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Veikkolainen 5 Expires: September 7, 2012 Nokia 6 March 6, 2012 8 Session Description Protocol (SDP) Extension For Setting Up Audio and 9 Video Media Streams Over Circuit-Switched Bearers In The Public Switched 10 Telephone Network (PSTN) 11 draft-ietf-mmusic-sdp-cs-10 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 September 7, 2012. 37 Copyright Notice 39 Copyright (c) 2012 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 This document may contain material from IETF Documents or IETF 53 Contributions published or made publicly available before November 54 10, 2008. The person(s) controlling the copyright in some of this 55 material may not have granted the IETF Trust the right to allow 56 modifications of such material outside the IETF Standards Process. 57 Without obtaining an adequate license from the person(s) controlling 58 the copyright in such materials, this document may not be modified 59 outside the IETF Standards Process, and derivative works of it may 60 not be created outside the IETF Standards Process, except to format 61 it for publication as an RFC or to translate it into languages other 62 than English. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 67 2. Conventions Used in This Document . . . . . . . . . . . . . . 6 68 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 6 69 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7 70 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 7 71 5. Protocol Description . . . . . . . . . . . . . . . . . . . . . 9 72 5.1. Level of Compliance . . . . . . . . . . . . . . . . . . . 9 73 5.2. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 9 74 5.2.1. Connection Data . . . . . . . . . . . . . . . . . . . 9 75 5.2.2. Media Descriptions . . . . . . . . . . . . . . . . . . 10 76 5.2.3. Correlating the PSTN Circuit-Switched Bearer with 77 SDP . . . . . . . . . . . . . . . . . . . . . . . . . 12 78 5.2.3.1. The "cs-correlation" attribute . . . . . . . . . . 12 79 5.2.3.2. Caller-ID Correlation Mechanism . . . . . . . . . 13 80 5.2.3.3. User-User Information Element Correlation 81 Mechanism . . . . . . . . . . . . . . . . . . . . 14 82 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . . 15 83 5.3. Negotiating the correlation mechanisms . . . . . . . . . . 16 84 5.3.1. Determining the Direction of the Circuit-Switched 85 Bearer Setup . . . . . . . . . . . . . . . . . . . . . 16 86 5.3.2. Populating the cs-correlation attribute . . . . . . . 17 87 5.3.3. Considerations on successful correlation . . . . . . . 17 88 5.4. Considerations for Usage of Existing SDP . . . . . . . . . 18 89 5.4.1. Originator of the Session . . . . . . . . . . . . . . 18 90 5.4.2. Contact information . . . . . . . . . . . . . . . . . 19 91 5.5. Considerations for Usage of Third Party Call Control 92 (3PCC) . . . . . . . . . . . . . . . . . . . . . . . . . . 19 93 5.6. Offer/Answer mode extensions . . . . . . . . . . . . . . . 19 94 5.6.1. Generating the Initial Offer . . . . . . . . . . . . . 19 95 5.6.2. Generating the Answer . . . . . . . . . . . . . . . . 21 96 5.6.3. Offerer processing the Answer . . . . . . . . . . . . 24 97 5.6.4. Modifying the session . . . . . . . . . . . . . . . . 25 98 5.7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 25 99 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 100 6.1. Single PSTN audio stream . . . . . . . . . . . . . . . . . 27 101 6.2. Advanced SDP example: Circuit-Switched Audio and Video 102 Streams . . . . . . . . . . . . . . . . . . . . . . . . . 29 103 7. Security Considerations . . . . . . . . . . . . . . . . . . . 30 104 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 105 8.1. Registration of new cs-correlation SDP attribute . . . . . 31 106 8.2. Registration of a new "nettype" value . . . . . . . . . . 31 107 8.3. Registration of new "addrtype" values . . . . . . . . . . 32 108 8.4. Registration of a new "proto" value . . . . . . . . . . . 32 109 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32 110 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32 111 10.1. Normative References . . . . . . . . . . . . . . . . . . . 32 112 10.2. Informative References . . . . . . . . . . . . . . . . . . 33 113 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34 115 1. Introduction 117 The Session Description Protocol (SDP) [RFC4566] is intended for 118 describing multimedia sessions for the purposes of session 119 announcement, session invitation, and other forms of multimedia 120 session initiation. SDP is most commonly used for describing media 121 streams that are transported over the Real-Time Transport Protocol 122 (RTP) [RFC3550], using the profiles for audio and video media defined 123 in RTP Profile for Audio and Video Conferences with Minimal Control 124 [RFC3551]. 126 However, SDP can be used to describe other transport protocols than 127 RTP. Previous work includes SDP conventions for describing ATM 128 bearer connections [RFC3108] and the Message Session Relay Protocol 129 [RFC4975]. 131 SDP is commonly carried in Session Initiation Protocol (SIP) 132 [RFC3261] messages in order to agree on a common media description 133 among the endpoints. An Offer/Answer Model with Session Description 134 Protocol (SDP) [RFC3264] defines a framework by which two endpoints 135 can exchange SDP media descriptions and come to an agreement as to 136 which media streams should be used, along with the media related 137 parameters. 139 In some scenarios it might be desirable to establish the media stream 140 over a circuit-switched bearer connection even if the signaling for 141 the session is carried over an IP bearer. An example of such a 142 scenario is illustrated with two mobile devices capable of both 143 circuit-switched and packet-switched communication over a low- 144 bandwidth radio bearer. The radio bearer may not be suitable for 145 carrying real-time audio or video media, and using a circuit-switched 146 bearer would offer a better perceived quality of service. So, 147 according to this scenario, SDP and its higher layer session control 148 protocol (e.g., the Session Initiation Protocol (SIP) [RFC3261]) are 149 used over regular IP connectivity, while the audio or video is 150 received through the classical circuit-switched bearer. 152 Setting up a signaling relationship in the IP domain instead of just 153 setting up a circuit-switched call offers also the possibility of 154 negotiating in the same session other IP based media that is not 155 sensitive to jitter and delay, for example, text messaging or 156 presence information. 158 At a later point in time the mobile device might move to an area 159 where a high-bandwidth packet-switched bearer, for example a Wireless 160 Local Area Network (WLAN) connection, is available. At this point 161 the mobile device may perform a handover and move the audio or video 162 media streams over to the high-speed bearer. This implies a new 163 exchange of SDP Offer/Answer that lead to a re-negotiation of the 164 media streams. 166 Other use cases exist. For example, and endpoint might have at its 167 disposal circuit-switched and packet-switched connectivity, but the 168 same audio or video codecs are not feasible for both access networks. 169 For example, the circuit-switched audio or video stream supports 170 narrow-bandwidth codecs, while the packet-switched access allows any 171 other audio or video codec implemented in the endpoint. In this 172 case, it might be beneficial for the endpoint to describe different 173 codecs for each access type and get an agreement on the bearer 174 together with the remote endpoint. 176 There are additional use cases related to third party call control 177 where the session setup time is improved when the circuit-switched 178 bearer in the PSTN is described together with one or more codecs. 180 The rest of the document is structured as follows: Section 2 provides 181 the document conventions, Section 3 introduces the requirements, 182 Section 4 presents an overview of the proposed solutions, and 183 Section 5 contains the protocol description. Section 6 provides an 184 example of descriptions of circuit-switched audio or video streams in 185 SDP. Section 8 and Section 7 contain the IANA and Security 186 considerations, respectively. 