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'ITU.Q931.1998' ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) == Outdated reference: A later version (-17) exists of draft-ietf-cuss-sip-uui-10 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC WG M. Garcia-Martin 3 Internet-Draft Ericsson 4 Intended status: Standards Track S. Veikkolainen 5 Expires: December 05, 2013 Nokia 6 June 03, 2013 8 Session Description Protocol (SDP) Extension For Setting Audio and Video 9 Media Streams Over Circuit-Switched Bearers In The Public Switched 10 Telephone Network (PSTN) 11 draft-ietf-mmusic-sdp-cs-19 13 Abstract 15 This memo describes use cases, requirements, and protocol extensions 16 for using the Session Description Protocol (SDP) Offer/Answer model 17 for establishing audio and video media streams over circuit-switched 18 bearers in the Public Switched Telephone Network (PSTN). 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 December 05, 2013. 37 Copyright Notice 39 Copyright (c) 2013 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. Conventions Used in This Document . . . . . . . . . . . . . . 4 56 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 57 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5 58 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 5 59 5. Protocol Description . . . . . . . . . . . . . . . . . . . . 7 60 5.1. Level of Compliance . . . . . . . . . . . . . . . . . . . 7 61 5.2. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 7 62 5.2.1. Connection Data . . . . . . . . . . . . . . . . . . . 7 63 5.2.2. Media Descriptions . . . . . . . . . . . . . . . . . 8 64 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 10 65 5.2.3.1. The "cs-correlation" attribute . . . . . . . . . 11 66 5.2.3.2. Caller-ID Correlation Mechanism . . . . . . . . . 11 67 5.2.3.3. User-User Information Element Correlation 68 Mechanism . . . . . . . . . . . . . . . . . . . . 12 69 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . 14 70 5.2.3.5. Extensions to correlation mechanisms . . . . . . 15 71 5.3. Negotiating the correlation mechanisms . . . . . . . . . 15 72 5.3.1. Determining the Direction of the Circuit-Switched 73 Bearer Setup . . . . . . . . . . . . . . . . . . . . 15 74 5.3.2. Populating the cs-correlation attribute . . . . . . . 16 75 5.3.3. Considerations on correlations . . . . . . . . . . . 17 76 5.4. Considerations for Usage of Existing SDP . . . . . . . . 18 77 5.4.1. Originator of the Session . . . . . . . . . . . . . . 18 78 5.4.2. Contact information . . . . . . . . . . . . . . . . . 18 79 5.5. Considerations for Usage of Third Party Call Control 80 (3PCC) . . . . . . . . . . . . . . . . . . . . . . . . . 18 81 5.6. Offer/Answer mode extensions . . . . . . . . . . . . . . 19 82 5.6.1. Generating the Initial Offer . . . . . . . . . . . . 19 83 5.6.2. Generating the Answer . . . . . . . . . . . . . . . . 21 84 5.6.3. Offerer processing the Answer . . . . . . . . . . . . 24 85 5.6.4. Modifying the session . . . . . . . . . . . . . . . . 26 86 5.7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 26 87 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 27 88 6.1. Single PSTN audio stream . . . . . . . . . . . . . . . . 27 89 6.2. Advanced SDP example: Circuit-Switched Audio and 90 Video Streams . . . . . . . . . . . . . . . . . . . . . . 29 91 7. Security Considerations . . . . . . . . . . . . . . . . . . . 31 92 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 93 8.1. Registration of new cs-correlation SDP attribute . . . . 32 94 8.2. Registration of a new "nettype" value . . . . . . . . . . 33 95 8.3. Registration of new "addrtype" values . . . . . . . . . . 33 96 8.4. Registration of a new "proto" value . . . . . . . . . . . 33 98 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 99 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 100 10.1. Normative References . . . . . . . . . . . . . . . . . . 34 101 10.2. Informative References . . . . . . . . . . . . . . . . . 34 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 104 1. Introduction 106 The Session Description Protocol (SDP) [RFC4566] is intended for 107 describing multimedia sessions for the purposes of session 108 announcement, session invitation, and other forms of multimedia 109 session initiation. SDP is most commonly used for describing media 110 streams that are transported over the Real-Time Transport Protocol 111 (RTP) [RFC3550], using the profiles for audio and video media defined 112 in RTP Profile for Audio and Video Conferences with Minimal Control 113 [RFC3551]. 115 However, SDP can be used to describe other transport protocols than 116 RTP. Previous work includes SDP conventions for describing ATM 117 bearer connections [RFC3108] and the Message Session Relay Protocol 118 [RFC4975]. 120 SDP is commonly carried in Session Initiation Protocol (SIP) 121 [RFC3261] messages in order to agree on a common media description 122 among the endpoints. An Offer/Answer Model with Session Description 123 Protocol (SDP) [RFC3264] defines a framework by which two endpoints 124 can exchange SDP media descriptions and come to an agreement as to 125 which media streams should be used, along with the media related 126 parameters. 128 In some scenarios it might be desirable to establish the media stream 129 over a circuit-switched bearer connection even if the signaling for 130 the session is carried over an IP bearer. An example of such a 131 scenario is illustrated with two mobile devices capable of both 132 circuit-switched and packet-switched communication over a low- 133 bandwidth radio bearer. The radio bearer may not be suitable for 134 carrying real-time audio or video media, and using a circuit-switched 135 bearer would offer a better perceived quality of service. So, 136 according to this scenario, SDP and its higher layer session control 137 protocol (e.g., the Session Initiation Protocol (SIP) [RFC3261]) are 138 used over regular IP connectivity, while the audio or video is 139 received through the classical circuit-switched bearer. 141 Setting up a signaling relationship in the IP domain instead of just 142 setting up a circuit-switched call offers also the possibility of 143 negotiating in the same session other IP based media that is not 144 sensitive to jitter and delay, for example, text messaging or 145 presence information. 147 At a later point in time the mobile device might move to an area 148 where a high-bandwidth packet-switched bearer, for example a Wireless 149 Local Area Network (WLAN) connection, is available. At this point 150 the mobile device may perform a handover and move the audio or video 151 media streams over to the high-speed bearer. This implies a new 152 exchange of SDP Offer/Answer that leads to a re-negotiation of the 153 media streams. 155 Other use cases exist. For example, an endpoint might have at its 156 disposal circuit-switched and packet-switched connectivity, but the 157 same audio or video codecs are not feasible for both access networks. 158 For example, the circuit-switched audio or video stream supports 159 narrow-bandwidth codecs, while the packet-switched access allows any 160 other audio or video codec implemented in the endpoint. In this 161 case, it might be beneficial for the endpoint to describe different 162 codecs for each access type and get an agreement on the bearer 163 together with the remote endpoint. 165 There are additional use cases related to third party call control 166 where the session setup time is improved when the circuit-switched 167 bearer in the PSTN is described together with one or more codecs. 169 The rest of the document is structured as follows: Section 2 provides 170 the document conventions, Section 3 introduces the requirements, 171 Section 4 presents an overview of the proposed solutions, and 172 Section 5 contains the protocol description. Section 6 provides an 173 example of descriptions of circuit-switched audio or video streams in 174 SDP. Section 7 and Section 8 contain the Security and IANA 175 considerations, respectively. 177 2. Conventions Used in This Document 179 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 180 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 181 "OPTIONAL" in this document are to be interpreted as described in BCP 182 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant 183 implementations. 185 3. Requirements 187 This section presents the general requirements that are specific for 188 the audio or video media streams over circuit-switched bearers. 190 REQ-1: A mechanism for endpoints to negotiate and agree on an audio 191 or video media stream established over a circuit-switched bearer 192 MUST be available. 194 REQ-2: The mechanism MUST allow the endpoints to combine circuit- 195 switched audio or video media streams with other complementary 196 media streams, for example, text messaging. 198 REQ-3: The mechanism MUST allow the endpoint to negotiate the 199 direction of the circuit-switched bearer, i.e., which endpoint is 200 active when initiating the circuit-switched bearer. 