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Holmberg 3 Internet-Draft Ericsson 4 Intended status: Standards Track March 3, 2011 5 Expires: September 4, 2011 7 Session Initiation Protocol (SIP) Response Code for Indication of 8 Terminated Dialog 9 draft-ietf-sipcore-199-06.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 September 4, 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 . . . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . . . . . . . . 12 67 10. Security Considerations . . . . . . . . . . . . . . . . . . . 13 68 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 69 11.1. IANA Registration of the 199 response code . . . . . . . . 14 70 11.2. IANA Registration of the 199 option-tag . . . . . . . . . 14 71 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 72 13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 14 73 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 74 14.1. Normative References . . . . . . . . . . . . . . . . . . . 15 75 14.2. Informational References . . . . . . . . . . . . . . . . . 16 76 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 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. For example, a media gate 145 controlling SIP entity might use the 199 response when deciding for 146 which early dialogs media will be passed. 148 Section 9 contains signalling examples that show when and how a 149 forking proxy generates 199 responses in different situations. 151 2. Terminology 153 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 154 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 155 document are to be interpreted as described in RFC 2119 [RFC2119]. 157 3. Applicability and Limitation 159 The 199 response code is an optimization, and it only optimizes how 160 quickly recipients might be informed about terminated early dialogs. 161 The achieved optimization is limited. Since the response is normally 162 not sent reliably by an UAS, and can not be sent reliably when 163 generated and sent by a proxy, it is possible that some or all of the 164 199 responses get lost before they reach the recipients. In such 165 cases, recipients will behave the same as if the 199 response code 166 were not used at all. 168 One example for which a UAC could use the 199 response, is that when 169 it receives a 199 response it releases resources associated with the 170 terminated early dialog. The UAC could also use the 199 response to 171 make policy related decisions related to early dialogs. For example, 172 if a UAC is playing media associated with an early dialog, and it 173 then receives a 199 response indicating the early dialog has been 174 terminated, it could start playing media associated with a different 175 early dialog. 177 Applications designers utilizing the 199 response code MUST ensure 178 that the application's user experience is acceptable if all 199 179 responses are lost, and not delivered to the recipients. 181 4. User Agent Client behavior 183 When a UAC sends an initial dialog initiation request, and if it is 184 willing to receive 199 responses, it MUST insert a "199" option-tag 185 in the Supported header field [RFC3261] of the request. The option- 186 tag indicates that the UAC supports, and is willing to receive, 199 187 responses. A UAC SHOULD NOT insert a "199" option-tag in the Require 188 or the Proxy-Require header field [RFC3261] of the request, since in 189 many cases it would result in unnecessary session establishment 190 failures. 192 NOTE: The UAC always needs to insert a "199" option-tag in the 193 Supported header field, in order to indicate that it supports, and is 194 willing to receive, 199 responses, even if it also inserts the 195 option-tag in the Require or Proxy-Require header field. 197 It is RECOMMENDED that a UAC does not insert a "100rel" option-tag 198 [RFC3262] in the Require header field when it also indicates support 199 of 199 responses, unless the UAC also uses some other SIP extension 200 or procedure that mandates it to do so. The reason is that proxies 201 are not allowed to generate and send 199 responses when the UAC has 202 required provisional responses to be sent reliably. 204 When a UAC receives a 199 response, it might release resources 205 associated with the terminated early dialog. A UAC might also use 206 the 199 response to make policy related decisions related to early 207 dialogs. 