188 2. Conventions Used in This Document 190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 192 "OPTIONAL" in this document are to be interpreted as described in BCP 193 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant 194 implementations. 196 3. Requirements 198 This section presents the general requirements that are specific for 199 the audio or video media streams over circuit-switched bearers. 201 REQ-1: A mechanism for endpoints to negotiate and agree on an audio 202 or video media stream established over a circuit-switched 203 bearer MUST be available. 205 REQ-2: The mechanism MUST allow the endpoints to combine circuit- 206 switched audio or video media streams with other 207 complementary media streams, for example, text messaging. 209 REQ-3: The mechanism MUST allow the endpoint to negotiate the 210 direction of the circuit-switched bearer, i.e., which 211 endpoint is active when initiating the circuit-switched 212 bearer. 214 REQ-4: The mechanism MUST be independent of the type of the circuit- 215 switched access (e.g., Integrated Services Digital Network 216 (ISDN), Global System for Mobile Communication (GSM), etc.) 218 REQ-5: There MUST be a mechanism that helps an endpoint to correlate 219 an incoming circuit-switched bearer with the one negotiated 220 in SDP, as opposed to another incoming call that is not 221 related to that. 223 REQ-6: It MUST be possible for endpoints to advertise different list 224 of audio or video codecs in the circuit-switched audio or 225 video stream from those used in a packet-switched audio or 226 video stream. 228 REQ-7: It MUST be possible for endpoints to not advertise the list 229 of available codecs for circuit-switched audio or video 230 streams. 232 4. Overview of Operation 234 The mechanism defined in this memo extends SDP and allows describing 235 an audio or video media stream established over a circuit-switched 236 bearer. New tokens are registered in the "c=" and "m=" lines to be 237 able to describe a media stream over a circuit-switched bearer. 238 These SDP extensions are described in Section 5.2. Since circuit- 239 switched bearers are connection-oriented media streams, the mechanism 240 re-uses the connection-oriented extensions defined in RFC 4145 241 [RFC4145] to negotiate the active and passive sides of a connection 242 setup. This is further described in Section 5.3.1. 244 4.1. Example Call Flow 246 Consider the example presented in Figure 1. In this example, Alice 247 is located in an environment where she has access to both IP and 248 circuit-switched bearers for communicating with other endpoints. 249 Alice decides that the circuit-switched bearer offers a better 250 perceived quality of service for voice, and issues an SDP Offer 251 containing the description of an audio media stream over circuit- 252 switched bearer. 254 Alice Bob 255 | (1) SDP Offer (PSTN audio) | 256 |----------------------------------->| 257 | | 258 | (2) SDP Answer (PSTN audio) | 259 |<-----------------------------------| 260 | | 261 | PSTN call setup | 262 |<-----------------------------------| 263 | | 264 | | 265 |<===== media over PSTN bearer =====>| 266 | | 268 Figure 1: Example Flow 270 Bob receives the SDP offer and determines that he is located in an 271 environment where the IP based bearer is not suitable for real-time 272 audio media. However he also has PSTN circuit-switched bearer 273 available for audio. Bob generates an SDP answer containing a 274 description of the audio media stream over a circuit-switched bearer. 276 During the offer-answer exchange Alice and Bob also agree the 277 direction in which the circuit-switched bearer should be established. 278 In this example, Bob becomes the active party, in other words, he 279 establishes the circuit-switched call to the other endpoint. The 280 Offer/Answer exchange contains identifiers or references that can be 281 used on the circuit-switched network for addressing the other 282 endpoint, as well as information that is used to determine that the 283 incoming circuit-switched bearer establishment is related to the 284 ongoing session between Alice and Bob. 286 Bob establishes a circuit-switched bearer towards Alice using 287 whatever mechanisms are defined for the network type in question. 288 When receiving the incoming circuit-switched connection attempt, 289 Alice is able to determine that the attempt is related to the session 290 she is just establishing with Bob. 292 Alice accepts the circuit-switched connection; the circuit-switched 293 bearer setup is completed. Bob and Alice can now use the circuit- 294 switched connection for two-way audio media. 296 If, for some reason, Bob would like to reject the offered stream, he 297 would set the port number of the specific stream to zero, as 298 specified in RFC3264 [RFC3264]. Also, if Bob does not understand 299 some of the SDP attributes specified in this document, he would 300 ignore them, as specified in RFC4566 [RFC4566]. 302 5. Protocol Description 304 5.1. Level of Compliance 306 Implementations according to this specification MUST implement the 307 SDP extensions described in Section 5.2, and MUST implement the 308 considerations discussed in Section 5.3, Section 5.4 and Section 5.6. 310 5.2. Extensions to SDP 312 This section provides the syntax and semantics of the extensions 313 required for providing a description of audio or video media streams 314 over circuit-switched bearers in SDP. 316 5.2.1. Connection Data 318 According to SDP [RFC4566], the connection data line in SDP has the 319 following syntax: 321 c= 323 where indicates the network type, indicates the 324 address type, and the is the connection address, 325 which is dependent on the address type. 327 At the moment, the only network type defined is "IN", which indicates 328 Internet network type. The address types "IP4" and "IP6" indicate 329 the type of IP addresses. 331 This memo defines a new network type for describing a circuit- 332 switched bearer network type in the PSTN. The mnemonic "PSTN" is 333 used for this network type. 335 For the address type, we initially consider the possibility of 336 describing E.164 telephone numbers. We define a new "E164" address 337 type. When used, the "E164" address type indicates that the 338 connection address contains an international E.164 number represented 339 according to the ITU-T E.164 [ITU.E164.1991] recommendation. 341 It is a common convention that an international E.164 number contains 342 a leading '+' sign. For consistency's sake, we also require the 343 E.164 telephone is prepended with a '+', even if that is not 344 necessary for routing of the call in the PSTN network. 346 There are cases, though, when the endpoint is merely aware of a 347 circuit-switched bearer, without having further information about the 348 address type or the E.164 number allocated to it. In these cases a 349 dash "-" is used to indicate an unknown address type or connection 350 address. This makes the connection data line be according to the SDP 351 syntax. 353 Note that and/or should not be 354 omitted without being set to a "-" since this would violate basic 355 syntax of SDP [RFC4566]. 357 The following are examples of the extension to the connection data 358 line: 360 c=PSTN E164 +35891234567 362 c=PSTN - - 364 When the is PSTN, the connection address is defined as 365 follows: 367 o an international E.164 number 369 When the is "-", the connection address is defined as 370 follows: 372 o the value "-", signifying that the address is unknown 374 o any syntactically valid value, which is to be ignored 376 5.2.2. Media Descriptions 378 According to SDP [RFC4566], the media descriptions line in SDP has 379 the following syntax: 381 m= ... 383 The sub-field carries the media type. For establishing an 384 audio bearer, the existing "audio" media type is used. For 385 establishing a video bearer, the existing "video" media type is used. 387 The sub-field is the transport port to which the media stream 388 is sent. Circuit-switched access lacks the concept of a port number, 389 and therefore the sub-field does not carry any meaningful 390 value. In order to be compliant with SDP syntax, implementations 391 SHOULD set the sub-field to the discard port value "9" and 392 MUST ignore it on reception. 