202 REQ-4: The mechanism MUST be independent of the type of the circuit- 203 switched access (e.g., Integrated Services Digital Network (ISDN), 204 Global System for Mobile Communication (GSM), etc.) 206 REQ-5: There MUST be a mechanism that helps an endpoint to correlate 207 an incoming circuit-switched bearer with the one negotiated in 208 SDP, as opposed to another incoming call that is not related to 209 that. 211 REQ-6: It MUST be possible for endpoints to advertise different 212 lists of audio or video codecs in the circuit-switched audio or 213 video stream from those used in a packet-switched audio or video 214 stream. 216 REQ-7: It MUST be possible for endpoints to not advertise the list 217 of available codecs for circuit-switched audio or video streams. 219 4. Overview of Operation 221 The mechanism defined in this memo extends SDP and allows describing 222 an audio or video media stream established over a circuit-switched 223 bearer. A new network type ("PSTN") and a new protocol type ("PSTN") 224 are defined for the "c=" and "m=" lines to be able to describe a 225 media stream over a circuit-switched bearer. These SDP extensions 226 are described in Section 5.2. Since circuit-switched bearers are 227 connection-oriented media streams, the mechanism re-uses the 228 connection-oriented extensions defined in RFC 4145 [RFC4145] to 229 negotiate the active and passive sides of a connection setup. This 230 is further described in Section 5.3.1. 232 4.1. Example Call Flow 234 Consider the example presented in Figure 1. In this example, 235 Endpoint A is located in an environment where it has access to both 236 IP and circuit-switched bearers for communicating with other 237 endpoints. Endpoint A decides that the circuit-switched bearer 238 offers a better perceived quality of service for voice, and issues an 239 SDP Offer containing the description of an audio media stream over 240 circuit-switched bearer. 242 Endpoint A Endpoint B 243 | (1) SDP Offer (PSTN audio) | 244 |----------------------------------->| 245 | | 246 | (2) SDP Answer (PSTN audio) | 247 |<-----------------------------------| 248 | | 249 | PSTN call setup | 250 |<-----------------------------------| 251 | | 252 | | 253 |<===== media over PSTN bearer =====>| 254 | | 256 Figure 1: Example Flow 258 Endpoitn B receives the SDP offer and determines that it is located 259 in an environment where the IP based bearer is not suitable for real- 260 time audio media. However, Endpoint B also has PSTN circuit-switched 261 bearer available for audio. Endpoint B generates an SDP answer 262 containing a description of the audio media stream over a circuit- 263 switched bearer. 265 During the offer-answer exchange Endpoints A and B also agree the 266 direction in which the circuit-switched bearer should be established. 267 In this example, Endpoint B becomes the active party, in other words, 268 it establishes the circuit-switched call to the other endpoint. The 269 Offer/Answer exchange contains identifiers or references that can be 270 used on the circuit-switched network for addressing the other 271 endpoint, as well as information that is used to determine that the 272 incoming circuit-switched bearer establishment is related to the 273 ongoing session between the two endpoints. 275 Endpoint B establishes a circuit-switched bearer towards Endpoint A 276 using whatever mechanisms are defined for the network type in 277 question. When receiving the incoming circuit-switched connection 278 attempt, Endpoint A is able to determine that the attempt is related 279 to the session it is just establishing with B. 281 Endpoint A accepts the circuit-switched connection; the circuit- 282 switched bearer setup is completed. The two endpoints can now use 283 the circuit-switched connection for two-way audio media. 285 If, for some reason, Endpoint B would like to reject the offered 286 stream, it would set the port number of the specific stream to zero, 287 as specified in RFC3264 [RFC3264]. Also, if B does not understand 288 some of the SDP attributes specified in this document, it would 289 ignore them, as specified in RFC4566 [RFC4566]. 291 5. Protocol Description 293 5.1. Level of Compliance 295 Implementations according to this specification MUST implement the 296 SDP extensions described in Section 5.2, and MUST implement the 297 considerations discussed in Section 5.3, Section 5.4 and Section 5.6. 299 5.2. Extensions to SDP 301 This section provides the syntax and semantics of the extensions 302 required for providing a description of audio or video media streams 303 over circuit-switched bearers in SDP. 305 5.2.1. Connection Data 307 According to SDP [RFC4566], the connection data line in SDP has the 308 following syntax: 310 c= 312 where indicates the network type, indicates the 313 address type, and the is the connection address, 314 which is dependent on the address type. 316 At the moment, the only network type defined is "IN", which indicates 317 Internet network type. The address types "IP4" and "IP6" indicate 318 the type of IP addresses. 320 This memo defines a new network type for describing a circuit- 321 switched bearer network type in the PSTN. The mnemonic "PSTN" is 322 used for this network type. 324 For the address type, we initially consider the possibility of 325 describing E.164 telephone numbers. We define a new "E164" address 326 type to be used within the context of a "PSTN" network type. The 327 "E164" address type indicates that the connection address contains an 328 E.164 number represented according to the ITU-T E.164 [ITU.E164.1991] 329 recommendation. 331 It is a common convention that an international E.164 number contains 332 a leading '+' sign. For consistency's sake, we also require the 333 E.164 telephone is prepended with a '+', even if that is not 334 necessary for routing of the call in the PSTN network. 336 There are cases, though, when the endpoint is merely aware of a 337 circuit-switched bearer, without having further information about the 338 address type or the E.164 number allocated to it. In these cases a 339 dash ("-") is used to indicate an unknown address type or connection 340 address. This makes the connection data line be according to the SDP 341 syntax. 343 Please note that these "E164" and "-" address types defined in this 344 memo are exclusively defined to be used in conjunction with the 345 "PSTN" network type in accordance with [RFC4566]. Usage of "E164" or 346 "-" address types in conjunction with other network types may be 347 defined elsewhere. 349 This memo exclusively uses the international representation of E.164 350 numbers, i.e., those including a country code and, as described above 351 prepended with a '+' sign. Implementations conforming to this 352 specification and using the "E164" address type together with the 353 "PSTN" network type MUST use the 'global-number-digits' construction 354 specified in RFC 3966 [RFC3966] for representing international E.164 355 numbers. This representation requires the presence of the '+' sign, 356 and additionally allows for the presence of one or more 'visual- 357 separator' constructions for easier human readability (see 358 Section 5.7). 360 Note that and/or MUST NOT be omitted 361 when unknown since this would violate basic syntax of SDP [RFC4566]. 362 In such cases, they MUST be set to a "-". 364 The following are examples of the extension to the connection data 365 line: 367 c=PSTN E164 +441134960123 369 c=PSTN - - 371 When the is E164, the connection address is defined as 372 follows: 374 o an international E.164 number 376 When the is "-", the connection address is defined as 377 follows: 379 o the value "-", signifying that the address is unknown 381 o any value resulting from the production rule of connection-address 382 in RFC4566 [RFC4566], but in all cases any value encountered will 383 be ignored. 385 5.2.2. Media Descriptions 386 According to SDP [RFC4566], the media description line in SDP has the 387 following syntax: 389 m= ... 391 The subfield carries the media type. For establishing an 392 audio bearer, the existing "audio" media type is used. For 393 establishing a video bearer, the existing "video" media type is used. 395 The subfield is the transport port to which the media stream 396 is sent. Circuit-switched access lacks the concept of a port number, 397 and therefore the subfield does not carry any meaningful 398 value. In order to be compliant with SDP syntax, implementations 399 SHOULD set the subfield to the discard port value "9" and MUST 400 ignore it on reception. 402 According to RFC 3264 [RFC3264], a port number of zero in the offer 403 of a unicast stream indicates that the stream is offered but must not 404 be used. If a port number of zero is present in the answer of a 405 unicast stream, it indicates that the stream is rejected. These 406 rules are still valid when the media line in SDP represents a 407 circuit-switched bearer. 409 The subfield is the transport protocol. The circuit-switched 410 bearer uses whatever transport protocol it has available. This 411 subfield SHOULD be set to the mnemonic "PSTN" to be syntactically 412 correct with SDP [RFC4566] and to indicate the usage of circuit- 413 switched protocols in the PSTN. 415 The subfield is the media format description. In the classical 416 usage of SDP to describe RTP-based media streams, when the 417 subfield is set to "RTP/AVP" or "RTP/SAVP", the subfield 418 contains the payload types as defined in the RTP audio profile 419 [RFC3551]. 