209 NOTE: The 199 response indicates that the early dialog has been 210 terminated, so there is no need for the UAC to send a BYE request in 211 order to terminate the early dialog when it receives the 199 212 response. 214 NOTE: The 199 response does not affect other early dialogs associated 215 with the session establishment. For those the normal SIP rules, 216 regarding transaction timeout etc, still apply. 218 Once a UAC has received and accepted a 199 response, it MUST NOT send 219 Any media associated with the early dialog. In addition, if the UAC 220 is able to associate received media with early dialogs, it MUST NOT 221 process any received media associated with the early dialog that was 222 terminated. 224 If multiple usages [RFC5057] are used within an early dialog, and it 225 is not clear which dialog usage the 199 response terminates, SIP 226 entities that keep dialog state SHALL NOT release resources 227 associated with the early dialog when they receive the 199 response. 229 If a UAC receives an unreliably sent 199 response on a dialog which 230 has not previously been established (this can happen if a 199 231 response reaches the client before the 18x response that would 232 establish the early dialog) it SHALL discard the 199 responses. If a 233 UAC receives a reliably sent 199 response on a dialog which has not 234 previously been created, it MUST acknowledge the 199 response, as 235 described in RFC 3262 [RFC3262]. 237 If a UAC has received a 199 response for all early dialogs, and no 238 early dialog associated session establishment remains, it maintains 239 the "Proceeding" state [RFC3261] and waits for possible subsequent 240 early dialogs to be established, and eventually for a final response 241 to be received. 243 5. User Agent Server behavior 245 If a UAS receives an initial dialog initiation request, with a 246 Supported header field that contains a "199" option-tag, it SHOULD 247 NOT send a 199 response on an early dialog associated with the 248 request, before it sends a non-2xx final response. Cases where a UAS 249 might send a 199 response are if it has been configured to do so due 250 to lack of support of the 199 response code by forking proxies or 251 other intermediate SIP entities, or it is used in an environment that 252 specifies that it shall send a 199 response before sending a non-2xx 253 response. 255 NOTE: If a UAS has created multiple early dialogs associated with an 256 initial dialog initiation request (the UAS is acting similar to a 257 forking proxy), it does not always intend to send a final response on 258 all of those early dialogs. 260 NOTE: If the Require header field of an initial dialog initiation 261 request contains a "100rel" option-tag, proxies will not be able to 262 generate and send 199 responses. In such cases the UAS might choose 263 to send a 199 response on an early dialog, before it sends a non-2xx 264 final response, even if it would not do so in other cases. 266 If the Supported header field of an initial dialog initiation request 267 does not contain a "199" option-tag, the UAC MUST NOT send a 199 268 response on any early dialog associated with the request. 270 When a UAS generates a 199 response, the response MUST contain a To 271 header field tag parameter [RFC3261], in order for other entities to 272 identify the early dialog that has been terminated. The UAS MUST 273 also insert a Reason header field [RFC3326] that contains a response 274 code which describes the reason why the early dialog was terminated. 275 The UAS MUST NOT insert a "199" option-tag in the Supported, Require 276 or Proxy-Require header field of the 199 response. 278 If a UAS intends to send 199 responses, and if it supports the 279 procedures defined in RFC 3840 [RFC3840], it MAY during the 280 registration procedure use the sip.extensions feature tag [RFC3840] 281 to indicate support of the 199 response code. 283 A 199 response SHOULD NOT contain an SDP offer/answer message body, 284 unless required by the rules in RFC 3264 [RFC3264]. 