394 According to RFC 3264 [RFC3264], a port number of zero in the offer 395 of a unicast stream indicates that the stream is offered but must not 396 be used. If a port number of zero is present in the answer of a 397 unicast stream, it indicates that the stream is rejected. These 398 rules are still valid when the media line in SDP represents a 399 circuit-switched bearer. 401 The sub-field is the transport protocol. The circuit- 402 switched bearer uses whatever transport protocol it has available. 403 This subfield SHOULD be set to the mnemonic "PSTN" to be 404 syntactically correct with SDP [RFC4566] and to indicate the usage of 405 circuit-switched protocols in the PSTN. 407 The sub-field is the media format description. In the 408 classical usage of SDP to describe RTP-based media streams, when the 409 sub-field is set to "RTP/AVP" or "RTP/SAVP", the sub- 410 field contains the payload types as defined in the RTP audio profile 411 [RFC3551]. 413 When "RTP/AVP" is used in the field, the sub-field 414 contains the RTP payload type numbers. We use the sub-field to 415 indicate the list of available codecs over the circuit-switched 416 bearer, by re-using the conventions and payload type numbers defined 417 for RTP/AVP. The RTP audio and video media types, which, when 418 applied to PSTN circuit-switched bearers, represent merely an audio 419 or video codec. 421 In some cases, the endpoint is not able to determine the list of 422 available codecs for circuit-switched media streams. In this case, 423 in order to be syntactically compliant with SDP [RFC4566], the 424 endpoint MUST include a single dash "-" in the sub-field. 426 As per RFC 4566 [RFC4566], the media format descriptions are listed 427 in priority order. 429 Examples of media descriptions for circuit-switched audio streams 430 are: 432 m=audio 9 PSTN 3 0 8 434 m=audio 9 PSTN - 436 Similarly, an example of a media description for circuit-switched 437 video stream is: 439 m=video 9 PSTN 34 441 m=video 9 PSTN - 443 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 445 The endpoints should be able to correlate the circuit-switched bearer 446 with the session negotiated with SDP in order to avoid ringing for an 447 incoming circuit-switched bearer that is related to the session 448 controlled with SDP (and SIP). 450 Several alternatives exist for performing this correlation. This 451 memo provides three mutually non-exclusive correlation mechanisms. 452 Other correlation mechanisms may exist, and their usage will be 453 specified when need arises. All mechanisms share the same principle: 454 some unique information is sent in the SDP and in the circuit- 455 switched signaling protocol. If these pieces of information match, 456 then the circuit-switched bearer is part of the session described in 457 the SDP exchange. Otherwise, there is no guarantee that the circuit- 458 switched bearer is related to such session. 460 The first mechanism is based on the exchange of PSTN caller-ID 461 between the endpoints. The caller-ID is also available as the 462 Calling Party ID in the circuit-switched signaling. 464 The second mechanism is based on the inclusion in SDP of a value that 465 is also sent in the User-to-User Information Element that is part of 466 the bearer setup signaling in the PSTN. 468 The third mechanism is based on sending in SDP a string that 469 represents Dual Tone MultiFrequency (DTMF) digits that will be later 470 sent right after the circuit-switched bearer is established. 471 Implementations MAY use any of these mechanisms and MAY use two or 472 more mechanisms simultaneously. 474 5.2.3.1. The "cs-correlation" attribute 476 In order to provide support for the correlation mechanisms, we define 477 a new SDP attribute called "cs-correlation". This "cs-correlation" 478 attribute can include any of the "callerid", "uuie", or "dtmf" sub- 479 fields, which specify additional information required by the 480 Caller-ID, User to User Information, or DTMF correlation mechanisms, 481 respectively. The list of correlation mechanisms may be extended by 482 other specifications. 484 The following sections provide more detailed information of these 485 subfields. The "cs-correlation" attribute has the following format: 487 a=cs-correlation: 488 correlation-mechanisms = corr-mech *(SP corr-mech) 489 corr-mech = caller-id-mech / uuie-mech / 490 dfmt-mech / ext-mech 491 caller-id-mech = "callerid" [":" caller-id-value] 492 uuie-mech = "uuie" [":" uuie-value] 493 dtmf-mech = "dtmf" [":" dtmf-value] 494 ext-mech = [":"] 496 The values "callerid", "uuie" and "dtmf" refer to the correlation 497 mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and 498 Section 5.2.3.4, respectively. The formal Augmented Backus-Naur 499 Format (ABNF) syntax of the "cs-correlation" attribute is presented 500 in Section 5.7. 502 5.2.3.2. Caller-ID Correlation Mechanism 504 The Caller-ID correlation mechanisms consists of an exchange of the 505 calling party number as an international E.164 number in SDP, 506 followed by the availability of the Calling Party Number information 507 element in the call setup signaling of the circuit switched 508 connection. If both pieces of information match, the circuit- 509 switched bearer is correlated to the session described in SDP. 511 Example of inclusion of an international E.164 number in the "cs- 512 correlation" attribute is: 514 a=cs-correlation:callerid:+35891234567 516 The presence of the "callerid" sub-field indicates that the endpoint 517 supports use of the calling party number as a means of correlating a 518 PSTN call with the session being negotiated. The "callerid" sub- 519 field MAY be accompanied by the international E.164 number of the 520 party inserting the parameter. 522 Note that there are no warranties that this correlation mechanism 523 works or is even available, due a number of problems: 525 o The endpoint might not be aware of its own E.164 number, in which 526 case it cannot populate the SDP appropriately. 528 o The Calling Party Number information element in the circuit- 529 switched signaling might not be available, e.g., due to policy 530 restrictions of the network operator or caller restriction due to 531 privacy. 533 o The Calling Party Number information element in the circuit- 534 switched signaling might be available, but the digit 535 representation of the E.164 number might differ from the one 536 expressed in the SDP. To mitigate this problem implementations 537 should consider only some of the rightmost digits from the E.164 538 number for correlation. For example, the numbers +358-9-123-4567 539 and 09-123-4567 could be considered as the same number. This is 540 also the behavior of some cellular phones, which correlate the 541 incoming calling party with a number stored in the phone book, for 542 the purpose of displaying the caller's name. 544 5.2.3.3. User-User Information Element Correlation Mechanism 546 A second correlation mechanism is based on including in SDP a string 547 that represents the User-User Information Element that is part of the 548 call setup signaling of the circuit-switched bearer. The User-User 549 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 550 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 551 Element has a maximum size of 35 or 131 octets, depending on the 552 actual message of the PSTN protocol where it is included. 