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Holmberg 3 Internet-Draft Ericsson 4 Intended status: Standards Track February 1, 2011 5 Expires: August 5, 2011 7 Session Initiation Protocol (SIP) Response Code for Indication of 8 Terminated Dialog 9 draft-ietf-sipcore-199-05.txt 11 Abstract 13 This specification defines a new Session Initiation Protocol (SIP) 14 response code, 199 Early Dialog Terminated, that a SIP forking proxy 15 and a User Agent Server (UAS) can use to indicate towards upstream 16 SIP entities (including the User Agent Client (UAC)) that an early 17 dialog has been terminated, before a final response is sent towards 18 the SIP entities. 20 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on August 5, 2011. 37 Copyright Notice 39 Copyright (c) 2011 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. Applicability and Limitation . . . . . . . . . . . . . . . . . 4 57 4. User Agent Client behavior . . . . . . . . . . . . . . . . . . 5 58 5. User Agent Server behavior . . . . . . . . . . . . . . . . . . 6 59 6. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . . 7 60 7. Backward compatibility . . . . . . . . . . . . . . . . . . . . 9 61 8. Usage with SDP offer/answer . . . . . . . . . . . . . . . . . 9 62 9. Message Flow Examples . . . . . . . . . . . . . . . . . . . . 10 63 9.1. Example with a forking proxy which generates 199 . . . . . 10 64 9.2. Example with a forking proxy which receives 200 OK . . . . 11 65 9.3. Example with two forking proxies, of which one 66 generates 199 . . . . . . . . . . . . . . . . . . . . . . 11 67 10. Security Considerations . . . . . . . . . . . . . . . . . . . 12 68 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 69 11.1. IANA Registration of the 199 response code . . . . . . . . 13 70 11.2. IANA Registration of the 199 option-tag . . . . . . . . . 13 71 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 72 13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13 73 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 74 14.1. Normative References . . . . . . . . . . . . . . . . . . . 14 75 14.2. Informational References . . . . . . . . . . . . . . . . . 15 76 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 78 1. Introduction 80 As defined in RFC 3261 [RFC3261], a Session Initiation Protocol (SIP) 81 early dialog is created when a non-100 provisional response is sent 82 to the initial dialog initiation request (e.g. INVITE, outside an 83 existing dialog). The dialog is considered to be in early state 84 until a final response is sent. 86 When a proxy receives an initial dialog initiation request, it can 87 forward the request towards multiple remote destinations. When the 88 proxy does that, it performs forking [RFC3261]. 90 When a forking proxy receives a non-100 provisional response, or a 91 2xx final response, it forwards the response upstream towards the 92 sender of the associated request. After a forking proxy has 93 forwarded a 2xx final response, it normally generates and sends 94 CANCEL requests downstream towards all remote destinations where it 95 previously forked the request associated with the 2xx final response 96 and from which it has yet not received a final response. The CANCEL 97 requests are sent in order to terminate any outstanding early dialogs 98 associated with the request. 100 Upstream SIP entities might receive multiple 2xx final responses. 101 When a SIP entity receives the first 2xx final response, and it does 102 not intend to accept any subsequent 2xx final response, it will 103 automatically terminate any other outstanding early dialog associated 104 with the request. If the SIP entity receives a subsequent 2xx final 105 response, it will normally generate and send an ACK request, followed 106 with a BYE request, using the dialog identifier retrieved from the 107 2xx final response. 109 NOTE: A User Agent Client (UAC) can use the Request-Disposition 110 header field [RFC3841] to request that proxies do not generate and 111 send CANCEL requests downstream once they have received the first 2xx 112 final response. 