286 According to RFC 3264, if an INVITE request does not contain an SDP 287 offer, and the 199 response is the first reliably sent response 288 associated with the request, the 199 response is required to contain 289 an SDP offer. In this case the UAS SHOULD send the 199 response 290 unreliably, or send the 199 response reliably and include an SDP 291 offer with no m- lines in the response. 293 Since a 199 response is only used for information purpose, the UAS 294 SHOULD send it unreliably, unless the "100rel" option-tag is present 295 in the Require header field of the associated request. 297 6. Proxy behavior 299 When a proxy receives a 199 response to an initial dialog initiation 300 request, it MUST process the response as any other non-100 301 provisional response. The proxy will forward the response upstream 302 towards the sender of the associated request. The proxy MAY release 303 resources it has reserved associated with the early dialog that is 304 terminated. If a proxy receives a 199 response out of dialog, it 305 MUST process it as other non-100 provisional responses received out 306 of dialog. 308 When a forking proxy receives a non-2xx final response to an initial 309 dialog initiation request, that it recognizes as terminating one or 310 more early dialogs associated with the request, it MUST generate and 311 send a 199 response upstream for each of the terminated early dialogs 312 that satisfy each of the following conditions: 314 - the forking proxy does not intend to forward the final response 315 immediately (in accordance with rules for a forking proxy) 317 - the UAC has indicated support (by inserting the "199" option-tag in 318 a Supported header field) of the 199 response code in the associated 319 request 321 - the UAC has not required provisional responses to be sent reliably 322 (by inserting the "100rel" option-tag in a Require or Proxy-Require 323 header field) in the associated request 325 - the forking proxy has not already received and forwarded a 199 326 response for the early dialog 328 - the forking proxy has not already sent a final response for any of 329 the early dialogs 331 As a consequence, once a final response to an initial dialog 332 initiation request has been issued by the proxy, no further 199 333 responses associated with the request will be generated or forwarded 334 by the proxy. 336 When a forking proxy forks an initial dialog initiation request, it 337 generates a unique Via header branch parameter value for each forked 338 leg. A proxy can determine whether additional forking has occurred 339 downstream of the proxy by storing the top Via branch value from each 340 response which creates an early dialog. If the same top Via branch 341 value is received for multiple early dialogs, the proxy knows that 342 additional forking has occurred downstream of the proxy. A non-2xx 343 final response received for a specific early dialog also terminates 344 all other early dialog for which the same top Via branch value was 345 received in the responses which created those early dialogs. 347 Based on implementation policy, a forking proxy MAY wait before 348 sending the 199 response, e.g. if it expects to receive a 2xx final 349 response on another dialog shortly after it received the non-2xx 350 final response which triggered the 199 response. 352 When a forking proxy generates a 199 response, the response MUST 353 contain a To header field tag parameter, that identifies the 354 terminated early dialog. A proxy MUST also insert a Reason header 355 field that contains the SIP response code of the response that 356 triggered the 199 response. The SIP response code in the Reason 357 header field informs the receiver of the 199 response about the SIP 358 response code that was used by the UAS to terminate the early dialog, 359 and the receiver might use that information for triggering different 360 types of actions and procedures. The proxy MUST NOT insert a "199" 361 option-tag in the Supported, Require or Proxy-Require header field of 362 the 199 response. 364 A forking proxy that supports generating of 199 responses MUST keep 365 track of early dialogs, in order to determine whether to generate a 366 199 response when the proxy receives a non-2xx final response. In 367 addition, a proxy MUST keep track on which early dialogs it has 368 received and forwarded 199 responses, in order to not generate 369 additional 199 responses for those early dialogs. 