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Garcia-Martin 3 Internet-Draft Ericsson 4 Intended status: Standards Track S. Veikkolainen 5 Expires: October 27, 2013 Nokia 6 April 25, 2013 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-18 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 October 27, 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 . . . . . . . . . . . . . . . . 25 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 . . . . . . . . . . . . . . . . . . . 30 92 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 93 8.1. Registration of new cs-correlation SDP attribute . . . . 31 94 8.2. Registration of a new "nettype" value . . . . . . . . . . 32 95 8.3. Registration of new "addrtype" values . . . . . . . . . . 32 96 8.4. Registration of a new "proto" value . . . . . . . . . . . 33 98 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 99 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 100 10.1. Normative References . . . . . . . . . . . . . . . . . . 33 101 10.2. Informative References . . . . . . . . . . . . . . . . . 34 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 104 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, Alice 235 is located in an environment where she has access to both IP and 236 circuit-switched bearers for communicating with other endpoints. 237 Alice decides that the circuit-switched bearer offers a better 238 perceived quality of service for voice, and issues an SDP Offer 239 containing the description of an audio media stream over circuit- 240 switched bearer. 242 Alice Bob 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 Bob receives the SDP offer and determines that he is located in an 259 environment where the IP based bearer is not suitable for real-time 260 audio media. However he also has PSTN circuit-switched bearer 261 available for audio. Bob generates an SDP answer containing a 262 description of the audio media stream over a circuit-switched bearer. 264 During the offer-answer exchange Alice and Bob also agree the 265 direction in which the circuit-switched bearer should be established. 266 In this example, Bob becomes the active party, in other words, he 267 establishes the circuit-switched call to the other endpoint. The 268 Offer/Answer exchange contains identifiers or references that can be 269 used on the circuit-switched network for addressing the other 270 endpoint, as well as information that is used to determine that the 271 incoming circuit-switched bearer establishment is related to the 272 ongoing session between Alice and Bob. 274 Bob establishes a circuit-switched bearer towards Alice using 275 whatever mechanisms are defined for the network type in question. 276 When receiving the incoming circuit-switched connection attempt, 277 Alice is able to determine that the attempt is related to the session 278 she is just establishing with Bob. 280 Alice accepts the circuit-switched connection; the circuit-switched 281 bearer setup is completed. Bob and Alice can now use the circuit- 282 switched connection for two-way audio media. 284 If, for some reason, Bob would like to reject the offered stream, he 285 would set the port number of the specific stream to zero, as 286 specified in RFC3264 [RFC3264]. Also, if Bob does not understand 287 some of the SDP attributes specified in this document, he would 288 ignore them, as specified in RFC4566 [RFC4566]. 290 5. Protocol Description 292 5.1. Level of Compliance 294 Implementations according to this specification MUST implement the 295 SDP extensions described in Section 5.2, and MUST implement the 296 considerations discussed in Section 5.3, Section 5.4 and Section 5.6. 298 5.2. Extensions to SDP 300 This section provides the syntax and semantics of the extensions 301 required for providing a description of audio or video media streams 302 over circuit-switched bearers in SDP. 304 5.2.1. Connection Data 306 According to SDP [RFC4566], the connection data line in SDP has the 307 following syntax: 309 c= 311 where indicates the network type, indicates the 312 address type, and the is the connection address, 313 which is dependent on the address type. 315 At the moment, the only network type defined is "IN", which indicates 316 Internet network type. The address types "IP4" and "IP6" indicate 317 the type of IP addresses. 319 This memo defines a new network type for describing a circuit- 320 switched bearer network type in the PSTN. The mnemonic "PSTN" is 321 used for this network type. 323 For the address type, we initially consider the possibility of 324 describing E.164 telephone numbers. We define a new "E164" address 325 type to be used within the context of a "PSTN" network type. The 326 "E164" address type indicates that the connection address contains an 327 E.164 number represented according to the ITU-T E.164 [ITU.E164.1991] 328 recommendation. 330 It is a common convention that an international E.164 number contains 331 a leading '+' sign. For consistency's sake, we also require the 332 E.164 telephone is prepended with a '+', even if that is not 333 necessary for routing of the call in the PSTN network. 