114 When a forking proxy receives a non-2xx final response, it does not 115 always immediately forward the response upstream towards the sender 116 of the associated request. Instead, the proxy "stores" the response 117 and waits for subsequent final responses from other remote 118 destinations where the associated request was forked. At some point 119 the proxy uses a specified mechanism to determine the "best" final 120 response code, and forwards a final response using that response code 121 upstream towards the sender of the associated request. When an 122 upstream SIP entity receives the non-2xx final response it will 123 release resources associated with the session. The UAC will 124 terminate, or retry, the session setup. 126 Since the forking proxy does not always immediately forward non-2xx 127 final responses, upstream SIP entities (including the UAC that 128 initiated the request) are not immediately informed that an early 129 dialog has been terminated, and will therefore maintain resources 130 associated with the early dialog reserved until a final response is 131 sent by the proxy, even if the early dialog has already been 132 terminated. A SIP entity could use the resources for other things, 133 e.g. to accept subsequent early dialogs that it otherwise would 134 reject. 136 This specification defines a new SIP response code, 199 Early Dialog 137 Terminated. A forking proxy can send a 199 provisional response to 138 inform upstream SIP entities that an early dialog has been 139 terminated. A UAS can send a 199 response code, prior to sending a 140 non-2xx final response, for the same purpose. SIP entities that 141 receive the 199 response can use it to trigger the release of 142 resources associated with the terminated early dialog. In addition, 143 SIP entities might also use the 199 response to make policy related 144 decisions related to early dialogs. 146 2. Terminology 148 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 149 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 150 document are to be interpreted as described in RFC 2119 [RFC2119]. 152 3. Applicability and Limitation 154 The 199 response code is an optimization, and it only optimizes how 155 quickly recipients might be informed about terminated early dialogs. 156 The achieved optimization is limited. Since the response is normally 157 not sent reliably by an UAS, and can not be sent reliably when 158 generated and sent by a proxy, it is possible that some or all of the 159 199 responses get lost before they reach the recipients. In such 160 cases, recipients will behave the same as if the 199 response code 161 were not used at all. 163 One example for which a UAC could use the 199 response, is that when 164 it receives a 199 response it releases resources associated with the 165 terminated early dialog. The UAC could also use the 199 response to 166 make policy related decisions related to early dialogs. For example, 167 if a UAC is playing media associated with an early dialog, and the it 168 receives a 199 response indicating the early dialog has been 169 terminated, it could start playing media associated with a different 170 early dialog. 172 Applications designers utilizing the 199 response code MUST ensure 173 that the application's user experience is acceptable if all 199 174 responses are lost, and not delivered to the recipients. 176 4. User Agent Client behavior 178 When a UAC sends an initial dialog initiation request, and if it is 179 willing to receive 199 responses, it MUST insert an "199" option-tag 180 in the Supported header field [RFC3261] of the request. The option- 181 tag indicates that the UAC supports, and is willing to receive, 199 182 responses. A UAC SHOULD NOT insert an "199" option-tag in the 183 Require or the Proxy-Require header field [RFC3261] of the request, 184 since in many cases it would result in unnecessary session 185 establishment failures. 187 NOTE: The UAC always needs to insert an "199" option-tag in the 188 Supported header field, in order to indicate that it supports, and is 189 willing to receive, 199 responses, even if it also inserts the 190 option-tag in the Require or Proxy-Require header field. 192 It is RECOMMENDED that a UAC does not insert an "100rel" option-tag 193 [RFC3262] in the Require header field when it also indicates support 194 of 199 responses, unless the UAC also uses some other SIP extension 195 or procedure that mandates it to do so. The reason is that proxies 196 are not allowed to generate and send 199 responses when the UAC has 197 required provisional responses to be sent reliably. 