371 If a forking proxy receives a reliably sent 199 response for a 372 dialog, for which it has previously generated and sent a 199 373 response, it MUST forward the 199 response. If a proxy receives an 374 unreliably sent 199 response, for which it has previously generated 375 and sent a 199 response, it MAY forward the response, or it MAY 376 discard it. 378 When a forking proxy generates and sends a 199 response, the response 379 SHOULD NOT contain a Contact header field or a Record-Route header 380 field [RFC3261]. 382 If the Require header field of an initial dialog initiation request 383 contains a "100rel" option-tag, a proxy MUST NOT generate and send 384 199 responses associated with that request. The reason is that a 385 proxy is not allowed to generate and send 199 responses reliably. 387 7. Backward compatibility 389 Since all SIP entities involved in a session setup do not necessarily 390 support the specific meaning of the 199 Early Dialog Terminated 391 provisional response, the sender of the response MUST be prepared to 392 receive SIP requests and responses associated with the dialog for 393 which the 199 response was sent (a proxy can receive SIP messages 394 from either direction). If such request is received by a UA, it MUST 395 act in the same way as if it had received the request after sending 396 the final non-2xx response to the INVITE request, as specified in RFC 397 3261. A UAC that receives a 199 response for an early dialog MUST 398 NOT send any further requests on that dialog, except for requests 399 which acknowledge reliable responses. A proxy MUST forward requests 400 according to RFC 3261, even if the proxy has knowledge that the early 401 dialog has been terminated. 403 A 199 response does not "replace" a final response. RFC 3261 404 specifies when a final response is sent. 406 8. Usage with SDP offer/answer 408 A 199 response SHOULD NOT contain an SDP offer/answer [RFC3264] 409 message body, unless required by the rules in RFC 3264. 411 If an INVITE request does not contain an SDP offer, and the 199 412 response is the first reliably sent response, the 199 response is 413 required to contain an SDP offer. In this case the UAS SHOULD send 414 the 199 response unreliable, or include an SDP offer with no m- lines 415 in a reliable 199 response. 417 9. Message Flow Examples 419 9.1. Example with a forking proxy which generates 199 421 The figure shows an example, where a proxy (P1) forks an INVITE 422 received from UAC. The forked INVITE reaches UAS_2, UAS_3 and UAS_4, 423 which send 18x provisional responses in order to establish early 424 dialogs between themselves and the UAC. UAS_2 and UAS_3 reject the 425 INVITE by sending a 4xx error response each. When P1 receives the 426 4xx responses it immediately sends 199 responses towards the UAC, to 427 indicate that the early dialogs for which it received the 4xx 428 responses have been terminated. The early dialog leg is shown in 429 parenthesis. 431 UAC P1 UAS_2 UAS_3 UAS_4 432 | | | | | 433 |-- INVITE -->| | | | 434 | |--- INVITE (2) ->| | | 435 | |--- INVITE (3) --------->| | 436 | |--- INVITE (4) ----------------->| 437 | |<-- 18x (2) -----| | | 438 |<- 18x (2) --| | | | 439 | |<-- 18x (3) -------------| | 440 |<- 18x (3) --| | | | 441 | |<-- 18x (4) ---------------------| 442 |<- 18x (4) --| | | | 443 | |<-- 4xx (2) -----| | | 444 | |--- ACK (2) ---->| | | 445 |<- 199 (2) --| | | | 446 | |<-- 4xx (3) -------------| | 447 | |--- ACK (3) ------------>| | 448 |<- 199 (3) --| | | | 449 | |<-- 200 (4) ---------------------| 450 |<- 200 (4) --| | | | 451 |-- ACK (4) ->| | | | 452 | |--- ACK (4) -------------------->| 453 | | | | | 455 Figure 1: Example call flow 457 9.2. Example with a forking proxy which receives 200 OK 459 The figure shows an example, where a proxy (P1) forks an INVITE 460 request received from UAC. The forked request reaches UAS_2, UAS_3 461 and UAS_4, that all send 18x provisional responses in order to 462 establish early dialogs between themselves and the UAC. Later UAS_4 463 accepts the session and sends a 200 OK final response. When P1 464 receives the 200 OK responses it immediately forwards it towards the 465 UAC. P1 does not send 199 responses for the early dialogs from UAS_2 466 and UAS_3, since P1 has yet not received any final responses on those 467 early dialogs (even if P1 sends CANCEL requests to UAS_2 and UAS_3 P1 468 may still receive 200 OK final response from UAS_2 or UAS_3, that P1 469 would have to forward towards the UAC. The early dialog leg is shown 470 in parenthesis. 