335 There are cases, though, when the endpoint is merely aware of a 336 circuit-switched bearer, without having further information about the 337 address type or the E.164 number allocated to it. In these cases a 338 dash ("-") is used to indicate an unknown address type or connection 339 address. This makes the connection data line be according to the SDP 340 syntax. 342 Please note that these "E164" and "-" address types defined in this 343 memo are exclusively defined to be used in conjunction with the 344 "PSTN" network type in accordance with [RFC4566]. Usage of "E164" or 345 "-" address types in conjunction with other network types may be 346 defined elsewhere. 348 This memo exclusively uses the international representation of E.164 349 numbers, i.e., those including a country code and, as described above 350 prepended with a '+' sign. Implementations conforming to this 351 specification and using the "E164" address type together with the 352 "PSTN" network type MUST use the 'global-number-digits' construction 353 specified in RFC 3966 [RFC3966] for representing international E.164 354 numbers. This representation requires the presence of the '+' sign, 355 and additionally allows for the presence of one or more 'visual- 356 separator' constructions for easier human readability (see 357 Section 5.7). 359 Note that and/or MUST NOT be omitted 360 when unknown since this would violate basic syntax of SDP [RFC4566]. 361 In such cases, they MUST be set to a "-". 363 The following are examples of the extension to the connection data 364 line: 366 c=PSTN E164 +441134960123 368 c=PSTN - - 370 When the is E164, the connection address is defined as 371 follows: 373 o an international E.164 number 375 When the is "-", the connection address is defined as 376 follows: 378 o the value "-", signifying that the address is unknown 380 o any value resulting from the production rule of connection-address 381 in RFC4566 [RFC4566], but in all cases any value encountered will 382 be ignored. 384 5.2.2. Media Descriptions 385 According to SDP [RFC4566], the media description line in SDP has the 386 following syntax: 388 m= ... 390 The subfield carries the media type. For establishing an 391 audio bearer, the existing "audio" media type is used. For 392 establishing a video bearer, the existing "video" media type is used. 394 The subfield is the transport port to which the media stream 395 is sent. Circuit-switched access lacks the concept of a port number, 396 and therefore the subfield does not carry any meaningful 397 value. In order to be compliant with SDP syntax, implementations 398 SHOULD set the subfield to the discard port value "9" and MUST 399 ignore it on reception. 401 According to RFC 3264 [RFC3264], a port number of zero in the offer 402 of a unicast stream indicates that the stream is offered but must not 403 be used. If a port number of zero is present in the answer of a 404 unicast stream, it indicates that the stream is rejected. These 405 rules are still valid when the media line in SDP represents a 406 circuit-switched bearer. 408 The subfield is the transport protocol. The circuit-switched 409 bearer uses whatever transport protocol it has available. This 410 subfield SHOULD be set to the mnemonic "PSTN" to be syntactically 411 correct with SDP [RFC4566] and to indicate the usage of circuit- 412 switched protocols in the PSTN. 414 The subfield is the media format description. In the classical 415 usage of SDP to describe RTP-based media streams, when the 416 subfield is set to "RTP/AVP" or "RTP/SAVP", the subfield 417 contains the payload types as defined in the RTP audio profile 418 [RFC3551]. 420 When "RTP/AVP" is used in the field, the subfield 421 contains the RTP payload type numbers. We use the subfield to 422 indicate the list of available codecs over the circuit-switched 423 bearer, by re-using the conventions and payload type numbers defined 424 for RTP/AVP. The RTP audio and video media types, which, when 425 applied to PSTN circuit-switched bearers, represent merely an audio 426 or video codec. If the endpoint is able to determine the list of 427 available codecs for circuit-switched media streams, it MUST use the 428 corresponding payload type numbers in the subfield. 430 In some cases, the endpoint is not able to determine the list of 431 available codecs for circuit-switched media streams. In this case, 432 in order to be syntactically compliant with SDP [RFC4566], the 433 endpoint MUST include a single dash ("-") in the subfield. 435 As per RFC 4566 [RFC4566], the media format descriptions are listed 436 in priority order. 438 Examples of media descriptions for circuit-switched audio streams 439 are: 441 m=audio 9 PSTN 3 0 8 443 m=audio 9 PSTN - 445 Similarly, an example of a media description for circuit-switched 446 video stream is: 448 m=video 9 PSTN 34 450 m=video 9 PSTN - 452 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 454 The endpoints should be able to correlate the circuit-switched bearer 455 with the session negotiated with SDP in order to avoid ringing for an 456 incoming circuit-switched bearer that is related to the session 457 controlled with SDP (and SIP). 459 Several alternatives exist for performing this correlation. This 460 memo provides three mutually non-exclusive correlation mechanisms. 