199 When a UAC receives a 199 response, it might release resources 200 associated with the terminated early dialog. A UAC might also use 201 the 199 response to make policy related decisions related to early 202 dialogs. 204 NOTE: The 199 response indicates that the early dialog has been 205 terminated, so there is no need for the UAC to send a BYE request in 206 order to terminate the early dialog when it receives the 199 207 response. 209 NOTE: The 199 response does not affect other early dialogs associated 210 with the session establishment. For those the normal SIP rules, 211 regarding transaction timeout etc, still apply. 213 Once a UAC has received and accepted a 199 response, it MUST NOT send 214 Any media associated with the early dialog. In addition, if the UAC 215 is able to associate received media with early dialogs, it MUST NOT 216 process any received media associated with the early dialog that was 217 terminated. 219 If multiple usages [RFC5057] are used within an early dialog, and it 220 is not clear which dialog usage the 199 response terminates, SIP 221 entities that keep dialog state SHALL NOT release resources 222 associated with the early dialog when they receive the 199 response. 224 If a UAC receives an unreliably sent 199 response on a dialog which 225 has not previously been established (this can happen if a 199 226 response reaches the client before the 18x response that would 227 establish the early dialog) it SHALL discard the 199 responses. If a 228 UAC receives a reliably sent 199 response on a dialog which has not 229 previously been created, it MUST acknowledge the 199 response, as 230 described in RFC 3262 [RFC3262]. 232 If a UAC has received a 199 response for all early dialogs, and no 233 early dialog associated session establishment remains, it maintains 234 the "Proceeding" state [RFC3261] and waits for possible subsequent 235 early dialogs to be established, and eventually for a final response 236 to be received. 238 5. User Agent Server behavior 240 If a UAS receives an initial dialog initiation request, with a 241 Supported header field that contains a "199" option-tag, it SHOULD 242 NOT send a 199 response on an early dialog associated with the 243 request, before it sends a non-2xx final response, unless it e.g. has 244 been configured to do so due to lack of support of the 199 response 245 code by forking proxies or other intermediate SIP entities, or it is 246 used in an environment that specifies that it shall send a 199 247 response before sending a non-2xx response. 249 NOTE: If a UAS has created multiple early dialogs associated with an 250 initial dialog initiation request (the UAS is acting similar to a 251 forking proxy), it does not always intend to send a final response on 252 all of those early dialogs. 254 NOTE: If the Require header field of an initial dialog initiation 255 request contains a "100rel" option-tag, proxies will not be able to 256 generate and send 199 responses. In such cases the UAS might choose 257 to send a 199 response on an early dialog, before it sends a non-2xx 258 final response, even if it would not do so in other cases. 260 If the Supported header field of an initial dialog initiation request 261 does not contain an "199" option-tag, the UAC MUST NOT send a 199 262 response on any early dialog associated with the request. 264 When a UAS generates a 199 response, the response MUST contain a To 265 header field tag parameter [RFC3261], in order for other entities to 266 identify the early dialog that has been terminated. The UAS MUST 267 also insert a Reason header field [RFC3326] that contains a response 268 code which describes the reason why the early dialog was terminated. 269 The UAS MUST NOT insert a "199" option-tag in the Supported, Require 270 or Proxy-Require header field of the 199 response. 272 If a UAS intends to send 199 responses, and if it supports the 273 procedures defined in RFC 3840 [RFC3840], it MAY during the 274 registration procedure use the sip.extensions feature tag [RFC3840] 275 to indicate support of the 199 response code. 277 A 199 response SHOULD NOT contain an SDP offer/answer message body, 278 unless required by the rules in RFC 3264 [RFC3264]. 