472 UAC P1 UAS_2 UAS_3 UAS_4 473 | | | | | 474 |-- INVITE -->| | | | 475 | |--- INVITE (2) ->| | | 476 | |--- INVITE (3) --------->| | 477 | |--- INVITE (4) ----------------->| 478 | |<-- 18x (2) -----| | | 479 |<- 18x (2) --| | | | 480 | |<-- 18x (3) -------------| | 481 |<- 18x (3) --| | | | 482 | |<-- 18x (4) ---------------------| 483 |<- 18x (4) --| | | | 484 | |<-- 200 (4) ---------------------| 485 |<- 200 (4) --| | | | 486 |-- ACK (4) ->| | | | 487 | |--- ACK (4) -------------------->| 488 | | | | | 490 Figure 2: Example call flow 492 9.3. Example with two forking proxies, of which one generates 199 494 The figure shows an example, where a proxy (P1) forks an INVITE 495 request received from UAC. One of the forked requests reaches UAS_2. 496 The other requests reach another proxy (P2), that forks the request 497 to UAS_3 and UAS_4. UAS_3 and UAS_4 send 18x provisional responses 498 in order to establish early dialogs between themselves and UAC. 499 Later UAS_3 and UAS_4 reject the INVITE request by sending a 4xx 500 error response each. P2 does not support the 199 response code, and 501 forwards a single 4xx response. P1 supports the 199 response code, 502 and when it receives the 4xx response from P2, it also manages to 503 associate the early dialogs from both UAS_3 and UAS_4 with the 504 response. Therefore it generates and sends two 199 responses to 505 indicate that the early dialogs from UAS_3 and UAS_4 have been 506 terminated. The early dialog leg is shown in parenthesis. 508 UAC P1 P2 UAS_2 UAS_3 UAS_4 509 | | | | | | 510 |-- INVITE -->| | | | | 511 | |-- INVITE (2) ------------------>| | | 512 | |-- INVITE ---->| | | | 513 | | |--- INVITE (3) --------->| | 514 | | |--- INVITE (4) ----------------->| 515 | | |<-- 18x (3) -------------| | 516 | |<- 18x (3) ----| | | | 517 |<- 18x (3) --| | | | | 518 | | |<-- 18x (4) ---------------------| 519 | |<- 18x (4) ----| | | | 520 |<- 18x (4) --| | | | | 521 | | |<-- 4xx (3) -------------| | 522 | | |--- ACK (3) ------------>| | 523 | | |<-- 4xx (4) ---------------------| 524 | | |--- ACK (4) -------------------->| 525 | |<- 4xx (3) ----| | | | 526 | |-- ACK (3) --->| | | | 527 |<- 199 (3) --| | | | | 528 |<- 199 (4) --| | | | | 529 | |<- 200 (2) ----------------------| | | 530 |<- 200 (2) --| | | | | 531 |-- ACK (2) ->| | | | | 532 | |-- ACK (2) --------------------->| | | 533 | | | | | | 535 Figure 3: Example call flow 537 10. Security Considerations 539 General security issues related to SIP responses are described in RFC 540 3261. Due to the nature of the 199 response, it may be attractive to 541 use it for launching attacks in order to terminate specific early 542 dialogs (other early dialogs will not be affected). In addition, if 543 a man-in-the-middle generates and sends a 199 response, which 544 terminates a specific dialog, towards the UAC, it can take a while 545 until the UAS finds out that the UAC, and possible stateful 546 intermediates, have terminated the dialog. SIP security mechanisms 547 (e.g. hop-to-hop TLS) can be used to minimize, or eliminate, the risk 548 for such attacks. 550 11. IANA Considerations 552 This section registers a new SIP response code and a new option-tag, 553 according to the procedures of RFC 3261. 555 11.1. IANA Registration of the 199 response code 557 This section registers a new SIP response code, 199. The required 558 information for this registration, as specified in RFC 3261, is: 560 RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the 561 RFC number of this specification]] 563 Response Code Number: 199 565 Default Reason Phrase: Early Dialog Terminated 567 11.2. IANA Registration of the 199 option-tag 569 This section registers a new SIP option-tag, 199. The required 570 information for this registration, as specified in RFC 3261, is: 572 Name: 199 574 Description: This option-tag is for indicating support of the 199 575 Early Dialog Terminated provisional response code. When present 576 in a Supported header of a request, it indicates that the UAC 577 supports the 199 response code. When present in a Require or 578 Proxy-Require header field of a request, it indicates that the 579 UAS, or proxies, MUST support the 199 response code. It does 580 not require the UAS, or proxies, to actually send 199 581 responses. 