421 When "RTP/AVP" is used in the field, the subfield 422 contains the RTP payload type numbers. We use the subfield to 423 indicate the list of available codecs over the circuit-switched 424 bearer, by re-using the conventions and payload type numbers defined 425 for RTP/AVP. The RTP audio and video media types, when applied to 426 PSTN circuit-switched bearers, represent merely an audio or video 427 codec. If the endpoint is able to determine the list of available 428 codecs for circuit-switched media streams, it MUST use the 429 corresponding payload type numbers in the subfield. 431 In some cases, the endpoint is not able to determine the list of 432 available codecs for circuit-switched media streams. In this case, 433 in order to be syntactically compliant with SDP [RFC4566], the 434 endpoint MUST include a single dash ("-") in the subfield. 436 As per RFC 4566 [RFC4566], the media format descriptions are listed 437 in priority order. 439 Examples of media descriptions for circuit-switched audio streams 440 are: 442 m=audio 9 PSTN 3 0 8 444 m=audio 9 PSTN - 446 Similarly, an example of a media description for circuit-switched 447 video stream is: 449 m=video 9 PSTN 34 451 m=video 9 PSTN - 453 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 455 The endpoints should be able to correlate the circuit-switched bearer 456 with the session negotiated with SDP in order to avoid ringing for an 457 incoming circuit-switched bearer that is related to the session 458 controlled with SDP (and SIP). 460 Several alternatives exist for performing this correlation. This 461 memo provides three mutually non-exclusive correlation mechanisms. 462 Other correlation mechanisms may exist, and their usage will be 463 specified when need arises. All mechanisms share the same principle: 464 some unique information is sent in the SDP and in the circuit- 465 switched signaling protocol. If these pieces of information match, 466 then the circuit-switched bearer is part of the session described in 467 the SDP exchange. Otherwise, there is no guarantee that the circuit- 468 switched bearer is related to such session. 470 The first mechanism is based on the exchange of PSTN caller-ID 471 between the endpoints. The caller-ID is also available as the 472 Calling Party ID in the circuit-switched signaling. 474 The second mechanism is based on the inclusion in SDP of a value that 475 is also sent in the User-to-User Information Element that is part of 476 the bearer setup signaling in the PSTN. 478 The third mechanism is based on sending in SDP a string that 479 represents Dual Tone MultiFrequency (DTMF) digits that will be later 480 sent right after the circuit-switched bearer is established. 481 Implementations MAY use any of these mechanisms and MAY use two or 482 more mechanisms simultaneously. 484 5.2.3.1. The "cs-correlation" attribute 486 In order to provide support for the correlation mechanisms, we define 487 a new media-level SDP attribute called "cs-correlation". This "cs- 488 correlation" attribute can include any of the "callerid", "uuie", or 489 "dtmf" subfields, which specify additional information required by 490 the Caller-ID, User to User Information, or DTMF correlation 491 mechanisms, respectively. The list of correlation mechanisms may be 492 extended by other specifications, see Section 5.2.3.5 for more 493 details. There MUST be at most one "cs-correlation" attribute per 494 media description. 496 The following sections provide more detailed information of these 497 subfields. 499 The values "callerid", "uuie" and "dtmf" refer to the correlation 500 mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and 501 Section 5.2.3.4, respectively. The formal Augmented Backus-Naur 502 Format (ABNF) syntax of the "cs-correlation" attribute is presented 503 in Section 5.7. 505 5.2.3.2. Caller-ID Correlation Mechanism 507 The Caller-ID correlation mechanisms consists of an exchange of the 508 calling party number as an international E.164 number in SDP, 509 followed by the availability of the Calling Party Number information 510 element in the call setup signaling of the circuit switched 511 connection. If both pieces of information match, the circuit- 512 switched bearer is correlated to the session described in SDP. 514 Example of inclusion of an international E.164 number in the "cs- 515 correlation" attribute is: 517 a=cs-correlation:callerid:+441134960123 519 The presence of the "callerid" subfield indicates that the endpoint 520 supports use of the calling party number as a means of correlating a 521 PSTN call with the session being negotiated. The "callerid" subfield 522 MAY be accompanied by the international E.164 number of the party 523 inserting the parameter. 525 Note that there are no guarantees that this correlation mechanism 526 works or is even available, due a number of problems: 528 o The endpoint might not be aware of its own E.164 number, in which 529 case it cannot populate the SDP appropriately. 531 o The Calling Party Number information element in the circuit- 532 switched signaling might not be available, e.g., due to policy 533 restrictions of the network operator or caller restriction due to 534 privacy. 536 o The Calling Party Number information element in the circuit- 537 switched signaling might be available, but the digit 538 representation of the E.164 number might differ from the one 539 expressed in the SDP, due to, e.g., lack of of country code. To 540 mitigate this problem implementations should consider only some of 541 the rightmost digits from the E.164 number for correlation. For 542 example, the numbers +44-113-496-0123 and 0113-496-0123 could be 543 considered as the same number. This is also the behavior of some 544 cellular phones, which correlate the incoming calling party with a 545 number stored in the phone book, for the purpose of displaying the 546 caller's name. Please refer to ITU-T E.164 reccommendation 547 [ITU.E164.1991] for consideration of the relevant number of digits 548 to consider. 550 5.2.3.3. User-User Information Element Correlation Mechanism 552 A second correlation mechanism is based on including in SDP a string 553 that represents the User-User Information Element that is part of the 554 call setup signaling of the circuit-switched bearer. The User-User 555 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 556 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 557 Element has a maximum size of 35 or 131 octets, depending on the 558 actual message of the PSTN protocol where it is included and the 559 network settings. 561 The mechanism works as follows: An endpoint creates a User-User 562 Information Element, according to the requirements of the call setup 563 signaling protocol. The same value is included in the SDP offer or 564 SDP answer, in the "uuie" subfield of the "cs-correlation" attribute. 565 When the SDP Offer/Answer exchange is completed, each endpoint has 566 become aware of the value that will be used in the User-User 567 Information Element of the call setup message of the PSTN protocol. 568 The endpoint that initiates the call setup attempt includes this 569 value in the User-User Information Element. The recipient of the 570 call setup attempt can extract the User-User Information Element and 571 correlate it with the value previously received in the SDP. If both 572 values match, then the call setup attempt corresponds to that 573 indicated in the SDP. 575 According to ITU-T Q.931 [ITU.Q931.1998], the User-User Information 576 Element (UUIE) identifier is composed of a first octet identifying 577 this as a User-User Information Element, a second octet containing 578 the Length of the user-user contents, a third octet containing a 579 Protocol Discriminator, and a value of up to 32 or 128 octets 580 (depending on network settings) containing the actual User 581 Information (see Figure 4-36 in ITU-T Q.931). The first two octets 582 of the UUIE MUST NOT be used for correlation, only the octets 583 carrying the Protocol Discriminator and the User Information value 584 are input to the creation of the value of the "uuie" subfield in the 585 "cs-correlation" attribute. Therefore, the value of the "uuie" 586 subfield in the "cs-correlation" attribute MUST start with the 587 Protocol Discriminator octet, followed by the User Information 588 octets. The value of the Protocol Discriminator octet is not 589 specified in this document; it is expected that organizations using 590 this technology will allocate a suitable value for the Protocol 591 Discriminator. 593 Once the binary value of the "uuie" subfield in the "cs-correlation" 594 attribute is created, it MUST be base 16 (also known as "hex") 595 encoded before it is inserted in SDP. Please refer to RFC 4648 596 [RFC4648] for a detailed description of base 16 encoding. The 597 resulting encoded value needs to have an even number of hexadecimal 598 digits, and MUST be considered invalid if it has an odd number. 600 Note that the encoding of the "uuie" subfield of the "cs- 601 correlation" attribute is largely inspired by the encoding of the 602 same value in the User-to-User header field in SIP, according to 603 the document "A Mechanism for Transporting User to User Call 604 Control Information in SIP" [I-D.ietf-cuss-sip-uui]. 