280 According to RFC 3264, if an INVITE request does not contain an SDP 281 offer, and the 199 response is the first reliably sent response 282 associated with the request, the 199 response is required to contain 283 an SDP offer. In this case the UAS SHOULD send the 199 response 284 unreliably, or send the 199 response reliably and include an SDP 285 offer with no m- lines in the response. 287 Since a 199 response is only used for information purpose, the UAS 288 SHOULD send it unreliably, unless the "100rel" option-tag is present 289 in the Require header field of the associated request. 291 Once a UAS has sent a 199 response, it MUST NOT send or process any 292 media associated with the terminated early dialog. 294 6. Proxy behavior 296 When a proxy receives a 199 response to an initial dialog initiation 297 request, it MUST process the response as any other non-100 298 provisional response. The proxy will forward the response upstream 299 towards the sender of the associated request. The proxy MAY release 300 resources it has reserved associated with the early dialog that is 301 terminated. If a proxy receives a 199 response out of dialog, it 302 MUST process it as other non-100 provisional responses received out 303 of dialog. 305 When a forking proxy receives a non-2xx final response to an initial 306 dialog initiation request, that it recognizes as terminating one or 307 more early dialogs associated with the request, it MUST generate and 308 send a 199 response upstream for each of the terminated early dialogs 309 that satisfy each of the following conditions: 311 - the forking proxy does not intend to forward the final response 312 immediately (in accordance with rules for a forking proxy) 313 - the UAC has indicated support (by inserting the "199" option-tag in 314 a Supported header field) of the 199 response code in the associated 315 request 317 - the UAC has not required provisional responses to be sent reliably 318 (by inserting the "100rel" option-tag in a Require or Proxy-Require 319 header field) in the associated request 321 - the forking proxy has not already received and forwarded a 199 322 response for the early dialog 324 - the forking proxy has not already sent a final response for any of 325 the early dialogs 327 As a consequence, once a final response to an initial dialog 328 initiation request has been issued by the proxy, no further 199 329 responses associated with the request will be generated or forwarded 330 by the proxy. 332 When a forking proxy forks an initial dialog initiation request, it 333 generates a unique Via header branch parameter value for each forked 334 leg. A proxy can determine whether additional forking has occurred 335 downstream of the proxy by storing the top Via branch value from each 336 response which creates an early dialog. If the same top Via branch 337 value is received for multiple early dialogs, the proxy knows that 338 additional forking has occurred downstream of the proxy. A non-2xx 339 final response received for a specific early dialog also terminates 340 all other early dialog for which the same top Via branch value was 341 received in the responses which created those early dialogs. 343 Based on implementation policy, a forking proxy MAY wait before 344 sending the 199 response, e.g. if it expects to receive a 2xx final 345 response on another dialog shortly after it received the non-2xx 346 final response which triggered the 199 response. 348 When a forking proxy generates a 199 response, the response MUST 349 contain a To header field tag parameter, that identifies the 350 terminated early dialog. A proxy MUST also insert a Reason header 351 field that contains the SIP response code of the response that 352 triggered the 199 response. The SIP response code in the Reason 353 header field informs the receiver of the 199 response about the SIP 354 response code that was used by the UAS to terminate the early dialog, 355 and the receiver might use that information for triggering different 356 types of actions and procedures. The proxy MUST NOT insert a "199" 357 option-tag in the Supported, Require or Proxy-Require header field of 358 the 199 response. 360 A forking proxy that supports generating of 199 responses MUST keep 361 track of early dialogs, in order to determine whether to generate a 362 199 response when the proxy receives a non-2xx final response. In 363 addition, a proxy MUST keep track on which early dialogs it has 364 received and forwarded 199 responses, in order to not generate 365 additional 199 responses for those early dialogs. 