554 The mechanism works as follows: An endpoint creates a User-User 555 Information Element, according to the requirements of the call setup 556 signaling protocol. The same value is included in the SDP offer or 557 SDP answer, in a "cs-correlation:uuie" attribute. When the SDP 558 Offer/Answer exchange is completed, each endpoint has become aware of 559 the value that will be used in the User-User Information Element of 560 the call setup message of the PSTN protocol. The endpoint that 561 initiates the call setup attempt includes this value in the User-User 562 Information Element. The recipient of the call setup attempt can 563 extract the User-User Information Element and correlate it with the 564 value previously received in the SDP. If both values match, then the 565 call setup attempt corresponds to that indicated in the SDP. 567 The first three octets of the User-User Information Element specified 568 in ITU-T Q.931 [ITU.Q931.1998] are the UUIE identifier, lenght of the 569 user-user contents, and a protocol discriminator, followed by the 570 actual User information. The first two octets of the UUIE MUST NOT 571 be used for correlation, only the octets carrying the Protocol 572 Discriminator and the User information value are compared the value 573 of the "cs-correlation:uuie" attribute. The value of the Protocol 574 Discriminator octet is not specified in this document; it is expected 575 that organizations using this technology will allocate a suitable 576 value for the Protocol Discriminator. 578 Note that, for correlation purposes, the value of the User-User 579 Information Element is considered as a opaque string and only used 580 for correlation purposes. Typically call signaling protocols impose 581 requirements on the creation of User-User Information Element for 582 end-user protocol exchange. The details regarding the generation of 583 the User-User Information Element are outside the scope of this 584 specification. 586 Please note that there are no warranties that this correlation 587 mechanism works. On one side, policy restrictions might not make the 588 User-User information available end to end in the PSTN. On the other 589 hand, the generation of the User-User Information Element is 590 controlled by the PSTN circuit-switched call protocol, which might 591 not offer enough freedom for generating different values from one 592 endpoint to another one, or from one call to another in the same 593 endpoint. This might result in the same value of the User-User 594 Information Element for all calls. 596 5.2.3.4. DTMF Correlation Mechanism 598 We introduce a third mechanism for correlating the circuit-switched 599 bearer with the session described with SDP. This is based on 600 agreeing on a sequence of digits that are negotiated in the SDP 601 Offer/Answer exchange and sent as Dual Tone Multifrequency (DTMF) 602 tones over the circuit-switched bearer once this bearer is 603 established. If the DTMF digit sequence received through the 604 circuit-switched bearer matches the digit string negotiated in the 605 SDP, the circuit-switched bearer is correlated with the session 606 described in the SDP. The mechanism is similar to many voice 607 conferencing systems which require the user to enter a PIN code using 608 DTMF tones in order to be accepted in a voice conference. 610 The mechanism works as follows: An endpoint selects a DTMF digit 611 sequence. The same sequence is included in the SDP offer or SDP 612 answer, in a "cs-correlation:dtmf" attribute. When the SDP Offer/ 613 Answer exchange is completed, each endpoint has become aware of the 614 DTMF sequence that will be sent right after the circuit-switched 615 bearer is set up. The endpoint that initiates the call setup attempt 616 sends the DTMF digits according to the procedures defined for the 617 circuit-switched bearer technology used. The recipient (passive side 618 of the bearer setup) of the call setup attempt collects the digits 619 and compares them with the value previously received in the SDP. If 620 the digits match, then the call setup attempt corresponds to that 621 indicated in the SDP. 623 Implementations are advised to select a number of DTMF digits that 624 provide enough assurance that the call is related, but on the 625 other hand do not prolong the bearer setup time unnecessarily. 627 As an example, an endpoint willing to send DTMF tone sequence "14D*3" 628 would include a "cs-correlation" attribute line as follows: 630 a=cs-correlation:dtmf:14D*3 632 If the endpoints successfully agree on the usage of the DTMF digit 633 correlation mechanism, but the passive side does not receive any DTMF 634 digits after successful circuit-switched bearer setup, or receives a 635 set of DTMF digits that do not match the value of the "dtmf" 636 attribute (including receiving too many digits), the passive side 637 SHOULD treat the circuit-switched bearer as not correlated to the 638 ongoing session. 640 DTMF digits can only be sent once the circuit-switched bearer is 641 set up. In order to suppress alerting for an incoming circuit- 642 switched call, implementations may choose various mechanisms. For 643 example, alerting may be suppressed for a certain time period for 644 incoming call attempts that originate from the number that was 645 observed during the offer/answer negotiation. 647 5.3. Negotiating the correlation mechanisms 649 The three correlation mechanisms presented above (based on called 650 party number, User-User Information Element and DTMF digit sending) 651 are non-exclusive, and can be used independently of each other. In 652 order to know how to populate the "a=cs-correlation" attribute, the 653 endpoints need to agree which endpoint will become the active party, 654 i.e. the one that will set up the circuit-switched bearer. 656 5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup 658 In order to avoid a situation where both endpoints attempt to 659 initiate a connection simultaneously, the direction in which the 660 circuit-switched bearer is set up should be negotiated during the 661 Offer/Answer exchange. 663 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 664 agree which endpoint acts as the active endpoint when initiating a 665 TCP connection. While RFC 4145 [RFC4145] was originally designed for 666 establishing TCP connections, it can be easily extrapolated to the 667 connection establishment of circuit-switched bearers. This 668 specification uses the concepts specified in RFC 4145 [RFC4145] for 669 agreeing on the direction of establishment of a circuit-switched 670 bearer. 672 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 673 "connection". The "setup" attribute indicates which of the endpoints 674 should initiate the connection establishment of the PSTN circuit- 675 switched bearer. Four values are defined in Section 4 of RFC 4145 676 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 677 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 678 attribute. 680 The "connection" attribute indicates whether a new connection is 681 needed or an existing connection is reused. The attribute can take 682 the values "new" or "existing". Please refer to Section 5 of RFC 683 4145 [RFC4145] for a detailed description of this attribute. 685 Implementations according to this specification MUST support the 686 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 687 but applied to circuit-switched bearers in the PSTN. 689 We define the active party as the one that initiates the circuit- 690 switched bearer after the Offer/Answer process. The passive party is 691 the one receiving the circuit-switched bearer. Either party may 692 indicate its desire to become the active or passive party during the 693 Offer/Answer exchange using the procedures described in Section 5.6. 695 5.3.2. Populating the cs-correlation attribute 697 By defining values for the sub-fields in the "a=cs-correlation" 698 attribute, the endpoint indicates that it is willing to become the 699 active party, and that it can use those values in the Calling party 700 number, User-User Information Element, or as DTMF tones during the 701 circuit-switched bearer setup. 703 Thus, the following rules apply: 705 An endpoint that can only become the active party in the circuit- 706 switched bearer setup MUST include all correlation mechanisms it 707 supports in the "a=cs-correlation" attribute, and MUST also 708 specify values for the sub-fields. 710 An endpoint that can only become the passive party in the circuit- 711 switched bearer setup MUST include all correlation mechanisms it 712 supports in the "a=cs-correlation" attribute, but MUST NOT specify 713 values for the sub-fields. 715 An endpoint that is willing to become either the active or passive 716 party (by including the "a=setup:actpass" attribute in the Offer), 717 MUST include all correlation mechanisms it supports in the "a=cs- 718 correlation" attribute, and MUST also specify values for the sub- 719 fields. 