606 As an example, an endpoint willing to send a UUIE containing a 607 protocol discriminator with the hexadecimal value of %x56 and an 608 hexadecimal User Information value of %xA390F3D2B7310023 would 609 include a "cs-correlation" attribute line as follows: 611 a=cs-correlation:uuie:56A390F3D2B7310023 613 Note that the value of the User-User Information Element is 614 considered as an opaque string and only used for correlation 615 purposes. Typically call signaling protocols impose requirements on 616 the creation of User-User Information Element for end-user protocol 617 exchange. The details regarding the generation of the User-User 618 Information Element are outside the scope of this specification. 620 Please note that there are no guarantees that this correlation 621 mechanism works. On one side, policy restrictions might not make the 622 User-User information available end to end in the PSTN. On the other 623 hand, the generation of the User-User Information Element is 624 controlled by the PSTN circuit-switched call protocol, which might 625 not offer enough freedom for generating different values from one 626 endpoint to another one, or from one call to another in the same 627 endpoint. This might result in the same value of the User-User 628 Information Element for all calls. 630 5.2.3.4. DTMF Correlation Mechanism 632 We introduce a third mechanism for correlating the circuit-switched 633 bearer with the session described with SDP. This is based on 634 agreeing on a sequence of digits that are negotiated in the SDP Offer 635 /Answer exchange and sent as Dual Tone Multifrequency (DTMF) ITU-T 636 Recommendation Q.23 [ITU.Q23.1988] tones over the circuit-switched 637 bearer once this bearer is established. If the DTMF digit sequence 638 received through the circuit-switched bearer matches the digit string 639 negotiated in the SDP, the circuit-switched bearer is correlated with 640 the session described in the SDP. The mechanism is similar to many 641 voice conferencing systems which require the user to enter a PIN code 642 using DTMF tones in order to be accepted in a voice conference. 644 The mechanism works as follows: An endpoint selects a DTMF digit 645 sequence. The same sequence is included in the SDP offer or SDP 646 answer, in a "dtmf" subfield of the "cs-correlation" attribute. When 647 the SDP Offer/Answer exchange is completed, each endpoint has become 648 aware of the DTMF sequence that will be sent right after the circuit- 649 switched bearer is set up. The endpoint that initiates the call 650 setup attempt sends the DTMF digits according to the procedures 651 defined for the circuit-switched bearer technology used. The 652 recipient (passive side of the bearer setup) of the call setup 653 attempt collects the digits and compares them with the value 654 previously received in the SDP. If the digits match, then the call 655 setup attempt corresponds to that indicated in the SDP. 657 Implementations are advised to select a number of DTMF digits that 658 provide enough assurance that the call is related, but on the 659 other hand do not prolong the bearer setup time unnecessarily. A 660 number of 5 to 10 digits is a good compromise. 662 As an example, an endpoint willing to send DTMF tone sequence "14D*3" 663 would include a "cs-correlation" attribute line as follows: 665 a=cs-correlation:dtmf:14D*3 667 If the endpoints successfully agree on the usage of the DTMF digit 668 correlation mechanism, but the passive side does not receive any DTMF 669 digits after successful circuit-switched bearer setup, or receives a 670 set of DTMF digits that do not match the value of the "dtmf" 671 attribute (including receiving too many digits), the passive side 672 SHOULD consider that this DTMF mechanism has failed to correlate the 673 incoming call. 675 5.2.3.5. Extensions to correlation mechanisms 677 New values for the "cs-correlation" attribute may be specified. The 678 registration policy for new values is "Specification Required", see 679 Section 8. Any such specification MUST include a description of how 680 SDP Offer/Answer mechanism is used to negotiate the use of the new 681 values, taking into account how endpoints determine which side will 682 become active or passive (see Section 5.3 for more details). 684 If, during the Offer/Answer negotiation, either endpoint encounters 685 an unknown value in the "cs-correlation" attribute, it MUST consider 686 that mechanism as unsupported, and MUST NOT include that value in 687 subsequent Offer/Answer negotiation. 689 5.3. Negotiating the correlation mechanisms 691 The three correlation mechanisms presented above (based on called 692 party number, User-User Information Element and DTMF digit sending) 693 are non-exclusive, and can be used independently of each other. In 694 order to know how to populate the "cs-correlation" attribute, the 695 endpoints need to agree which endpoint will become the active party, 696 i.e., the one that will set up the circuit-switched bearer. 698 5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup 700 In order to avoid a situation where both endpoints attempt to 701 initiate a connection simultaneously, the direction in which the 702 circuit-switched bearer is set up MUST be negotiated during the Offer 703 /Answer exchange. 705 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 706 agree which endpoint acts as the active endpoint when initiating a 707 TCP connection. While RFC 4145 [RFC4145] was originally designed for 708 establishing TCP connections, it can be easily extrapolated to the 709 connection establishment of circuit-switched bearers. This 710 specification uses the concepts specified in RFC 4145 [RFC4145] for 711 agreeing on the direction of establishment of a circuit-switched 712 bearer. 714 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 715 "connection". The "setup" attribute indicates which of the endpoints 716 should initiate the connection establishment of the PSTN circuit- 717 switched bearer. Four values are defined in Section 4 of RFC 4145 718 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 719 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 720 attribute. 722 The "connection" attribute indicates whether a new connection is 723 needed or an existing connection is reused. The attribute can take 724 the values "new" or "existing". Please refer to Section 5 of RFC 725 4145 [RFC4145] for a detailed description of this attribute. 727 Implementations according to this specification MUST support the 728 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 729 but applied to circuit-switched bearers in the PSTN. 731 We define the active party as the one that initiates the circuit- 732 switched bearer after the Offer/Answer exchange. The passive party 733 is the one receiving the circuit-switched bearer. Either party may 734 indicate its desire to become the active or passive party during the 735 Offer/Answer exchange using the procedures described in Section 5.6. 737 5.3.2. Populating the cs-correlation attribute 739 By defining values for the subfields in the "a=cs-correlation" 740 attribute, the endpoint indicates that it is willing to become the 741 active party, and that it can use those values in the Calling party 742 number, User-User Information Element, or as DTMF tones during the 743 circuit-switched bearer setup. 745 Thus, the following rules apply: 747 An endpoint that can only become the active party in the circuit- 748 switched bearer setup MUST include all correlation mechanisms it 749 supports in the "a=cs-correlation" attribute, and MUST also 750 specify values for the subfields. 752 An endpoint that can only become the passive party in the circuit- 753 switched bearer setup MUST include all correlation mechanisms it 754 supports in the "a=cs-correlation" attribute, but MUST NOT specify 755 values for the subfields. 757 An endpoint that is willing to become either the active or passive 758 party (by including the "a=setup:actpass" attribute in the Offer), 759 MUST include all correlation mechanisms it supports in the "a=cs- 760 correlation" attribute, and MUST also specify values for the 761 subfields. 763 5.3.3. Considerations on correlations 765 Passive endpoints should expect an incoming CS call for setting up 766 the audio bearer. Passive endpoints MAY suppress the incoming CS 767 alert during a certain time periods. Additional restrictions can be 768 applied, such as the passive endpoint not alerting incoming calls 769 originated from the number that was observed during the offer/answer 770 negotiation. 772 Note that it cannot be guaranteed that any given correlation 773 mechanism will succeed even if the usage of those was agreed 774 beforehand. This is due to the fact that the correlation mechanisms 775 require support from the circuit-switched bearer technology used. 777 Therefore, even a single positive indication using any of these 778 mechanisms SHOULD be interpreted by the passive endpoint so that the 779 circuit-switched bearer establishment is related to the ongoing 780 session, even if the other correlation mechanisms fail. 782 If, after negotiating one or more correlation mechanisms in the SDP 783 offer/answer exchange, an endpoint receives a circuit-switched bearer 784 with no correlation information present, the endpoint has two 785 choices: it can either treat the call as unrelated, or treat the call 786 as related to the ongoing session in the IP domain. 