367 If a forking proxy receives a reliably sent 199 response for a 368 dialog, for which it has previously generated and sent a 199 369 response, it MUST forward the 199 response. If a proxy receives an 370 unreliably sent 199 response, for which it has previously generated 371 and sent a 199 response, it MAY forward the response, or it MAY 372 discard it. 374 When a forking proxy generates and sends a 199 response, the response 375 SHOULD NOT contain a Contact header field or a Record-Route header 376 field [RFC3261]. 378 If the Require header field of an initial dialog initiation request 379 contains an "100rel" option-tag, a proxy MUST NOT generate and send 380 199 responses associated with that request. The reason is that a 381 proxy is not allowed to generate and send 199 responses reliably. 383 7. Backward compatibility 385 Since all SIP entities involved in a session setup do not necessarily 386 support the specific meaning of the 199 Early Dialog Terminated 387 provisional response, the sender of the response MUST be prepared to 388 receive SIP requests and responses associated with the dialog for 389 which the 199 response was sent (a proxy can receive SIP messages 390 from either direction). If such request is received by a UA, it MUST 391 act in the same way as if it had received the request after sending 392 the final non-2xx response to the INVITE request, as specified in RFC 393 3261. A UAC that receives a 199 response for an early dialog MUST 394 NOT send any further requests on that dialog, except for requests 395 which acknowledge reliable responses. A proxy MUST forward requests 396 according to RFC 3261, even if the proxy has knowledge that the early 397 dialog has been terminated. 399 A 199 response does not "replace" a final response. RFC 3261 400 specifies when a final response is sent. 402 8. Usage with SDP offer/answer 404 A 199 response SHOULD NOT contain an SDP offer/answer [RFC3264] 405 message body, unless required by the rules in RFC 3264. 407 If an INVITE request does not contain an SDP offer, and the 199 408 response is the first reliably sent response, the 199 response is 409 required to contain an SDP offer. In this case the UAS SHOULD send 410 the 199 response unreliable, or include an SDP offer with no m- lines 411 in a reliable 199 response. 413 9. Message Flow Examples 415 9.1. Example with a forking proxy which generates 199 417 The figure shows an example, where a proxy (P1) forks an INVITE 418 received from UAC. The forked INVITE reaches UAS_2, UAS_3 and UAS_4, 419 which send 18x provisional responses in order to establish early 420 dialogs between themselves and the UAC. UAS_2 and UAS_3 reject the 421 INVITE by sending a 4xx error response each. When P1 receives the 422 4xx responses it immediately sends 199 responses towards the UAC, to 423 indicate that the early dialogs for which it received the 4xx 424 responses have been terminated. The early dialog leg is shown in 425 parenthesis. 427 UAC P1 UAS_2 UAS_3 UAS_4 428 | | | | | 429 |-- INVITE -->| | | | 430 | |--- INVITE (2) ->| | | 431 | |--- INVITE (3) --------->| | 432 | |--- INVITE (4) ----------------->| 433 | |<-- 18x (2) -----| | | 434 |<- 18x (2) --| | | | 435 | |<-- 18x (3) -------------| | 436 |<- 18x (3) --| | | | 437 | |<-- 18x (4) ---------------------| 438 |<- 18x (4) --| | | | 439 | |<-- 4xx (2) -----| | | 440 | |--- ACK (2) ---->| | | 441 |<- 199 (2) --| | | | 442 | |<-- 4xx (3) -------------| | 443 | |--- ACK (3) ------------>| | 444 |<- 199 (3) --| | | | 445 | |<-- 200 (4) ---------------------| 446 |<- 200 (4) --| | | | 447 |-- ACK (4) ->| | | | 448 | |--- ACK (4) -------------------->| 449 | | | | | 451 Figure 1: Example call flow 453 9.2. Example with a forking proxy which receives 200 OK 455 The figure shows an example, where a proxy (P1) forks an INVITE 456 request received from UAC. The forked request reaches UAS_2, UAS_3 457 and UAS_4, that all send 18x provisional responses in order to 458 establish early dialogs between themselves and the UAC. Later UAS_4 459 accepts the session and sends a 200 OK final response. When P1 460 receives the 200 OK responses it immediately forwards it towards the 461 UAC. P1 does not send 199 responses for the early dialogs from UAS_2 462 and UAS_3, since P1 has yet not received any final responses on those 463 early dialogs (even if P1 sends CANCEL requests to UAS_2 and UAS_3 P1 464 may still receive 200 OK final response from UAS_2 or UAS_3, that P1 465 would have to forward towards the UAC. The early dialog leg is shown 466 in parenthesis. 