721 5.3.3. Considerations on successful correlation 723 Note that, as stated above, it cannot be guaranteed that any given 724 correlation mechanism will succeed even if the usage of those was 725 agreed beforehand. This is due to the fact that the correlation 726 mechanisms require support from the circuit-switched bearer 727 technology used. 729 Therefore, even a single positive indication using any of these 730 mechanisms SHOULD be interpreted by the passive endpoint so that the 731 circuit-switched bearer establishment is related to the ongoing 732 session, even if the other correlation mechanisms fail. 734 If, after negotiating one or more correlation mechanisms in the SDP 735 offer/answer exchange, an endpoint receives a circuit-switched bearer 736 with no correlation information present, the endpoint has two 737 choices: it can either treat the call as unrelated, or treat the call 738 as related to the ongoing session in the IP domain. 740 An endpoint may for example specify a time window after SDP offer/ 741 answer exchange during which received calls are treated as correlated 742 even if the signaling in the circuit-switched domain does not carry 743 any correlation information. In this case, there is a chance that 744 the call is erroneously treated as related to the ongoing session. 746 An endpoint may also choose to always treat an incoming call as 747 unrelated if the signaling in the circuit-switched domain does not 748 carry any correlation information. In this case, there is a chance 749 that the call is erroneously treated as unrelated. 751 Since, in these cases, no correlation information can be deduced from 752 the signaling, it is up to the implementation to decide how to 753 behave. One option is also to let the user decide whether to accept 754 the call as related, or to treat the call as unrelated. 756 5.4. Considerations for Usage of Existing SDP 758 5.4.1. Originator of the Session 760 According to SDP [RFC4566], the origin line in SDP has the following 761 syntax: 763 o= 764 766 Of interest here are the and fields, which 767 indicate the type of network and type of address, respectively. 768 Typically, this field carries the IP address of the originator of the 769 session. Even if the SDP was used to negotiate an audio or video 770 media stream transported over a circuit-switched bearer, the 771 originator is using SDP over an IP bearer. Therefore, and 772 fields in the "o=" line should be populated with the IP 773 address identifying the source of the signaling. 775 5.4.2. Contact information 777 SDP [RFC4566] defines the "p=" line which may include the phone 778 number of the person responsible for the conference. Even though 779 this line can carry a phone number, it is not suited for the purpose 780 of defining a connection address for the media. Therefore, we have 781 selected to define the PSTN specific connection addresses in the "c=" 782 line. 784 5.5. Considerations for Usage of Third Party Call Control (3PCC) 786 Best Current Practices for Third Party Call Control (3pcc) in the 787 Session Initiation Protocol (SIP) [RFC3725] outlines several flows 788 which are possible in third party call control scenarios and 789 recommends some flows for specific situations. 791 One of the assumptions in [RFC3725] is that an SDP Offer may include 792 a "black hole" connection address, which has the property that 793 packets sent to it will never leave the host which sent them. For 794 IPv4, this "black hole" connection address is 0.0.0.0, or a domain 795 name within the .invalid DNS top level domain. 797 When using an E.164 address scheme in the context of third-party call 798 control, when the User Agent needs to indicate an unknown phone 799 number, it MUST populate the of the SDP "c=" line with a 800 "-" string. 802 Note that this may result in the recipient of the initial Offer in 803 rejecting the Offer if the recipient of the Offer is not aware of 804 its own E.164 number, and thus concluding that it will not be 805 possible to establish a circuit-switched bearer since neither 806 party is aware of their E.164 number. 808 5.6. Offer/Answer mode extensions 810 In this section, we define extensions to the Offer/Answer model 811 defined in The Offer/Answer Model in SDP [RFC3264] and extended in 812 the SDP Capability Negotiation [RFC5939] to allow for PSTN addresses 813 to be used with the Offer/Answer model. 815 5.6.1. Generating the Initial Offer 817 The Offerer, wishing to use PSTN audio or video stream, MUST populate 818 the "c=" and "m=" lines as follows. 820 The endpoint MUST set the in the "c=" line to "PSTN", and 821 the to "E164". Furthermore, the endpoint SHOULD set the 822 field to its own international E.164 number 823 (with a leading "+"). If the endpoint is not aware of its own E.164 824 number, it MUST set the to "-". 826 In the "m=" line, the endpoint MUST set the subfield to 827 "audio" or "video", depending on the media type, and the sub- 828 field to "PSTN". The sub-field SHOULD be set to "9" (the 829 discard port). 831 The sub-field carries the payload type number(s) the endpoint 832 is wishing to use. Payload type numbers in this case refer to the 833 codecs that the endpoint wishes to use. For example, if the endpoint 834 wishes to use the GSM codec, it would add payload type number 3 in 835 the list of codecs. 837 For dynamic payload types, the endpoint MUST define the set of valid 838 encoding names and related parameters using the "a=rtpmap" attribute 839 line. See Section 6 of SDP [RFC4566] for details. 841 When generating the Offer, if the Offerer supports any of the 842 correlation mechanisms defined in this memo, it MUST include an 843 attribute line "a=cs-correlation" in the SDP offer. The "a=cs- 844 correlation" line contains an enumeration of the correlation 845 mechanisms supported by the Offerer, in the format of sub-fields. 847 The current list of sub-fields include "callerid", "uuie" and "dtmf" 848 and they refer to the correlation mechanisms defined in 849 Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. 851 If the Offerer supports any of the correlation mechanisms defined in 852 this memo, and is willing to become the active party, the Offerer 853 MUST add the "callerid", "uuie", and/or "dtmf" sub-fields and MUST 854 specify values for those sub-fields. 856 o the international E.164 number as the value in the "callerid" sub- 857 field, 859 o the contents of the User-User information element as the value of 860 the "uuie" sub-field, and/or 862 o the DTMF tone string as the value of the "dtmf" sub-field 864 If the Offerer is only able to become the passive party in the 865 circuit-switched bearer setup, it MUST add the "callerid", "uuie" 866 and/or "dtmf" sub-fields, but MUST NOT specify values for those sub- 867 fields. 869 For example, if the Offerer is willing to use the User-User 870 Information element and DTMF digit sending mechanisms, but can only 871 become the passive party, it includes the following lines to the SDP: 873 a=cs-correlation:uuie dtmf 875 a=setup:passive 877 If, on the other hand, the Offerer is willing to use the User-User 878 Information element and the DTMF correlation mechanisms, and is able 879 to become the active or passive side, it includes to following lines 880 to the SDP: 882 a=cs-correlation:uuie:2890W284hAT452612908awudfjang908 dtmf:14D*3 884 a=setup:actpass 886 The negotiation of the value of the 'setup' attribute takes place as 887 defined in Section 4.1 of TCP-Based Media Transport in the SDP 888 [RFC4145]. 890 The Offerer states which role or roles it is willing to perform; and 891 the Answerer, taking the Offerer's willingness into consideration, 892 chooses which roles both endpoints will actually perform during the 893 circuit-switched bearer setup. 895 By 'active' endpoint, we refer to an endpoint that will establish the 896 circuit-switched bearer; and by 'passive' endpoint, we refer to an 897 endpoint that will receive a circuit-switched bearer. 899 If an Offerer does not know its international E.164 number, it MUST 900 set the 'a=setup' attribute to the value 'active'. If the Offerer 901 knows its international E.164 number, it MUST set the value to either 902 'actpass' or 'passive'. 