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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 CUSS WG A. Johnston 3 Internet-Draft Avaya 4 Intended status: Informational L. Liess 5 Expires: November 29, 2011 Deutsche Telekom AG 6 May 28, 2011 8 Problem Statement and Requirements for Transporting User to User Call 9 Control Information in SIP 10 draft-ietf-cuss-sip-uui-reqs-02 12 Abstract 14 This document introduces the transport of call control related User 15 to User Information (UUI) using the Session Initiation Protocol 16 (SIP), and develops several requirements for a new SIP mechanism. 17 Some SIP sessions are established by or related to a non-SIP 18 application. This application may have information that needs to be 19 transported between the SIP User Agents during session establishment. 20 A common example in another protocol is the ISDN User to User 21 Information Service. As networks move to SIP it is important that 22 applications requiring this data can continue to function in SIP 23 networks as well as the ability to interwork with this ISDN service 24 for end-to-end transparency. This document discusses requirements 25 and approaches to achieve this. In addition, the extension will also 26 be used for native SIP endpoints implementing similar services and 27 interworking with ISDN services. Example use cases include an 28 exchange between two user agents, retargeting by a proxy, and 29 redirection. An example application is an Automatic Call Distributor 30 (ACD) in a contact center. 32 Status of this Memo 34 This Internet-Draft is submitted to IETF in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on November 29, 2011. 49 Copyright Notice 51 Copyright (c) 2011 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 68 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 3.1. User Agent to User Agent . . . . . . . . . . . . . . . . . 4 70 3.2. Proxy Retargeting . . . . . . . . . . . . . . . . . . . . 5 71 3.3. Redirection . . . . . . . . . . . . . . . . . . . . . . . 5 72 3.4. Referral . . . . . . . . . . . . . . . . . . . . . . . . . 7 73 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7 74 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 75 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 76 7. Informative References . . . . . . . . . . . . . . . . . . . . 10 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 79 1. Overview 81 This document describes the transport of User to User Information 82 (UUI) during session setup. This section introduces UUI and explains 83 how it relates to SIP. 85 We define SIP UUI information as application-specific information 86 that is related to a session being established using SIP. It is 87 assumed that the application is running in both the originator of the 88 session and the terminator of the session. That is, the application 89 interacts with the User Agent Client (UAC) and User Agent Server 90 (UAS). In order to function properly, the application needs a small 91 piece of information, the UUI, to be transported at the time of 92 session establishment. This information is essentially opaque data 93 to SIP - it is unrelated to SIP routing, authentication, or any other 94 SIP function. This application can be considered to be operating at 95 a higher layer on the protocol stack. As a result, SIP should not 96 interpret, understand, or perform any operations on the UUI. Should 97 this not be the case, then the information being transported is not 98 considered UUI, and another SIP-specific mechanism will be needed to 99 transport the information (such as a new header field). 101 UUI is defined this way for two reasons. Firstly, this supports a 102 strict layering of protocols and data. Providing information and 103 understanding of the UUI to the transport layer (SIP in this case) 104 would not provide any benefits and instead could create cross layer 105 coupling. Secondly, it is neither feasible nor desirable for a SIP 106 User Agent (UA) to understand the information; instead the goal is 107 for the UA to simply pass the information as efficiently as possible 108 to the application which does understand the information. 110 An important application is the interworking with User to User 111 Information (UUI) in ISDN, specifically, the transport of the call 112 control related ITU-T Q.931 User to User Information Element (UU IE) 113 [Q931] and ITU-T Q.763 User to User Information Parameter [Q763] data 114 in SIP. ISDN UUI is widely used in the PSTN today in contact centers 115 and call centers. These applications are currently transitioning 116 away from using ISDN for session establishment to using SIP. Native 117 SIP endpoints will need to implement a similar service and be able to 118 interwork with this ISDN service. 