolution is beyond the scope of this document, and 1015 will require a new normative document. 1017 UAC UAS 1018 | session established | 1019 |<===================>| 1020 | | 1021 | F1 re-INVITE (SDP) | 1022 |-------------------->| 1023 | F2 1xx-rel (SDP) | 1024 |<--------------------| 1025 | F3 PRACK | 1026 |-------------------->| 1027 | F4 2xx PRA | 1028 |<--------------------| 1029 | | 1030 |UPDATE(SDP) 4xx INV | 1031 |---------\ /--------| 1032 | \/ | 1033 | /\ | 1034 |<--------/ \------->| 1035 | | 1037 Figure 7 Commit/Rollback Issue with Race Condition 1039 6.3. Offer in a Reliable Response 1041 In RFC 3261, it is stated that when an INVITE is sent without an 1042 offer, the first reliable response MUST contain an offer. There was 1043 discussion on whether this rule can be loosened up. There is no 1044 clear explanation why this restriction is defined. However, this 1045 rule will be left as it is, unless the strong necessity to loosen 1046 it up is raised in the future. 1048 6.4. Requesting Hold while already on Hold 1050 RFC 3264, section 8.4, contains procedures for putting a unicast 1051 media stream on hold. Of particular note, it states: 1053 "If the stream to be placed on hold was previously a recvonly 1054 media stream, it is placed on hold by marking it inactive." 1056 Section 5.3. of the current document makes a recommendation for 1057 this case which conflicts with that, and explains why. Some 1058 concerns have been raised that such a recommendation is invalid 1059 because RFC 3264 is normative on this subject. 1061 This document takes the position that Section 8.4 of RFC 3264 is 1062 non-normative, and so may be overridden. It is further recommended 1063 that RFC 3264 be revised to avoid the confusion. 1065 7. Add New Offer/Answer Usage in SIP 1067 This document recommends against the addition of new offer/answer 1068 methods using SIP. However, it may be necessary to define new 1069 offer/answer exchange methods as SIP extensions evolve. This 1070 section recommends some things that should be taken into 1071 considerations in that case. 1073 7.1. Explicit Usage 1075 New method definitions should define offer/answer usage explicitly 1076 without any ambiguity. 1078 7.2. Rejection of an Offer 1080 New method definitions should define how to reject an offer where 1081 possible. 1083 7.3. Backward Compatibility 1085 New methods must keep backward compatibility. 1087 7.4. Exceptional Case Handling 1089 New methods should take care of how to handle exceptional cases, 1090 message crossing case and glare case. 1092 8. IANA Considerations 1094 This document has no actions for IANA. 1096 9. Security Considerations 1098 There are not any security issues beyond the referenced RFCs. 1100 10. Acknowledgement 1102 The authors would like to thank Christer Holmberg, Rajeev Seth, 1103 Nataraju A B, Byron Campen and Jonathan Rosenberg for their 1104 thorough reviews and comments. Many of their suggestions and ideas 1105 are incorporated to complete this document. 1107 11. References 1109 11.1. Normative References 1111 [1] Bradner, S., "Key words for use in RFCs to Indicate 1112 Requirement Levels", BCP 14, RFC 2119, March 1997. 1114 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1115 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1116 Session Initiation Protocol", RFC 3261, June 2002. 1118 [3] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional 1119 Responses in the Session Initiation Protocol (SIP)", RFC 3262, 1120 June 2002. 1122 [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1123 SDP", RFC 3264, June 2002. 1125 [5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE 1126 Method", RFC 3311, September 2002. 1128 [6] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration 1129 of Resource Management and Session Initiation Protocol (SIP)", 1130 RFC 3312, October 2002. 1132 11.2. Informative References 1134 [7] G. Camarillo, "The Early Session Disposition Type for the 1135 Session Initiation Protocol (SIP)", RFC 3959, December 2004. 1137 Author's Addresses 1139 Takuya Sawada 1140 KDDI Corporation 1141 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 1143 Email: tu-sawada@kddi.com 1145 Paul H. Kyzivat 1146 Cisco Systems, Inc. 1147 1414 Massachusetts Avenue 1148 Boxborough, MA 01719 1149 USA 1151 Email: pkyzivat@cisco.com 1153 Full Copyright Statement 1155 Copyright (C) The IETF Trust (2008). 1157 This document is subject to the rights, licenses and restrictions 1158 contained in BCP 78, and except as set forth therein, the authors 1159 retain all their rights. 1161 This document and the information contained herein are provided on 1162 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 1163 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 1164 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 1165 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 1166 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 1167 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1168 FOR A PARTICULAR PURPOSE. 1170 Intellectual Property Statement 1172 The IETF takes no position regarding the validity or scope of any 1173 Intellectual Property Rights or other rights that might be claimed 1174 to pertain to the implementation or use of the technology described 1175 in this document or the extent to which any license under such 1176 rights might or might not be available; nor does it represent that 1177 it has made any independent effort to identify any such rights. 1178 Information on the procedures with respect to rights in RFC 1179 documents can be found in BCP 78 and BCP 79. 1181 Copies of IPR disclosures made to the IETF Secretariat and any 1182 assurances of licenses to be made available, or the result of an 1183 attempt made to obtain a general license or permission for the use 1184 of such proprietary rights by implementers or users of this 1185 specification can be obtained from the IETF on-line IPR repository 1186 at 1187 http://www.ietf.org/ipr. 1189 The IETF invites any interested party to bring to its attention any 1190 copyrights, patents or patent applications, or other proprietary 1191 rights that may cover technology that may be required to implement 1192 this standard. Please address the information to the IETF at 1193 ietf-ipr@ietf.org. 1195 Acknowledgment 1197 Funding for the RFC Editor function is provided by the IETF 1198 Administrative Support Activity (IASA).