904 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 905 It indicates that the connection is not established for the time 906 being. 908 The Offerer uses the "a=connection" attribute to decide whether a new 909 circuit-switched bearer is to be established or not. For the initial 910 Offer, the Offerer MUST use value 'new'. 912 5.6.2. Generating the Answer 914 If the Offer contained a circuit-switched audio or video stream, the 915 Answerer first determines whether it is able to accept and use such 916 streams. If the Answerer is not willing to use circuit-switched 917 media for the session, it MUST construct an Answer where the port 918 number for such media stream(s) is set to zero, according to Section 919 6 of An Offer/Answer Model with the Session Description Protocol 920 (SDP) [RFC3264]. If the Answerer is willing to use circuit-switched 921 media for the session, it MUST ignore the received port number 922 (unless the port number is set to zero). 924 If the Offer included a "-" as the payload type number, it indicates 925 that the Offerer is not willing or able to define any specific 926 payload type. Most often, a "-" is expected to be used instead of 927 the payload type when the endpoint is not aware of or not willing to 928 define the codecs which will eventually be used on the circuit- 929 switched bearer. The circuit-switched signaling protocols have their 930 own means of negotiating or indicating the codecs, therefore an 931 Answerer SHOULD accept such Offers, and SHOULD set the payload type 932 to "-" also in the Answer. 934 If the Answerer explicitly wants to specify a codec for the circuit- 935 switched media, it MAY set the respective payload numbers in the 936 sub-field in the answer. This behavior, however, is NOT 937 RECOMMENDED. 939 When receiving the Offer, the Answerer MUST determine whether it 940 becomes the active or passive party. 942 If the SDP in the Offer indicates that the Offerer is only able to 943 become the active party, the Answerer needs to determine whether it 944 is able to become the passive party. If this is not possible e.g. 945 due to the Answerer not knowing its international E.164 number, the 946 Answerer MUST reject the circuit-switched media by setting the port 947 number to zero on the Answer. If the Answerer is aware of its 948 international E.164 number, it MUST include the "a=setup" attribute 949 in the Answer and set it to value "passive" or "holdconn". 951 If the SDP in the Offer indicates that the Offerer is only able to 952 become the passive party, the Answerer MUST verify that the Offerer's 953 E.164 number is included in the "c=" line of the Offer. If the 954 number is included, the Answerer MUST include the "a=setup" attribute 955 in the Answer and set it to value "active" or "holdconn". If the 956 number is not included, call establishment is not possible, and the 957 Answerer MUST reject the circuit-switched media by setting the port 958 number to zero in the Answer. 960 If the SDP in the Offer indicates that the Offerer is able to become 961 either the active or passive party, the Answerer needs to determine 962 which role it would like to take. If the Offer includes an 963 international E.164 number in the "c=" line, the Answerer SHOULD 964 become the active party. If the Offer does not include an E.164 965 number, and if the Answerer is aware of its international E.164 966 number, it MUST become the passive party. If the Offer does not 967 include an E.164 number in the "c=" line and the Answerer is not 968 aware of its E.164 number, it MUST reject the circuit-switched media 969 by setting the port number to zero in the Answer. 971 The Answerer MUST select those correlation mechanisms from the Offer 972 it supports, and include an "a=cs-correlation" attribute line in the 973 Answer containing those mechanisms it supports. The Answerer MUST 974 NOT add any mechanisms which were not included in the offer. 976 If the Answerer becomes the active party, it MUST add parameter 977 values to the "callerid", "uuie" or "dtmf" sub-fields. 979 If the Answerer becomes the passive party, it MUST NOT add values to 980 the "callerid", "uuie" and/or "dtmf" sub-fields. 982 After generating and sending the Answer, if the Answerer became the 983 active party, it 985 o MUST extract the E.164 number from the "c=" line of the Offer and 986 MUST establish a circuit-switched bearer to that address. 988 o if the SDP Answer contained a value for the "callerid" sub-field, 989 must set the Calling Party Number Information Element to that 990 number, 992 o if the SDP Answer contained a value for the "uuie" sub-field, MUST 993 send the User-User Information element according to the rules 994 defined for the circuit-switched technology used, and set the 995 value of the Information Element to that received in the SDP 996 Offer, 998 o if the SDP Answer contained a value for the "dtmf" sub-field, MUST 999 send those DTMF digits according to the circuit-switched 1000 technology used. 1002 If, on the other hand, the Answerer became the passive party, it 1004 o MUST be prepared to receive a circuit-switched bearer, 1006 o if the Offer contained a value for the "callerid" sub-field, MUST 1007 compare that value to the Calling Party Number Information Element 1008 of the circuit-switched bearer, 1010 o if the Offer contained a value for the "dtmf" sub-field, MUST be 1011 prepared to receive and collect DTMF digits once the circuit- 1012 switched bearer is set up. The Answerer MUST compare the received 1013 DTMF digits to the value of the "dtmf" sub-field. If the received 1014 DTMF digits match the value of the "dtmf" sub-field in the "cs- 1015 correlation" attribute, the call SHOULD be treated as correlated 1016 to the ongoing session. 1018 o if the Offer contained a value for the "uuie" sub-field, MUST be 1019 prepared to receive a User-User Information element once the 1020 circuit-switched bearer is set up. The Answerer MUST compare the 1021 received UUI to the value of the "uuie" sub-field. If the value 1022 of the received UUI matches the value of the "uuie" sub-field, the 1023 call SHOULD be treated as correlated to the ongoing session. 1025 5.6.3. Offerer processing the Answer 1027 When receiving the Answer, if the SDP does not contain "a=cs- 1028 correlation" attribute line, the Offerer should take that as an 1029 indication that the other party does not support or is not willing to 1030 use the procedures defined in the document for this session, and MUST 1031 revert to normal processing of SDP. 1033 When receiving the Answer, the Offerer MUST first determine whether 1034 it becomes the active or passive party, as described in Section 1035 Section 5.3.1. 1037 If the Offerer becomes the active party, it 1039 o MUST extract the E.164 number from the "c=" line and MUST 1040 establish a circuit-switched bearer to that address. 1042 o if the SDP Answer contained a value for the "uuie" sub-field, MUST 1043 send the User-User Information element according to the rules 1044 defined for the circuit-switched technology used, and set the 1045 value of the Information Element to that received in the SDP 1046 Answer, 1048 o if the SDP Answer contained a value for the "dtmf" sub-field, MUST 1049 send those DTMF digits according to the circuit-switched 1050 technology used. 1052 If the Offerer becomes the passive party, it 1054 o MUST be prepared to receive a circuit-switched bearer, 1056 o if the Answer contained a value for the "dtmf" sub-field, MUST be 1057 prepared to receive and collect DTMF digits once the circuit- 1058 switched bearer is set up. The Offerer SHOULD compare the 1059 received DTMF digits to the value of the "dtmf" sub-field. If the 1060 received DTMF digits match the value of the "dtmf" sub-field in 1061 the "cs-correlation" attribute, the call SHOULD be treated as 1062 correlated to the ongoing session. 1064 o if the Answer contained a value for the "uuie" sub-field, MUST be 1065 prepared to receive a User-User Information element once the 1066 circuit-switched bearer is set up. The Offerer SHOULD compare the 1067 received UUI to the value of the "uuie" sub-field. If the value 1068 of the received UUI matches the value of the "uuie" sub-field, the 1069 call SHOULD be treated as correlated to the ongoing session. 