120 Note that the distinction between call control UUI and non-call 121 control UUI is very important. SIP already has a mechanism for 122 sending arbitrary UUI information between UAs during a session or 123 dialog - the SIP INFO [RFC2976] method. Call control UUI, in 124 contrast, must be exchanged at the time of setup and needs to be 125 carried in the INVITE and a few other methods and responses. 126 Applications that exchange UUI but do not have a requirement that it 127 be transported and processed during call setup can simply use SIP 128 INFO and do not need a new SIP extensions. 130 In this document, four different use case call flows are discussed. 131 Next, the requirements for call control UUI transport are discussed. 133 2. Terminology 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 137 document are to be interpreted as described in BCP 14, RFC 2119 138 [RFC2119]. 140 3. Use Cases 142 This section discusses four use cases for the transport of call 143 control related user to user information. What is not discussed here 144 is the transport of non-call control UUI which can be done using the 145 SIP INFO method. These use cases will help motivate the requirements 146 for SIP call control UUI. 148 3.1. User Agent to User Agent 150 In this scenario, the originator UA includes UUI in the INVITE sent 151 through a proxy to the terminating UA. The terminator can use the 152 UUI in any way. If it is an ISDN gateway, it could map the UUI into 153 the appropriate DSS1 information element or QSIG information element 154 or ISUP parameter. Alternatively, the using application might render 155 the information to the user, or use it during alerting or as a lookup 156 for a screen pop. In this case, the proxy does not need to 157 understand the UUI mechanism, but normal proxy rules should result in 158 the UUI being forwarded without modification. This call flow is 159 shown in Figure 1. 161 Originator Proxy Terminator 162 | | | 163 | INVITE (UUI) F1 | | 164 |------------------->| INVITE (UUI) F2 | 165 | 100 Trying F3 |------------------->| 166 |<-------------------| 200 OK F4 | 167 | 200 OK F5 |<-------------------| 168 |<-------------------| | 169 | ACK F6 | | 170 |------------------->| ACK F7 | 171 | |------------------->| 173 Figure 1. Call flow with UUI exchanged between Originator and 174 Terminator. 176 3.2. Proxy Retargeting 178 In this scenario, the originator UA includes UUI in the INVITE sent 179 through a proxy to the terminating UA. The proxy retargets the 180 INVITE, sending it to a different termination UA. The UUI 181 information is then received and processed by the terminating UA. 182 This call flow is shown in Figure 2. 184 Originator Proxy Terminator 2 185 | | | 186 | INVITE (UUI) F1 | | 187 |------------------->| INVITE (UUI) F2 | 188 | 100 Trying F3 |------------------->| 189 |<-------------------| 200 OK F4 | 190 | 200 OK F5 |<-------------------| 191 |<-------------------| | 192 | ACK F6 | | 193 |------------------->| ACK F7 | 194 | |------------------->| 196 Figure 2. Call flow with Proxy Retargeting. 198 The UUI in the INVITE needs to be passed unchanged through this proxy 199 retargeting operation. 201 3.3. Redirection 203 In this scenario, UUI is inserted by an application which utilizes a 204 SIP redirect server. The UUI is then included in the INVITE sent by 205 the Originator to the Terminator. In this case, the Originator does 206 not necessarily need to support the UUI mechanism but does need to 207 support the SIP redirection mechanism used to include the UUI 208 information. Two examples of UUI with redirection (transfer and 209 diversion) are defined in [ANSII] and [ETSI]. 211 Note that this case may not precisely map to an equivalent ISDN 212 service use case. This is because there is no one-to-one mapping 213 between elements in a SIP network and elements in an ISDN network. 214 Also, there is not an exact one-to-one mapping between SIP call 215 control and ISDN call control. However, this should not prevent the 216 usage of SIP call control UUI in these cases. Instead, these slight 217 differences between the SIP UUI service and the ISDN service need to 218 be carefully noted and discussed in an interworking specification. 220 Figure 3 shows this scenario, with the Redirect inserting UUI which 221 is then included in the INVITE F4 send to the Terminator. 223 Originator Redirect Server Terminator 224 | | | 225 | INVITE F1 | | 226 |------------------->| | 227 | 302 Moved (UUI) F2 | | 228 |<-------------------| | 229 | ACK F3 | | 230 |------------------->| | 231 | INVITE (UUI) F4 | | 232 |---------------------------------------->| 233 | 200 OK F5 | 234 |<----------------------------------------| 235 | ACK F6 | 236 |---------------------------------------->| 238 Figure 3. Call flow with UUI exchanged between Redirect Server and 239 Terminator. 