468 UAC P1 UAS_2 UAS_3 UAS_4 469 | | | | | 470 |-- INVITE -->| | | | 471 | |--- INVITE (2) ->| | | 472 | |--- INVITE (3) --------->| | 473 | |--- INVITE (4) ----------------->| 474 | |<-- 18x (2) -----| | | 475 |<- 18x (2) --| | | | 476 | |<-- 18x (3) -------------| | 477 |<- 18x (3) --| | | | 478 | |<-- 18x (4) ---------------------| 479 |<- 18x (4) --| | | | 480 | |<-- 200 (4) ---------------------| 481 |<- 200 (4) --| | | | 482 |-- ACK (4) ->| | | | 483 | |--- ACK (4) -------------------->| 484 | | | | | 486 Figure 2: Example call flow 488 9.3. Example with two forking proxies, of which one generates 199 490 The figure shows an example, where a proxy (P1) forks an INVITE 491 request received from UAC. One of the forked requests reaches UAS_2. 492 The other requests reach another proxy (P2), that forks the request 493 to UAS_3 and UAS_4. UAS_3 and UAS_4 send 18x provisional responses 494 in order to establish early dialogs between themselves and UAC. 495 Later UAS_3 and UAS_4 reject the INVITE request by sending a 4xx 496 error response each. P2 does not support the 199 response code, and 497 forwards a single 4xx response. P1 supports the 199 response code, 498 and when it receives the 4xx response from P2, it also manages to 499 associate the early dialogs from both UAS_3 and UAS_4 with the 500 response. Therefore it generates and sends two 199 responses to 501 indicate that the early dialogs from UAS_3 and UAS_4 have been 502 terminated. The early dialog leg is shown in parenthesis. 504 UAC P1 P2 UAS_2 UAS_3 UAS_4 505 | | | | | | 506 |-- INVITE -->| | | | | 507 | |-- INVITE (2) ------------------>| | | 508 | |-- INVITE ---->| | | | 509 | | |--- INVITE (3) --------->| | 510 | | |--- INVITE (4) ----------------->| 511 | | |<-- 18x (3) -------------| | 512 | |<- 18x (3) ----| | | | 513 |<- 18x (3) --| | | | | 514 | | |<-- 18x (4) ---------------------| 515 | |<- 18x (4) ----| | | | 516 |<- 18x (4) --| | | | | 517 | | |<-- 4xx (3) -------------| | 518 | | |--- ACK (3) ------------>| | 519 | | |<-- 4xx (4) ---------------------| 520 | | |--- ACK (4) -------------------->| 521 | |<- 4xx (3) ----| | | | 522 | |-- ACK (3) --->| | | | 523 |<- 199 (3) --| | | | | 524 |<- 199 (4) --| | | | | 525 | |<- 200 (2) ----------------------| | | 526 |<- 200 (2) --| | | | | 527 |-- ACK (2) ->| | | | | 528 | |-- ACK (2) --------------------->| | | 529 | | | | | | 531 Figure 3: Example call flow 533 10. Security Considerations 535 General security issues related to SIP responses are described in RFC 536 3261. Due to the nature of the 199 response, it may be attractive to 537 use it for launching attacks in order to terminate specific early 538 dialogs (other early dialogs will not be affected). In addition, if 539 a man-in-the-middle generates and sends a 199 response, which 540 terminates a specific dialog, towards the UAC, it can take a while 541 until the UAS finds out that the UAC, and possible stateful 542 intermediates, have terminated the dialog. SIP security mechanisms 543 (e.g. hop-to-hop TLS) can be used to minimize, or eliminate, the risk 544 for such attacks. 546 11. IANA Considerations 548 This section registers a new SIP response code and a new option-tag, 549 according to the procedures of RFC 3261. 551 11.1. IANA Registration of the 199 response code 553 This section registers a new SIP response code, 199. The required 554 information for this registration, as specified in RFC 3261, is: 556 RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the 557 RFC number of this specification]] 559 Response Code Number: 199 561 Default Reason Phrase: Early Dialog Terminated 563 11.2. IANA Registration of the 199 option-tag 565 This section registers a new SIP option-tag, 199. The required 566 information for this registration, as specified in RFC 3261, is: 568 Name: 199 570 Description: This option-tag is for indicating support of the 199 571 Early Dialog Terminated provisional response code. When present 572 in a Supported header of a request, it indicates that the UAC 573 supports the 199 response code. When present in a Require or 574 Proxy-Require header field of a request, it indicates that the 575 UAS, or proxies, MUST support the 199 response code. It does 576 not require the UAS, or proxies, to actually send 199 577 responses. 