1071 5.6.4. Modifying the session 1073 If, at a later time, one of the parties wishes to modify the session, 1074 e.g., by adding new media stream, or by changing properties used on 1075 an existing stream, it may do so via the mechanisms defined for An 1076 Offer/Answer Model with SDP [RFC3264]. 1078 If there is an existing circuit-switched bearer between the 1079 endpoints, and the Offerer wants to reuse that the Offerer MUST set 1080 the value of the "a=connection" attribute to 'existing'. 1082 If either party removes the circuit-switched media from the session 1083 (by setting the port number to zero), it MUST terminate the circuit- 1084 switched bearer using whatever mechanism is appropriate for the 1085 technology in question. 1087 If either party wishes to drop and reestablish an existing call, that 1088 party MUST first remove the circuit-switched media from the session 1089 by setting the port number to zero, and then use another Offer/Answer 1090 exchange where it MUST set the "a=connection" attribute to 'new'". 1091 If the media types are different (for example, a different codec will 1092 be used for the circuit-switched bearer), the media descriptions for 1093 terminating the existing bearer and the new bearer can be in the same 1094 Offer. 1096 5.7. Formal Syntax 1098 The following is the formal Augmented Backus-Naur Form (ABNF) 1099 [RFC5234] syntax that supports the extensions defined in this 1100 specification. The syntax is built above the SDP [RFC4566] grammar. 1101 Implementations according to this specification MUST be compliant 1102 with this syntax. 1104 Figure 2 shows the formal syntax of the extensions defined in this 1105 memo. 1107 ; extension to the connection field originally specified 1108 ; in RFC 4566 1110 connection-field = [%x63 "=" nettype SP addrtype SP 1111 connection-address CRLF] 1112 ;nettype and addrtype are defined in RFC 4566 1114 connection-address /= e164-address / "-" 1115 e164-address = "+" 1*15DIGIT 1116 ; DIGIT is specified in RFC 5234 1118 ;subrules for correlation attribute 1119 attribute /= cs-correlation-attr 1120 ; attribute defined in RFC 4566 1121 cs-correlation-attr= "cs-correlation:" corr-mechanisms 1122 corr-mechanisms = corr-mech *(SP corr-mech) 1123 corr-mech = caller-id-mech / uuie-mech / 1124 dtmf-mech / ext-mech 1125 caller-id-mech = "callerid" [":" caller-id-value] 1126 caller-id-value = "+" 1*15DIGIT 1127 uuie-mech = "uuie" [":" uuie-value] 1128 uuie-value = 1*32(ALPHA/DIGIT) 1129 dtmf-mech = "dtmf" [":" dtmf-value] 1130 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1131 ;0-9, A-D, '#' and '*' 1132 ext-mech = ext-mech-name[":" ext-mech-value] 1133 ext-mech-name = token 1134 ext-mech-value = token 1135 ; token is specified in RFC4566 1137 Figure 2: Syntax of the SDP extensions 1139 6. Examples 1141 In the examples below, where an SDP line is too long to be displayed 1142 as a single line, a braking character "\" indicates continuation in 1143 the following line. Note that this is character is included for 1144 displaying purposes. Implementation MUST write a single line without 1145 brakes. 1147 6.1. Single PSTN audio stream 1149 Alice Bob 1150 | | 1151 | (1) SDP Offer (PSTN audio) | 1152 |--------------------------------->| 1153 | | 1154 | (2) SDP Answer (PSTN audio) | 1155 |<---------------------------------| 1156 | | 1157 | PSTN call setup | 1158 |<---------------------------------| 1159 | | 1160 |<==== media over PSTN bearer ====>| 1161 | | 1163 Figure 3: Basic flow 1165 Figure 3 shows a basic example that describes a single audio media 1166 stream over a circuit-switched bearer. Alice generates a SDP Offer 1167 which is show in Figure 4. The Offer describes a PSTN circuit- 1168 switched bearer in the "m=" and "c=" line where it also indicates its 1169 international E.164 number format. Additionally, Alice expresses 1170 that she can initiate the circuit-switched bearer or be the recipient 1171 of it in the "a=setup" attribute line. The SDP Offer also includes a 1172 correlation identifiers that this endpoint will be inserting the 1173 Calling Party Number and/or User-User Information Element of the PSTN 1174 call setup if eventually this endpoint initiates the PSTN call. 1176 v=0 1177 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5 1178 s= 1179 t=0 0 1180 m=audio 9 PSTN - 1181 c=PSTN E164 +35891234567 1182 a=setup:actpass 1183 a=connection:new 1184 a=cs-correlation:callerid:+15551234 \ 1185 uuie:2890W284hAT452612908awudfjang908 1187 Figure 4: SDP offer (1) 1189 Bob generates a SDP Answer (Figure 5), describing a PSTN audio media 1190 on port 9 without information on the media sub-type on the "m=" line. 1191 The "c=" line contains Bob's international E.164 number. In the 1192 "a=setup" line Bob indicates that he is willing to become the active 1193 endpoint when establishing the PSTN call, and he also includes the 1194 "a=cs-correlation" attribute line containing the values he is going 1195 to include in the Calling Party Number and User-User IE of the PSTN 1196 call establishment. 1198 v=0 1199 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1200 s= 1201 t=0 0 1202 m=audio 9 PSTN - 1203 c=PSTN E164 +35897654321 1204 a=setup:active 1205 a=connection:new 1206 a=cs-correlation:callerid:+15554321 \ 1207 uuie:2127W49uThi455916adjfhtow9619361 1209 Figure 5: SDP Answer with circuit-switched media 1211 When Alice receives the Answer, she examines that Bob is willing to 1212 become the active endpoint when setting up the PSTN call. Alice 1213 temporarily stores Bob's E.164 number and the User-User IE value of 1214 the "cs-correlation" attribute, and waits for a circuit-switched 1215 bearer establishment. 1217 Bob initiates a circuit-switched bearer using whatever circuit- 1218 switched technology is available for him. The called party number is 1219 set to Alice's number, and calling party number is set to Bob's own 1220 number. Bob also sets the User-User Information Element value to the 1221 on contained in the SDP Answer. 1223 When Alice receives the circuit-switched bearer establishment, she 1224 examines the UUIE and the calling party number, and by comparing 1225 those received during O/A exchange determines that the call is 1226 related to the SDP session. 1228 It may also be that neither the UUIE nor the calling party number is 1229 received by the called party, or the format of the calling party 1230 number is changed by the PSTN. Implementations may still accept such 1231 call establishment attempts as being related to the session that was 1232 established in the IP network. As it cannot be guaranteed that the 1233 values used for correlation are always passed intact through the 1234 network, they should be treated as additional hints that the circuit- 1235 switched bearer is actually related to the session. 1237 6.2. Advanced SDP example: Circuit-Switched Audio and Video Streams 1239 Alice Bob 1240 | | 1241 | (1) SDP Offer (PSTN audio and video) | 1242 |------------------------------------------->| 1243 | | 1244 | (2) SDP Answer (PSTN audio) | 1245 |<-------------------------------------------| 1246 | | 1247 | PSTN call setup | 1248 |<-------------------------------------------| 1249 | | 1250 |<======== media over PSTN bearer ==========>| 1251 | | 1253 Figure 6: Circuit-Switched Audio and Video streams 1255 Figure 6 shows an example of negotiating audio and video media 1256 streams over circuit-switched bearers. 1258 v=0 1259 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5 1260 s= 1261 t=0 0 1262 a=setup:actpass 1263 a=connection:new 1264 a=cs-correlation:dtmf:112233 1265 c=PSTN E164 +35891234567 1266 m=audio 9 PSTN - 1267 m=video 9 PSTN 34 1268 a=rtpmap:34 H263/90000 1270 Figure 7: SDP offer with circuit-switched audio and video (1) 1272 Upon receiving the SDP offer descibed in Figure 7, Bob rejects the 1273 video stream as his device does not currently support video, but 1274 accepts the circuit-switched audio stream. As Alice indicated that 1275 she is able to become either the active, or passive party, Bob gets 1276 to select which role he would like to take. Since the Offer 1277 contained the international E.164 number of Alice, Bob decides that 1278 he becomes the active party in setting up the circuit-switched 1279 bearer. Bob includes a new value in the cs-correlation:dtmf sub- 1280 field, which he is going send as DTMF tones once the bearer setup is 1281 complete. The Answer is described in Figure 8 1282 v=0 1283 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1284 s= 1285 t=0 0 1286 a=setup:active 1287 a=connection:new 1288 a=cs-correlation:dtmf:332211 1289 c=PSTN E164 +35897654321 1290 m=audio 9 PSTN - 1291 m=video 0 PSTN 34 1293 Figure 8: SDP answer with circuit-switched audio and video (2) 1295 7. Security Considerations 1297 This document provides an extension on top of RFC 4566 [RFC4566], and 1298 RFC 3264 [RFC3264]. As such, the security considerations of those 1299 documents apply. 