241 If the Redirect Server is not in the same administrative domain as 242 the Terminator, the Redirect Server must not remove or replace any 243 UUI in the initial INVITE. In Figure 3, this means that if F1 244 included UUI, the Redirect Server could not modify or replace the UUI 245 in F2. However, if the Redirect Server and the Terminator are part 246 of the same administrative domain, they may have a policy allowing 247 the Redirect Server to modify or rewrite UUI information. In fact, 248 many UUI uses within an Enterprise rely on this feature to work today 249 in ISDN. 251 A common example application of this call flow is an Automatic Call 252 Distributer (ACD) in a PSTN contact center. The originator would be 253 a PSTN gateway. The ACD would act as a Redirect Server, inserting 254 UUI based on called number, calling number, time of day, and other 255 information. The resulting UUI would be passed to the agent's 256 handset which acts as the Terminator. The UUI could be used to 257 lookup information rendered to the agent at the time of call 258 answering. 260 This redirection scenario, and the referral scenario in the next 261 section, are the most important scenarios for contact center 262 applications. Incoming calls to a contact center almost always are 263 redirected or referred to a final destination, sometimes multiple 264 times, based on collected information and business logic. The 265 ability to maintain UUI in these scenarios is critical. 267 3.4. Referral 269 In this scenario, the application uses a UA to initiate a referral, 270 which causes an INVITE to be generated between the Originator and 271 Terminator with UUI information inserted by the Referrer UA. Note 272 that this REFER [RFC3515] could be part of a transfer operation or it 273 might be unrelated to an existing call, such as out-of-dialog REFER 274 call control. In some cases, this call flow is used in place of the 275 redirection call flow where immediately upon answer, the REFER is 276 sent. This scenario is shown in Figure 4. 278 Originator Referrer Terminator 279 | | | 280 | REFER (UUI) F1 | | 281 |<-------------------| | 282 | 202 Accepted F2 | | 283 |------------------->| | 284 | NOTIFY (100 Trying) F3 | 285 |------------------->| | 286 | 200 OK F4 | | 287 |<-------------------| | 288 | INVITE (UUI) F5 | | 289 |---------------------------------------->| 290 | 200 OK F6 | 291 |<----------------------------------------| 292 | ACK F7 | 293 |---------------------------------------->| 294 | NOTIFY (200 OK) F8 | | 295 |------------------->| | 296 | 200 OK F9 | | 297 |<-------------------| | 299 Figure 4. Call flow with Referral and UUI. 301 4. Requirements 303 This section discusses the requirements for the transport of call 304 control related user to user information (UUI). 306 REQ-1: The mechanism will allow UAs to insert and receive UUI data in 307 SIP call setup requests and responses. 309 SIP messages covered by this include INVITE requests and end-to- 310 end responses to the INVITE, which includes 18x and 200 responses. 312 REQ-2: The mechanism will allow UAs to insert and receive UUI data in 313 SIP dialog terminating requests and responses. 315 Q.931 UUI supports inclusion in release and release completion 316 messages. SIP messages covered by this include BYE and 200 OK 317 responses to a BYE. 319 REQ-3: The mechanism will allow UUI to be inserted and retrieved in 320 SIP redirects and referrals. 322 SIP messages covered by this include 3xx responses to INVITE and 323 REFER requests. 325 REQ-4: The mechanism will allow UUI to be able to survive proxy 326 retargeting or any other form of redirection of the request. 328 Retargeting is a common method of call routing in SIP, and must 329 not result in the loss of user to user information. 331 REQ-5: The mechanism should not require processing entities to 332 dereference a URL in order to retrieve the UUI information. 334 Passing a pointer or link to the UUI information will not meet the 335 real-time processing considerations and would complicate 336 interworking with the PSTN. 338 REQ-6 has been deleted. To avoid confusion, the number will not be 339 reused. 341 REQ-7: The mechanism will support interworking with call control 342 related DSS1 information elements or QSIG information elements or 343 ISUP parameters. 345 REQ-8: The mechanism will allow a UAC to learn or request that a UAS 346 understands the call control UUI mechanism and permit routing of 347 requests based on this. 349 This could be useful in ensuring that a request destined for the 350 PSTN is routed to a gateway that supports the ISDN UUI service 351 rather than an otherwise equivalent PSTN gateway that does not 352 support the ISDN UUI service. Note that support of the UUI 353 mechanism does not, by itself, imply that a particular user 354 application is supported - see REQ-10. 356 REQ-9: The mechanism will allow proxies to remove a particular type 357 of UUI information from a request or response. 