788 An endpoint may for example specify a time window after SDP offer/ 789 answer exchange during which received calls are treated as correlated 790 even if the signaling in the circuit-switched domain does not carry 791 any correlation information. In this case, there is a chance that 792 the call is erroneously treated as related to the ongoing session. 794 An endpoint may also choose to always treat an incoming call as 795 unrelated if the signaling in the circuit-switched domain does not 796 carry any correlation information. In this case, there is a chance 797 that the call is erroneously treated as unrelated. 799 Since, in these cases, no correlation information can be deduced from 800 the signaling, it is up to the implementation to decide how to 801 behave. One option is also to let the user decide whether to accept 802 the call as related, or to treat the call as unrelated. 804 5.4. Considerations for Usage of Existing SDP 806 5.4.1. Originator of the Session 808 According to SDP [RFC4566], the origin line in SDP has the following 809 syntax: 811 o= 812 814 Of interest here are the and fields, which 815 indicate the type of network and type of address, respectively. 816 Typically, this field carries the IP address of the originator of the 817 session. Even if the SDP was used to negotiate an audio or video 818 media stream transported over a circuit-switched bearer, the 819 originator is using SDP over an IP bearer. Therefore, and 820 fields in the "o=" line should be populated with the IP 821 address identifying the source of the signaling. 823 5.4.2. Contact information 825 SDP [RFC4566] defines the "p=" line which may include the phone 826 number of the person responsible for the conference. Even though 827 this line can carry a phone number, it is not suited for the purpose 828 of defining a connection address for the media. Therefore, we have 829 selected to define the PSTN specific connection addresses in the "c=" 830 line. 832 5.5. Considerations for Usage of Third Party Call Control (3PCC) 834 Best Current Practices for Third Party Call Control (3pcc) in the 835 Session Initiation Protocol (SIP) [RFC3725] outlines several flows 836 which are possible in third party call control scenarios and 837 recommends some flows for specific situations. 839 One of the assumptions in [RFC3725] is that an SDP Offer may include 840 a "black hole" connection address, which has the property that 841 packets sent to it will never leave the host which sent them. For 842 IPv4, this "black hole" connection address is 0.0.0.0, or a domain 843 name within the .invalid DNS top level domain. 845 When using an E.164 address scheme in the context of third-party call 846 control, when the User Agent needs to indicate an unknown phone 847 number, it MUST populate the of the SDP "c=" line with a 848 "-" string. 850 Note that this may result in the recipient of the initial offer 851 rejecting such offer if the recipient of the offer was not aware 852 of its own E.164 number. Consequently it will not be possible to 853 establish a circuit-switched bearer, since neither party is aware 854 of their E.164 number. 856 5.6. Offer/Answer mode extensions 858 In this section, we define extensions to the Offer/Answer model 859 defined in The Offer/Answer Model in SDP [RFC3264] to allow for PSTN 860 addresses to be used with the Offer/Answer model. 862 5.6.1. Generating the Initial Offer 864 The Offerer, wishing to use PSTN audio or video stream, MUST populate 865 the "c=" and "m=" lines as follows. 867 The endpoint MUST set the in the "c=" line to "PSTN", and 868 the to "E164". Furthermore, the endpoint SHOULD set the 869 field to its own international E.164 number 870 (with a leading "+"). If the endpoint is not aware of its own E.164 871 number, it MUST set the to "-". 873 In the "m=" line, the endpoint MUST set the subfield to 874 "audio" or "video", depending on the media type, and the 875 subfield to "PSTN". The subfield SHOULD be set to "9" (the 876 discard port). 878 The subfield carries the payload type number(s) the endpoint is 879 wishing to use. Payload type numbers in this case refer to the 880 codecs that the endpoint wishes to use on the PSTN media stream. For 881 example, if the endpoint wishes to use the GSM codec, it would add 882 payload type number 3 in the list of codecs. The list of payload 883 types MUST only contain those codecs the endpoint is able to use on 884 the PSTN bearer. In case the endpoint is not aware of the codecs 885 available for the circuit-switched media streams, it MUST include a 886 dash ("-") in the subfield. 888 The mapping table of static payload types numbers to payload types is 889 initally specified in [RFC3551] and maintained by IANA. For dynamic 890 payload types, the endpoint MUST define the set of valid encoding 891 names and related parameters using the "a=rtpmap" attribute line. 892 See Section 6 of SDP [RFC4566] for details. 894 When generating the Offer, if the Offerer supports any of the 895 correlation mechanisms defined in this memo, it MUST include an 896 attribute line "a=cs-correlation" in the SDP offer. The Offerer MUST 897 NOT include more than one "cs-correlation" attribute per media 898 decription. The "a=cs-correlation" line contains an enumeration of 899 the correlation mechanisms supported by the Offerer, in the format of 900 subfields. 902 The current list of subfields include "callerid", "uuie" and "dtmf" 903 and they refer to the correlation mechanisms defined in 904 Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. 906 If the Offerer supports any of the correlation mechanisms defined in 907 this memo, and is willing to become the active party, the Offerer 908 MUST add the "callerid", "uuie", and/or "dtmf" subfields and MUST 909 specify values for those subfields. 911 o the international E.164 number as the value in the "callerid" 912 subfield, 914 o the contents of the User-User information element as the value of 915 the "uuie" subfield, and/or 917 o the DTMF tone string as the value of the "dtmf" subfield 919 If the Offerer is only able to become the passive party in the 920 circuit-switched bearer setup, it MUST add at least one of the 921 possible correlation mechanisms, but MUST NOT specify values for 922 those subfields. 924 For example, if the Offerer is willing to use the User-User 925 Information element and DTMF digit sending mechanisms, but can only 926 become the passive party, it includes the following lines in the SDP: 928 a=cs-correlation:uuie dtmf 930 a=setup:passive 932 If, on the other hand, the Offerer is willing to use the User-User 933 Information element and the DTMF correlation mechanisms, and is able 934 to become the active or passive side, it includes the following lines 935 in the SDP: 937 a=cs-correlation:uuie:56A390F3D2B7310023 dtmf:14D*3 938 a=setup:actpass 940 The negotiation of the value of the 'setup' attribute takes place as 941 defined in Section 4.1 of TCP-Based Media Transport in the SDP 942 [RFC4145]. 944 The Offerer states which role or roles it is willing to perform; and 945 the Answerer, taking the Offerer's willingness into consideration, 946 chooses which roles both endpoints will actually perform during the 947 circuit-switched bearer setup. 949 By 'active' endpoint, we refer to an endpoint that will establish the 950 circuit-switched bearer; and by 'passive' endpoint, we refer to an 951 endpoint that will receive a circuit-switched bearer. 953 If an Offerer does not know its international E.164 number, it MUST 954 set the 'a=setup' attribute to the value 'active'. If the Offerer 955 knows its international E.164 number, it SHOULD set the value to 956 either 'actpass' or 'passive'. 958 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 959 It indicates that the connection should not be established for the 960 time being. 962 The Offerer uses the "a=connection" attribute to decide whether a new 963 circuit-switched bearer is to be established or not. For the initial 964 Offer, the Offerer MUST use value 'new'. 966 5.6.2. Generating the Answer 968 If the Offer contained a circuit-switched audio or video stream, the 969 Answerer first determines whether it is able to accept and use such 970 streams. If the Answerer does not support or is not willing to use 971 circuit-switched media for the session, it MUST construct an Answer 972 where the port number for such media stream(s) is set to zero, 973 according to Section 6 of An Offer/Answer Model with the Session 974 Description Protocol (SDP) [RFC3264]. If the Answerer is willing to 975 use circuit-switched media for the session, it MUST ignore the 976 received port number (unless the port number is set to zero). 978 If the Offer included a "-" as the payload type number, it indicates 979 that the Offerer is not willing or able to define any specific 980 payload type. Most often, a "-" is expected to be used instead of 981 the payload type when the endpoint is not aware of or not willing to 982 define the codecs which will eventually be used on the circuit- 983 switched bearer. The circuit-switched signaling protocols have their 984 own means of negotiating or indicating the codecs, therefore an 985 Answerer SHOULD accept such Offers, and SHOULD set the payload type 986 to "-" also in the Answer. 988 If the Answerer explicitly wants to specify a codec for the circuit- 989 switched media, it MAY set the respective payload numbers in the 990 subfield in the answer. This behavior, however, is NOT 991 RECOMMENDED. 