1301 This memo provides mechanisms to agree on a correlation identifier or 1302 identifiers that are used to evaluate whether an incoming circuit- 1303 switched bearer is related to an ongoing session in the IP domain. 1304 If an attacker replicates the correlation identifer and establishes a 1305 call within the time window the receiving endpoint is expecting a 1306 call, the attacker may be able to hijack the circuit-switched bearer. 1307 These types of attacks are not specific to the mechanisms presented 1308 in this memo. For example, caller ID spoofing is a well known attack 1309 in the PSTN. Users are advised to use the same caution before 1310 revealing sensitive information as they would on any other phone 1311 call. Furthermore, users are advised that mechanisms that may be in 1312 use in the IP domain for securing the media, like Secure RTP (SRTP) 1313 [RFC3711], are not available in the CS domain. 1315 For the purposes of establishing a circuit-switched bearer, the 1316 active endpoint needs to know the passive endpoint's phone number. 1317 Phone numbers are sensitive information, and some people may choose 1318 not to reveal their phone numbers when calling using supplementary 1319 services like Calling Line Identification Restriction (CLIR) in GSM. 1320 Implementations should take the caller's preferences regarding 1321 calling line identification into account if possible, by restricting 1322 the inclusion of the phone number in SDP "c=" line if the caller has 1323 chosen to use CLIR. If this is not possible, implementations may 1324 present a prompt informing the user that their phone number may be 1325 transmitted to the other party. 1327 Similarly as with IP addresses, if there is a desire the protect the 1328 SDP containing phone numbers carried in SIP, implementers are adviced 1329 to follow the security mechanisms defined in [RFC3261]. 1331 8. IANA Considerations 1333 This document instructs IANA to register a number of SDP tokens 1334 according to the following data. 1336 8.1. Registration of new cs-correlation SDP attribute 1338 Contact: Miguel Garcia 1340 Attribute name: cs-correlation 1342 Long-form attribute name: PSTN Correlation Identifier 1344 Type of attribute: media level only 1346 This attribute is subject to the charset attribute 1348 Description: This attribute provides the Correlation Identifier 1349 used in PSTN signaling 1351 Specification: RFC XXXX 1353 The IANA is requested the create a subregistry for 'cs-correlation' 1354 attribute under the Session Description Protocol (SDP) Parameters 1355 registry. The initial values for the subregistry are presented in 1356 the following, and IANA is requested to add them into its database: 1358 Value of 'cs-correlation' attribute Reference Description 1359 ----------------------------------- --------- ----------- callerid 1360 RFC XXXX Caller ID uuie RFC XXXX User-User Information Element dtmf 1361 RFC XXXX Dual-tone Multifrequency 1363 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1364 reference to the RFC number of this draft. 1366 As per the terminology in [RFC2434], the registration policy for new 1367 values of 'cs-correlation' parameter is 'Specification Required'. 1369 8.2. Registration of a new "nettype" value 1371 This memo provides instructions to IANA to register a new "nettype" 1372 in the Session Description Protocol Parameters registry [1]. The 1373 registration data, according to RFC 4566 [RFC4566] follows. 1375 Type SDP Name Reference 1376 ---- ------------------ --------- 1377 nettype PSTN [RFCxxxx] 1379 8.3. Registration of new "addrtype" values 1381 This memo provides instructions to IANA to register a new "addrtype" 1382 in the Session Description Protocol Parameters registry [1]. The 1383 registration data, according to RFC 4566 [RFC4566] follows. 1385 Type SDP Name Reference 1386 ---- ------------------ --------- 1387 addrtype E164 [RFCxxxx] 1388 - [RFCxxxx] 1390 8.4. Registration of a new "proto" value 1392 This memo provides instructions to IANA to register a new "proto" in 1393 the Session Description Protocol Parameters registry [1]. The 1394 registration data, according to RFC 4566 [RFC4566] follows. 1396 Type SDP Name Reference 1397 -------------- --------------------------- --------- 1398 proto PSTN [RFCxxxx] 1400 9. Acknowledgments 1402 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas 1403 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan 1404 Rosenberg, Ingemar Johansson, Christer Holmberg, and Alf Heidermark 1405 for providing their insight and comments on this document. 1407 10. References 1409 10.1. Normative References 1411 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1412 Requirement Levels", BCP 14, RFC 2119, March 1997. 1414 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1415 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 1416 October 1998. 1418 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1419 Session Description Protocol (SDP) for ATM Bearer 1420 Connections", RFC 3108, May 2001. 1422 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1423 with Session Description Protocol (SDP)", RFC 3264, 1424 June 2002. 1426 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1427 the Session Description Protocol (SDP)", RFC 4145, 1428 September 2005. 1430 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1431 Description Protocol", RFC 4566, July 2006. 1433 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1434 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1436 [RFC5939] Andreasen, F., "Session Description Protocol (SDP) 1437 Capability Negotiation", RFC 5939, September 2010. 1439 10.2. Informative References 1441 [ITU.E164.1991] 1442 International Telecommunications Union, "The International 1443 Public Telecommunication Numbering Plan", ITU- 1444 T Recommendation E.164, 1991. 1446 [ITU.Q931.1998] 1447 "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN 1448 User - Network Interface Layer 3 Specification for Basic 1449 Call Control", ISO Standard 9594-1, May 1998. 1451 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1452 A., Peterson, J., Sparks, R., Handley, M., and E. 1453 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1454 June 2002. 1456 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1457 Jacobson, "RTP: A Transport Protocol for Real-Time 1458 Applications", STD 64, RFC 3550, July 2003. 1460 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1461 Video Conferences with Minimal Control", STD 65, RFC 3551, 1462 July 2003. 1464 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1465 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1466 RFC 3711, March 2004. 1468 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1469 Camarillo, "Best Current Practices for Third Party Call 1470 Control (3pcc) in the Session Initiation Protocol (SIP)", 1471 BCP 85, RFC 3725, April 2004. 1473 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1474 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1476 [TS.24.008] 1477 3GPP, "Mobile radio interface Layer 3 specification; Core 1478 network protocols; Stage 3", 3GPP TS 24.008 3.20.0, 1479 December 2005. 1481 URIs 1483 [1] 1485 Authors' Addresses 1487 Miguel A. Garcia-Martin 1488 Ericsson 1489 Calle Via de los Poblados 13 1490 Madrid, ES 28033 1491 Spain 1493 Email: miguel.a.garcia@ericsson.com 1495 Simo Veikkolainen 1496 Nokia 1497 P.O. Box 407 1498 NOKIA GROUP, FI 00045 1499 Finland 1501 Phone: +358 50 486 4463 1502 Email: simo.veikkolainen@nokia.com