359 This is a common security function provided by border elements to 360 header fields such as Alert-Info or Call-Info URIs. 362 REQ-10: The mechanism will provide the ability for a UA to discover 363 which types or application usages of UUI another UA understands or 364 supports. 366 The creation of a registry of application usages for the SIP UUI 367 mechanism is implied by this requirement. For the ISDN Service, 368 there could be value in utilizing the protocol discriminator, 369 which is the first octet of the ISDN UUI information, for this 370 purpose. 372 REQ-11: The solution will provide a mechanism of transporting at 373 least 128 octets of user data and a one octet protocol discriminator, 374 i.e. 129 octets in total. 376 There is the potential for non-ISDN services to allow UUI to be 377 larger than 128 octets. However, users of the mechanism will need 378 be cognizant of the size of SIP messages and the ability of 379 parsers to handle extremely large values. 381 REQ-12: The recipient of UUI will be able to determine the entity 382 that inserted the UUI. It is acceptable that this is performed 383 implicitly where it is known that there is only one other end UA 384 involved in the dialog. Where that does not exist, some other 385 mechanism will need to be provided. 387 This requirement comes into play during redirection, retargeting, 388 and referral scenarios. 390 REQ-13: The mechanism will allow integrity protection of the UUI. 392 This allows the UAS to be able to know that the UUI has not been 393 modified or tampered with by intermediaries. This property is not 394 guaranteed by the protocol in the ISDN application. 396 REQ-14: The mechanism will allow end-to-end privacy of the UUI. 398 Some UUI may contain private or sensitive information and may 399 require different security handling from the rest of the SIP 400 message. Note that this property is not available in the ISDN 401 application. In some cases, this requirement could be met by a 402 SIP privacy mechanism. In other cases, the application may 403 provide this service, encrypting the UUI before passing to the SIP 404 layer and decrypting upon receipt after receiving the UUI from the 405 SIP layer. In no cases should an intermediary element assume that 406 it will be able to read or interpret the UUI, as it only has end- 407 to-end significance. 409 5. Security Considerations 411 The security requirements for the SIP UUI mechanism are described in 412 REQ-13 and REQ-14, providing integrity protection and/or privacy. In 413 addition, there is an identity requirement in REQ-12, which relates 414 to the ability for the UAS to know who inserted the UUI. 416 It is important to note that UUI security is jointly provided at the 417 application layer and at the SIP layer. As such, is important for 418 application users of SIP UUI to know the realistic level of security 419 used and deployed in SIP, and not assume that some rarely deployed 420 SIP level security mechanism is in place. 422 6. Acknowledgements 424 Thanks to Joanne McMillen who was a co-author of earlier versions of 425 this specification. Thanks to Spencer Dawkins, Keith Drage, and 426 Vijay Gurbani for their review of earlier versions of this document. 427 The authors wish to thank Christer Holmberg, Frederique Forestie, 428 Francois Audet, Denis Alexeitsev, Paul Kyzivat, Cullen Jennings, and 429 Mahalingam Mani for their comments on this topic. 431 7. Informative References 433 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 434 A., Peterson, J., Sparks, R., Handley, M., and E. 435 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 436 June 2002. 438 [Q931] "ITU-T Q.931 User to User Information Element (UU IE)", 439 http://www.itu.int/rec/T-REC-Q.931-199805-I/en . 441 [Q763] "ITU-T Q.763 Signaling System No. 7 - ISDN user part 442 formats and codes", 443 http://www.itu.int/rec/T-REC-Q.931-199805-I/en . 445 [ANSII] "ANSI T1.643-1995, Telecommunications-Integrated Services 446 Digital Network (ISDN)-Explicit Call Transfer 447 Supplementary Service". 449 [ETSI] "ETSI ETS 300 207-1 Ed.1 (1994), Integrated Services 450 Digital Network (ISDN); Diversion supplementary services". 452 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 453 Requirement Levels", BCP 14, RFC 2119, March 1997. 455 [RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976, 456 October 2000. 458 [RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol 459 for Telephones (SIP-T): Context and Architectures", 460 BCP 63, RFC 3372, September 2002. 462 [RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer 463 Method", RFC 3515, April 2003. 465 [RFC3324] Watson, M., "Short Term Requirements for Network Asserted 466 Identity", RFC 3324, November 2002. 468 Authors' Addresses 470 Alan Johnston 471 Avaya 472 St. Louis, MO 63124 474 Email: alan.b.johnston@gmail.com 476 Laura Liess 477 Deutsche Telekom AG 479 Email: laura.liess.dt@gmail.com