583 12. Acknowledgements 585 Thanks to Paul Kyzivat, Dale Worley, Gilad Shaham, Francois Audet, 586 Attila Sipos, Robert Sparks, Brett Tate, Ian Elz, Hadriel Kaplan, 587 Timothy Dwight, Dean Willis, Serhad Doken, John Elwell, Gonzalo 588 Camarillo, Adam Roach, Bob Penfield, Tom Taylor, Ya Ching Tan, Keith 589 Drage, Hans Erik van Elburg and Cullen Jennings for their feedback 590 and suggestions. 592 13. Change Log 594 [RFC EDITOR NOTE: Please remove this section when publishing] 596 Changes from draft-ietf-sipcore-199-04 597 o "Usage with 100rel" section removed based on comments from John 598 Elwell (31.01.2011) 599 o Editorial corrections based on comments from Paul Kyzivat 600 (31.01.2011) 602 Changes from draft-ietf-sipcore-199-03 603 o RFC 3262 update removed 604 o Functional modification: proxy must not send 199 in case of 605 Require:100rel 606 o Recommendation that UAC does not require reliable provisional 607 responses with 199 608 o Clarification that Require:199 does not mandate the UAS to send a 609 199 response 610 o Clarification that a UAC needs to insert the 199 option-tag in a 611 Supported header field, even if it also inserts the option-tag in 612 a Require or Proxy-Require header field 613 o Editorial corrections 615 Changes from draft-ietf-sipcore-199-02 616 o Usage example section rewritten and clarified 617 o Requirement has been removed 618 o SIP has been added to document title 619 o Acronyms expanded in the abstract and throughout the document 620 o Editorial fixes throughout the document 621 o Indication added that document is aimed for standards track 622 o Some references made informative 623 o Additional text added regarding the usage of the Reason header 624 o SBC latching text has been removed 625 o Usage of Require/Proxy-Require header removed 626 o Additional text added regarding sending SDP offer in 199 627 o Note added, which clarifies that 199 does not affect other early 628 dialogs 629 o References added to Security Considerations 630 o Clarification of local ringing tone 631 o Clarification that media must not be sent or processed after 199 632 o Text regarding sending media on terminated dialogs added to 633 security section 634 o Change: UAS must send 199 reliably in case of Require:100rel 635 o Change: Section 4 of RFC 3262 updated 637 14. References 639 14.1. Normative References 641 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 642 Requirement Levels", BCP 14, RFC 2119, March 1997. 644 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 645 A., Peterson, J., Sparks, R., Handley, M., and E. 646 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 647 June 2002. 649 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 650 Provisional Responses in Session Initiation Protocol 651 (SIP)", RFC 3262, June 2002. 653 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 654 with Session Description Protocol (SDP)", RFC 3264, 655 June 2002. 657 [RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason 658 Header Field for the Session Initiation Protocol (SIP)", 659 RFC 3326, December 2002. 661 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 662 "Indicating User Agent Capabilities in the Session 663 Initiation Protocol (SIP)", RFC 3840, August 2004. 665 14.2. Informational References 667 [RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller 668 Preferences for the Session Initiation Protocol (SIP)", 669 RFC 3841, August 2004. 671 [RFC5057] Sparks, R., "Multiple Dialog Usages in the Session 672 Initiation Protocol", RFC 5057, November 2007. 674 [3GPP.24.182] 675 3GPP, "IP Multimedia Subsystem (IMS) Customized Alerting 676 Tones (CAT); Protocol specification", 3GPP TS 24.182. 678 [3GPP.24.628] 679 3GPP, "Common Basic Communication procedures using IP 680 Multimedia (IM)Core Network (CN) subsystem; Protocol 681 specification", 3GPP TS 24.628. 683 Author's Address 685 Christer Holmberg 686 Ericsson 687 Hirsalantie 11 688 Jorvas 02420 689 Finland 691 Email: christer.holmberg@ericsson.com