579 12. Acknowledgements 581 Thanks to Paul Kyzivat, Dale Worley, Gilad Shaham, Francois Audet, 582 Attila Sipos, Robert Sparks, Brett Tate, Ian Elz, Hadriel Kaplan, 583 Timothy Dwight, Dean Willis, Serhad Doken, John Elwell, Gonzalo 584 Camarillo, Adam Roach, Bob Penfield, Tom Taylor, Ya Ching Tan, Keith 585 Drage, Hans Erik van Elburg and Cullen Jennings for their feedback 586 and suggestions. 588 13. Change Log 590 [RFC EDITOR NOTE: Please remove this section when publishing] 591 Changes from draft-ietf-sipcore-199-04 592 o "Usage with 100rel" section removed based on comments from John 593 Elwell (31.01.2011) 594 o Editorial corrections based on comments from Paul Kyzivat 595 (31.01.2011) 597 Changes from draft-ietf-sipcore-199-03 598 o RFC 3262 update removed 599 o Functional modification: proxy must not send 199 in case of 600 Require:100rel 601 o Recommendation that UAC does not require reliable provisional 602 responses with 199 603 o Clarification that Require:199 does not mandate the UAS to send a 604 199 response 605 o Clarification that a UAC needs to insert the 199 option-tag in a 606 Supported header field, even if it also inserts the option-tag in 607 a Require or Proxy-Require header field 608 o Editorial corrections 610 Changes from draft-ietf-sipcore-199-02 611 o Usage example section rewritten and clarified 612 o Requirement has been removed 613 o SIP has been added to document title 614 o Acronyms expanded in the abstract and throughout the document 615 o Editorial fixes throughout the document 616 o Indication added that document is aimed for standards track 617 o Some references made informative 618 o Additional text added regarding the usage of the Reason header 619 o SBC latching text has been removed 620 o Usage of Require/Proxy-Require header removed 621 o Additional text added regarding sending SDP offer in 199 622 o Note added, which clarifies that 199 does not affect other early 623 dialogs 624 o References added to Security Considerations 625 o Clarification of local ringing tone 626 o Clarification that media must not be sent or processed after 199 627 o Text regarding sending media on terminated dialogs added to 628 security section 629 o Change: UAS must send 199 reliably in case of Require:100rel 630 o Change: Section 4 of RFC 3262 updated 632 14. References 634 14.1. Normative References 636 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 637 Requirement Levels", BCP 14, RFC 2119, March 1997. 639 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 640 A., Peterson, J., Sparks, R., Handley, M., and E. 641 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 642 June 2002. 644 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 645 Provisional Responses in Session Initiation Protocol 646 (SIP)", RFC 3262, June 2002. 648 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 649 with Session Description Protocol (SDP)", RFC 3264, 650 June 2002. 652 [RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason 653 Header Field for the Session Initiation Protocol (SIP)", 654 RFC 3326, December 2002. 656 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 657 "Indicating User Agent Capabilities in the Session 658 Initiation Protocol (SIP)", RFC 3840, August 2004. 660 14.2. Informational References 662 [RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller 663 Preferences for the Session Initiation Protocol (SIP)", 664 RFC 3841, August 2004. 666 [RFC5057] Sparks, R., "Multiple Dialog Usages in the Session 667 Initiation Protocol", RFC 5057, November 2007. 669 [3GPP.24.182] 670 3GPP, "IP Multimedia Subsystem (IMS) Customized Alerting 671 Tones (CAT); Protocol specification", 3GPP TS 24.182. 673 [3GPP.24.628] 674 3GPP, "Common Basic Communication procedures using IP 675 Multimedia (IM)Core Network (CN) subsystem; Protocol 676 specification", 3GPP TS 24.628. 678 Author's Address 680 Christer Holmberg 681 Ericsson 682 Hirsalantie 11 683 Jorvas 02420 684 Finland 686 Email: christer.holmberg@ericsson.com