993 When receiving the Offer, the Answerer MUST determine whether it 994 becomes the active or passive party. 996 If the SDP in the Offer indicates that the Offerer is only able to 997 become the active party, the Answerer needs to determine whether it 998 is able to become the passive party. If this is not possible e.g. 999 due to the Answerer not knowing its international E.164 number, the 1000 Answerer MUST reject the circuit-switched media by setting the port 1001 number to zero on the Answer. If the Answerer is aware of its 1002 international E.164 number, it MUST include the "a=setup" attribute 1003 in the Answer and set it to value "passive" or "holdconn". The 1004 Answerer MUST also include its E.164 number on the "c=" line. 1006 If the SDP in the Offer indicates that the Offerer is only able to 1007 become the passive party, the Answerer MUST verify that the Offerer's 1008 E.164 number is included in the "c=" line of the Offer. If the 1009 number is included, the Answerer MUST include the "a=setup" attribute 1010 in the Answer and set it to value "active" or "holdconn". If the 1011 number is not included, or the recipient of the Offer is not willing 1012 to establish a connection the E.164 based on a priori knowledge of 1013 cost, or other reasons, call establishment is not possible, and the 1014 Answerer MUST reject the circuit-switched media by setting the port 1015 number to zero in the Answer. 1017 If the SDP in the Offer indicates that the Offerer is able to become 1018 either the active or passive party, the Answerer determines which 1019 role it will take. If the Offer includes an international E.164 1020 number in the "c=" line, the Answerer SHOULD become the active party. 1021 If the Answerer does not become the active party, and if the Answerer 1022 is aware of its E.164 number, it MUST become the passive party. If 1023 the Answerer does not become the active or the passive party, it MUST 1024 reject the circuit-switched media by setting the port number to zero 1025 in the Answer. 1027 For each media description where the Offer includes a "a=cs- 1028 correlation" attribute, the Answerer MUST select from the Offer those 1029 correlation mechanisms it supports, and include in the Answer one "a 1030 =cs-correlation" attribute line containing those mechanisms it is 1031 willing to use. The Answerer MUST only add one "a=cs-correlation" 1032 attribute in those media descriptions where also the Offer included a 1033 "a=cs-correlation" attribute. The Answerer MUST NOT add any 1034 mechanisms which were not included in the offer. If there are more 1035 than one "cs-correlation" attributes per media description in the 1036 Offer, the Answerer MUST discard all but the first for any media 1037 description. Also, the Answerer MUST discard all unknown "cs- 1038 correlation" attribute values. 1040 If the Answerer becomes the active party, it MUST add a value to any 1041 of the possible subfields. 1043 If the Answerer becomes the passive party, it MUST NOT add any values 1044 to the subfields in the "cs-correlation" attribute. 1046 After generating and sending the Answer, if the Answerer became the 1047 active party, it 1049 o MUST extract the E.164 number from the "c=" line of the Offer and 1050 MUST establish a circuit-switched bearer to that address. 1052 o if the SDP Answer contained a value for the "callerid" subfield, 1053 MUST set the Calling Party Number Information Element to that 1054 number, 1056 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1057 send the User-User Information element according to the rules 1058 defined for the circuit-switched technology used, and set the 1059 value of the Information Element to that received in the SDP 1060 Offer, 1062 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1063 send those DTMF digits according to the circuit-switched 1064 technology used. 1066 If, on the other hand, the Answerer became the passive party, it 1068 o MUST be prepared to receive a circuit-switched bearer, 1069 o if the Offer contained a value for the "callerid" subfield, MUST 1070 compare that value to the Calling Party Number Information Element 1071 of the circuit-switched bearer. If the received Calling Party 1072 Number Information Element matches the value of the "callerid" 1073 subfield, the call SHOULD be treated as correlated to the ongoing 1074 session. 1076 o if the Offer contained a value for the "dtmf" subfield, MUST be 1077 prepared to receive and collect DTMF digits once the circuit- 1078 switched bearer is set up. The Answerer MUST compare the received 1079 DTMF digits to the value of the "dtmf" subfield. If the received 1080 DTMF digits match the value of the "dtmf" subfield in the "cs- 1081 correlation" attribute, the call SHOULD be treated as correlated 1082 to the ongoing session. 1084 o if the Offer contained a value for the "uuie" subfield, MUST be 1085 prepared to receive a User-User Information element once the 1086 circuit-switched bearer is set up. The Answerer MUST compare the 1087 received UUI to the value of the "uuie" subfield. If the value of 1088 the received UUI matches the value of the "uuie" subfield, the 1089 call SHOULD be treated as correlated to the ongoing session. 1091 If the Answerer becomes the active party, generates an SDP answer, 1092 and then it finds out that the circuit-switched call cannot be 1093 established, then the Answerer MUST create a new SDP offer where 1094 circuit-switched stream is removed from the session (actually, by 1095 setting the corresponding port in the m= line to zero) and send it to 1096 its counterpart. This is to synchronize both parties (and potential 1097 intermediaries) on the state of the session. 1099 5.6.3. Offerer processing the Answer 1101 When receiving the Answer, if the SDP does not contain "a=cs- 1102 correlation" attribute line, the Offerer should take that as an 1103 indication that the other party does not support or is not willing to 1104 use the procedures defined in the document for this session, and MUST 1105 revert to normal processing of SDP. 1107 When receiving the Answer, the Offerer MUST first determine whether 1108 it becomes the active or passive party, as described in 1109 Section 5.3.1. 1111 If the Offerer becomes the active party, it 1113 o MUST extract the E.164 number from the "c=" line and MUST 1114 establish a circuit-switched bearer to that address. 1116 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1117 send the User-User Information element according to the rules 1118 defined for the circuit-switched technology used, and set the 1119 value of the Information Element to that received in the SDP 1120 Answer, 1122 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1123 send those DTMF digits according to the circuit-switched 1124 technology used. 1126 If the Offerer becomes the passive party, it 1128 o MUST be prepared to receive a circuit-switched bearer, 1130 o Note that if delivery of the Answer is delayed for some reason, 1131 the circuit-switched call attempt may arrive at the Offerer before 1132 the Answer has been processed. In this case, since the 1133 correlation mechanisms are negotiated as part of the Offer/Answer 1134 exchange, the Answerer cannot know whether or not the incoming 1135 circuit-switched call attempt is correlated with the session being 1136 negotiated, the Offerer SHOULD answer the circuit-switched call 1137 attempt only after it has received and processed the Answer. 1139 o If the Answer contained a value for the "dtmf" subfield, the 1140 Offerer MUST be prepared to receive and collect DTMF digits once 1141 the circuit-switched bearer is set up. The Offerer SHOULD compare 1142 the received DTMF digits to the value of the "dtmf" subfield. If 1143 the received DTMF digits match the value of the "dtmf" subfield in 1144 the "cs-correlation" attribute, the call SHOULD be treated as 1145 correlated to the ongoing session. 1147 o If the Answer contained a value for the "uuie" subfield, the 1148 Offerer MUST be prepared to receive a User-User Information 1149 element once the circuit-switched bearer is set up. The Offerer 1150 SHOULD compare the received UUI to the value of the "uuie" 1151 subfield. If the value of the received UUI matches the value of 1152 the "uuie" subfield, the call SHOULD be treated as correlated to 1153 the ongoing session. 1155 According the Offer/Answer Model with SDP [RFC3264], the Offerer 1156 needs to be ready to receive media as soon as the Offer has been 1157 sent. It may happen that the Answerer, if it became the active 1158 party, will initiate a circuit-switched bearer setup which will 1159 arrive at the Offerer before the Answer has arrived. However, the 1160 Offerer needs to receive the Answer and examine the information about 1161 the correlation mechanisms in order to successfully perform 1162 correlation of the circuit-switched call to the session. Therefore, 1163 if the Offerer receives an incoming circuit-switched call, it MUST 1164 NOT accept the call before the Answer has been received. If no 1165 Answer is received during an implementation specific time, the 1166 Offerer MUST either modify the session according to [RFC3264] or 1167 terminate it according to the session signaling procedures in 1168 question (for terminating a SIP session, see Section 15 of 1169 [RFC3261]). 1171 5.6.4. Modifying the session 1173 If, at a later time, one of the parties wishes to modify the session, 1174 e.g., by adding new media stream, or by changing properties used on 1175 an existing stream, it may do so via the mechanisms defined for An 1176 Offer/Answer Model with SDP [RFC3264]. 1178 If there is an existing circuit-switched bearer between the 1179 endpoints, and the Offerer wants to reuse that, the Offerer MUST set 1180 the value of the "a=connection" attribute to 'existing'. 1182 If either party removes the circuit-switched media from the session 1183 (by setting the port number to zero), it MUST terminate the circuit- 1184 switched bearer using whatever mechanism is appropriate for the 1185 technology in question. 1187 If either party wishes to drop and reestablish an existing call, that 1188 party MUST first remove the circuit-switched media from the session 1189 by setting the port number to zero, and then use another Offer/Answer 1190 exchange where it MUST set the "a=connection" attribute to 'new'". 1191 If the media types are different (for example, a different codec will 1192 be used for the circuit-switched bearer), the media descriptions for 1193 terminating the existing bearer and the new bearer can be in the same 1194 Offer. 1196 5.7. Formal Syntax 1198 The following is the formal Augmented Backus-Naur Form (ABNF) 1199 [RFC5234] syntax that supports the extensions defined in this 1200 specification. The syntax is built above the SDP [RFC4566] and the 1201 tel URI [RFC3966] grammars. Implementations according to this 1202 specification MUST be compliant with this syntax. 1204 Figure 2 shows the formal syntax of the extensions defined in this 1205 memo. 1207 ; extension to the connection field originally specified 1208 ; in RFC4566 1210 connection-field = [%x63 "=" nettype SP addrtype SP 1211 connection-address CRLF] 1212 ; CRLF defined in RFC5234 1214 ;nettype and addrtype are defined in RFC 4566 1216 connection-address /= global-number-digits / "-" 1217 ; global-number-digits specified in RFC3966 1219 ;subrules for correlation attribute 1220 attribute /= cs-correlation-attr 1221 ; attribute defined in RFC4566 1222 cs-correlation-attr = "cs-correlation:" corr-mechanisms 1223 corr-mechanisms = corr-mech *(SP corr-mech) 1224 corr-mech = caller-id-mech / uuie-mech / 1225 dtmf-mech / ext-mech 1226 caller-id-mech = "callerid" [":" caller-id-value] 1227 caller-id-value = "+" 1*15DIGIT 1228 ; DIGIT defined in RFC5234 1229 uuie-mech = "uuie" [":" uuie-value] 1230 uuie-value = 1*65(HEXDIG HEXDIG) 1231 ;This represents up to 130 HEXDIG 1232 ; (65 octets) 1233 ;HEXDIG defined in RFC5234 1234 ;HEXDIG defined as 0-9, A-F 1235 dtmf-mech = "dtmf" [":" dtmf-value] 1236 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1237 ;0-9, A-D, '#' and '*' 1238 ext-mech = ext-mech-name [":" ext-mech-value] 1239 ext-mech-name = token 1240 ext-mech-value = token 1241 ; token is specified in RFC4566 1243 Figure 2: Syntax of the SDP extensions 1245 6. Examples 1247 In the examples below, where an SDP line is too long to be displayed 1248 as a single line, a breaking character "\" indicates continuation in 1249 the following line. Note that this character is included for display 1250 purposes only. Implementations MUST write a single line without 1251 breaks. 1253 6.1. Single PSTN audio stream 1255 Endpoint A Endpoint B 1256 | | 1257 | (1) SDP Offer (PSTN audio) | 1258 |--------------------------------->| 1259 | | 1260 | (2) SDP Answer (PSTN audio) | 1261 |<---------------------------------| 1262 | | 1263 | PSTN call setup | 1264 |<---------------------------------| 1265 | | 1266 |<==== media over PSTN bearer ====>| 1267 | | 1269 Figure 3: Basic flow 1271 Figure 3 shows a basic example that describes a single audio media 1272 stream over a circuit-switched bearer. Endpoint A generates a SDP 1273 Offer which is shown in Figure 4. The Offer describes a PSTN 1274 circuit-switched bearer in the "m=" and "c=" line where it also 1275 indicates its international E.164 number format. Additionally, 1276 Endpoint A expresses that it can initiate the circuit-switched bearer 1277 or be the recipient of it in the "a=setup" attribute line. The SDP 1278 Offer also includes correlation identifiers that this endpoint will 1279 insert in the Calling Party Number and/or User-User Information 1280 Element of the PSTN call setup if eventually this endpoint initiates 1281 the PSTN call. 1283 v=0 1284 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1285 s= 1286 t=0 0 1287 m=audio 9 PSTN - 1288 c=PSTN E164 +441134960123 1289 a=setup:actpass 1290 a=connection:new 1291 a=cs-correlation:callerid:+441134960123 \ 1292 uuie:56A390F3D2B7310023 1294 Figure 4: SDP offer (1) 1296 Endpoint B generates a SDP Answer (Figure 5), describing a PSTN audio 1297 media on port 9 without information on the media sub-type on the "m=" 1298 line. The "c=" line contains B's international E.164 number. In the 1299 "a=setup" line Endpoint B indicates that it is willing to become the 1300 active endpoint when establishing the PSTN call, and it also includes 1301 the "a=cs-correlation" attribute line containing the values it is 1302 going to include in the Calling Party Number and User-User IE of the 1303 PSTN call establishment. 1305 v=0 1306 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1307 s= 1308 t=0 0 1309 m=audio 9 PSTN - 1310 c=PSTN E164 +441134960124 1311 a=setup:active 1312 a=connection:new 1313 a=cs-correlation:callerid:+441134960124 \ 1314 uuie:74B9027A869D7966A2 1316 Figure 5: SDP Answer with circuit-switched media 1318 When Endoint A receives the Answer, it examines that B is willing to 1319 become the active endpoint when setting up the PSTN call. Endoint A 1320 temporarily stores B's E.164 number and the User-User IE value of the 1321 "cs-correlation" attribute, and waits for a circuit-switched bearer 1322 establishment. 1324 Endpoint B initiates a circuit-switched bearer using whatever 1325 circuit-switched technology is available for it. The called party 1326 number is set to A's number, and calling party number is set to B's 1327 own number. Endpoint B also sets the User-User Information Element 1328 value to the one contained in the SDP Answer. 1330 When Endpoint A receives the circuit-switched bearer establishment, 1331 it examines the UUIE and the calling party number, and by comparing 1332 those received during O/A exchange determines that the call is 1333 related to the SDP session. 1335 It may also be that neither the UUIE nor the calling party number is 1336 received by the called party, or the format of the calling party 1337 number is changed by the PSTN. Implementations may still accept such 1338 call establishment attempts as being related to the session that was 1339 established in the IP network. As it cannot be guaranteed that the 1340 values used for correlation are always passed intact through the 1341 network, they should be treated as additional hints that the circuit- 1342 switched bearer is actually related to the session. 1344 6.2. Advanced SDP example: Circuit-Switched Audio and Video Streams 1346 Endpoint A Endpoint B 1347 | | 1348 | (1) SDP Offer (PSTN audio and video) | 1349 |------------------------------------------->| 1350 | | 1351 | (2) SDP Answer (PSTN audio) | 1352 |<-------------------------------------------| 1353 | | 1354 | PSTN call setup | 1355 |<-------------------------------------------| 1356 | | 1357 |<======== media over PSTN bearer ==========>| 1358 | | 1360 Figure 6: Circuit-Switched Audio and Video streams 1362 Figure 6 shows an example of negotiating audio and video media 1363 streams over circuit-switched bearers. 1365 v=0 1366 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1367 s= 1368 t=0 0 1369 a=setup:actpass 1370 a=connection:new 1371 c=PSTN E164 +441134960123 1372 m=audio 9 PSTN - 1373 a=cs-correlation:dtmf:1234536 1374 m=video 9 PSTN 34 1375 a=rtpmap:34 H263/90000 1376 a=cs-correlation:callerid:+441134960123 1378 Figure 7: SDP offer with circuit-switched audio and video (1) 1380 Upon receiving the SDP offer described in Figure 7, Endpoint B 1381 rejects the video stream as the device does not currently support 1382 video, but accepts the circuit-switched audio stream. As Endoint A 1383 indicated that it is able to become either the active or passive 1384 party, Endpoint B gets to select which role it would like to take. 1385 Since the Offer contained the international E.164 number of Endpoint 1386 A, Endpoint B decides that it becomes the active party in setting up 1387 the circuit-switched bearer. B includes a new value in the "dtmf" 1388 subfield of the "cs-correlation" attribute, which it is going to send 1389 as DTMF tones once the bearer setup is complete. The Answer is 1390 described in Figure 8 1392 v=0 1393 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1394 s= 1395 t=0 0 1396 a=setup:active 1397 a=connection:new 1398 c=PSTN E164 +441134960124 1399 m=audio 9 PSTN - 1400 a=cs-correlation:dtmf:654321 1401 m=video 0 PSTN 34 1402 a=cs-correlation:callerid:+441134960124 1404 Figure 8: SDP answer with circuit-switched audio and video (2) 1406 7. Security Considerations 1408 This document provides an extension on top of RFC 4566 [RFC4566], and 1409 RFC 3264 [RFC3264]. As such, the security considerations of those 1410 documents apply. 1412 This memo provides mechanisms to agree on a correlation identifier or 1413 identifiers that are used to evaluate whether an incoming circuit- 1414 switched bearer is related to an ongoing session in the IP domain. 1415 If an attacker replicates the correlation identifier and establishes 1416 a call within the time window the receiving endpoint is expecting a 1417 call, the attacker may be able to hijack the circuit-switched bearer. 1418 These types of attacks are not specific to the mechanisms presented 1419 in this memo. For example, caller ID spoofing is a well known attack 1420 in the PSTN. Users are advised to use the same caution before 1421 revealing sensitive information as they would on any other phone 1422 call. Furthermore, users are advised that mechanisms that may be in 1423 use in the IP domain for securing the media, like Secure RTP (SRTP) 1424 [RFC3711], are not available in the CS domain. 1426 For the purposes of establishing a circuit-switched bearer, the 1427 active endpoint needs to know the passive endpoint's phone number. 1428 Phone numbers are sensitive information, and some people may choose 1429 not to reveal their phone numbers when calling using supplementary 1430 services like Calling Line Identification Restriction (CLIR) in GSM. 1431 Implementations should take the caller's preferences regarding 1432 calling line identification into account if possible, by restricting 1433 the inclusion of the phone number in SDP "c=" line if the caller has 1434 chosen to use CLIR. If this is not possible, implementations may 1435 present a prompt informing the user that their phone number may be 1436 transmitted to the other party. 1438 Similarly as with IP addresses, if there is a desire to protect the 1439 SDP containing phone numbers carried in SIP, implementers are advised 1440 to follow the security mechanisms defined in [RFC3261]. 1442 It is possible that an attacker creates a circuit-switched session 1443 whereby the attacked endpoint should dial a circuit-switched number, 1444 perhaps even a premium-rate telephone number. To mitigate the 1445 consequences of this attack, endpoints MUST authenticate and trust 1446 remote endpoints users who try to remain passive in the circuit- 1447 switched connection establishment. It is RECOMMENDED that endpoints 1448 have local policies precluding the active establishment of circuit 1449 switched connections to certain numbers (e.g., international, 1450 premium, long distance). Additionally, it is strongly RECOMMENDED 1451 that the end user is asked for consent prior to the endpoint 1452 initiating a circuit-switched connection. 1454 8. IANA Considerations 1456 This document instructs IANA to register a number of SDP tokens 1457 according to the following data. 1459 8.1. Registration of new cs-correlation SDP attribute 1461 Contact: Miguel Garcia 1463 Attribute name: cs-correlation 1465 Long-form attribute name: PSTN Correlation Identifier 1467 Type of attribute: media level only 1469 Subject to charset: No 1471 Description: This attribute provides the Correlation Identifier 1472 used in PSTN signaling 1474 Appropriate values:see Section 5.2.3.1 1476 Specification: RFC XXXX 1478 The IANA is requested to create a subregistry for 'cs-correlation' 1479 attribute under the Session Description Protocol (SDP) Parameters 1480 registry. The initial values for the subregistry are presented in 1481 the following, and IANA is requested to add them into its database: 1483 Value of 'cs-correlation' attribute Reference Description 1484 ----------------------------------- --------- ----------- 1485 callerid RFC XXXX Caller ID 1486 uuie RFC XXXX User-User 1487 Information Element 1488 dtmf RFC XXXX Dual-tone 1489 Multifrequency 1491 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1492 reference to the RFC number of this draft. 1494 As per the terminology in [RFC5226], the registration policy for new 1495 values of 'cs-correlation' parameter is 'Specification Required'. 1497 8.2. Registration of a new "nettype" value 1499 This memo provides instructions to IANA to register a new "nettype" 1500 in the Session Description Protocol Parameters registry [1]. The 1501 registration data, according to RFC 4566 [RFC4566] follows. 1503 Type SDP Name Reference 1504 ---- ------------------ --------- 1505 nettype PSTN [RFCxxxx] 1507 8.3. Registration of new "addrtype" values 1509 This memo provides instructions to IANA to register two new 1510 "addrtype" in the Session Description Protocol Parameters registry 1511 [2]. The registration data, according to RFC 4566 [RFC4566] follows. 1513 Type SDP Name Reference 1514 ---- ------------------ --------- 1515 addrtype E164 [RFCxxxx] 1516 addrtype - [RFCxxxx] 1518 Note: RFC XXXX defines the "E164" and "-" addrtypes in the context of 1519 the "PSTN" nettype only. Please refer to the relevant RFC for a 1520 description of that representation. 1522 8.4. Registration of a new "proto" value 1524 This memo provides instructions to IANA to register a new "proto" in 1525 the Session Description Protocol Parameters registry [3]. The 1526 registration data, according to RFC 4566 [RFC4566] follows. 1528 Type SDP Name Reference 1529 -------------- --------------------------- --------- 1530 proto PSTN [RFCxxxx] 1532 The related "fmt" namespace re-uses the conventions and payload type 1533 number defined for RTP/AVP. In RFC XXXX, the RTP audio and video 1534 media types, when applied to PSTN circuit-switched bearers, represent 1535 merely an audio or video codec in its native format directly on top 1536 of a single PSTN bearer. 1538 In come cases, the endpoint is not able to determine the list of 1539 available codecs for circuit-switched media streams. In this case, 1540 in order to be syntactically compliant with SDP [RFC4566], the 1541 endpoint MUST include a single dash ("-") in the subfield. 1543 9. Acknowledgments 1545 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas 1546 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan 1547 Rosenberg, Ingemar Johansson, Christer Holmberg, Alf Heidermark, Tom 1548 Taylor, Thomas Belling, Keith Drage, and Andrew Allen for providing 1549 their insight and comments on this document. 1551 10. References 1553 10.1. Normative References 1555 [ITU.Q931.1998] 1556 , "Digital Subscriber Signalling System No. 1 (DSS 1) - 1557 ISDN User - Network Interface Layer 3 Specification for 1558 Basic Call Control", ISO Standard 9594-1, May 1998. 1560 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1561 Requirement Levels", BCP 14, RFC 2119, March 1997. 1563 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1564 with Session Description Protocol (SDP)", RFC 3264, June 1565 2002. 1567 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 1568 3966, December 2004. 1570 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1571 the Session Description Protocol (SDP)", RFC 4145, 1572 September 2005. 1574 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1575 Description Protocol", RFC 4566, July 2006. 1577 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1578 Encodings", RFC 4648, October 2006. 1580 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1581 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1582 May 2008. 1584 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1585 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1587 10.2. Informative References 1589 [I-D.ietf-cuss-sip-uui] 1590 Johnston, A. and J. Rafferty, "A Mechanism for 1591 Transporting User to User Call Control Information in 1592 SIP", draft-ietf-cuss-sip-uui-10 (work in progress), April 1593 2013. 1595 [ITU.E164.1991] 1596 International Telecommunications Union, "The International 1597 Public Telecommunication Numbering Plan", ITU-T 1598 Recommendation E.164, 1991. 1600 [ITU.Q23.1988] 1601 International Telecommunications Union, "Technical 1602 features of push-button telephone sets ", ITU-T Technical 1603 Recommendation Q.23, 1988. 1605 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1606 Session Description Protocol (SDP) for ATM Bearer 1607 Connections", RFC 3108, May 2001. 1609 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1610 A., Peterson, J., Sparks, R., Handley, M., and E. 1611 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1612 June 2002. 1614 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1615 Jacobson, "RTP: A Transport Protocol for Real-Time 1616 Applications", STD 64, RFC 3550, July 2003. 1618 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1619 Video Conferences with Minimal Control", STD 65, RFC 3551, 1620 July 2003. 1622 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1623 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1624 RFC 3711, March 2004. 1626 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1627 Camarillo, "Best Current Practices for Third Party Call 1628 Control (3pcc) in the Session Initiation Protocol (SIP)", 1629 BCP 85, RFC 3725, April 2004. 1631 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1632 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1634 [TS.24.008] 1635 3GPP , "Mobile radio interface Layer 3 specification; Core 1636 network protocols; Stage 3 ", 3GPP TS 24.008 3.20.0, 1637 December 2005. 1639 Authors' Addresses 1641 Miguel A. Garcia-Martin 1642 Ericsson 1643 Calle Via de los Poblados 13 1644 Madrid, ES 28033 1645 Spain 1647 Email: miguel.a.garcia@ericsson.com 1649 Simo Veikkolainen 1650 Nokia 1651 P.O. Box 226 1652 NOKIA GROUP, FI 00045 1653 Finland 1655 Phone: +358 50 486 4463 1656 Email: simo.veikkolainen@nokia.com