461 Other correlation mechanisms may exist, and their usage will be 462 specified when need arises. All mechanisms share the same principle: 463 some unique information is sent in the SDP and in the circuit- 464 switched signaling protocol. If these pieces of information match, 465 then the circuit-switched bearer is part of the session described in 466 the SDP exchange. Otherwise, there is no guarantee that the circuit- 467 switched bearer is related to such session. 469 The first mechanism is based on the exchange of PSTN caller-ID 470 between the endpoints. The caller-ID is also available as the 471 Calling Party ID in the circuit-switched signaling. 473 The second mechanism is based on the inclusion in SDP of a value that 474 is also sent in the User-to-User Information Element that is part of 475 the bearer setup signaling in the PSTN. 477 The third mechanism is based on sending in SDP a string that 478 represents Dual Tone MultiFrequency (DTMF) digits that will be later 479 sent right after the circuit-switched bearer is established. 480 Implementations MAY use any of these mechanisms and MAY use two or 481 more mechanisms simultaneously. 483 5.2.3.1. The "cs-correlation" attribute 485 In order to provide support for the correlation mechanisms, we define 486 a new media-level SDP attribute called "cs-correlation". This "cs- 487 correlation" attribute can include any of the "callerid", "uuie", or 488 "dtmf" subfields, which specify additional information required by 489 the Caller-ID, User to User Information, or DTMF correlation 490 mechanisms, respectively. The list of correlation mechanisms may be 491 extended by other specifications, see Section 5.2.3.5 for more 492 details. There MUST be at most one "cs-correlation" attribute per 493 media description. 495 The following sections provide more detailed information of these 496 subfields. 498 The values "callerid", "uuie" and "dtmf" refer to the correlation 499 mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and 500 Section 5.2.3.4, respectively. The formal Augmented Backus-Naur 501 Format (ABNF) syntax of the "cs-correlation" attribute is presented 502 in Section 5.7. 504 5.2.3.2. Caller-ID Correlation Mechanism 506 The Caller-ID correlation mechanisms consists of an exchange of the 507 calling party number as an international E.164 number in SDP, 508 followed by the availability of the Calling Party Number information 509 element in the call setup signaling of the circuit switched 510 connection. If both pieces of information match, the circuit- 511 switched bearer is correlated to the session described in SDP. 513 Example of inclusion of an international E.164 number in the "cs- 514 correlation" attribute is: 516 a=cs-correlation:callerid:+441134960123 518 The presence of the "callerid" subfield indicates that the endpoint 519 supports use of the calling party number as a means of correlating a 520 PSTN call with the session being negotiated. The "callerid" subfield 521 MAY be accompanied by the international E.164 number of the party 522 inserting the parameter. 524 Note that there are no guarantees that this correlation mechanism 525 works or is even available, due a number of problems: 527 o The endpoint might not be aware of its own E.164 number, in which 528 case it cannot populate the SDP appropriately. 530 o The Calling Party Number information element in the circuit- 531 switched signaling might not be available, e.g., due to policy 532 restrictions of the network operator or caller restriction due to 533 privacy. 535 o The Calling Party Number information element in the circuit- 536 switched signaling might be available, but the digit 537 representation of the E.164 number might differ from the one 538 expressed in the SDP, due to, e.g., lack of of country code. To 539 mitigate this problem implementations should consider only some of 540 the rightmost digits from the E.164 number for correlation. For 541 example, the numbers +44-113-496-0123 and 0113-496-0123 could be 542 considered as the same number. This is also the behavior of some 543 cellular phones, which correlate the incoming calling party with a 544 number stored in the phone book, for the purpose of displaying the 545 caller's name. Please refer to ITU-T E.164 reccommendation 546 [ITU.E164.1991] for consideration of the relevant number of digits 547 to consider. 549 5.2.3.3. User-User Information Element Correlation Mechanism 551 A second correlation mechanism is based on including in SDP a string 552 that represents the User-User Information Element that is part of the 553 call setup signaling of the circuit-switched bearer. The User-User 554 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 555 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 556 Element has a maximum size of 35 or 131 octets, depending on the 557 actual message of the PSTN protocol where it is included and the 558 network settings. 560 The mechanism works as follows: An endpoint creates a User-User 561 Information Element, according to the requirements of the call setup 562 signaling protocol. The same value is included in the SDP offer or 563 SDP answer, in the "uuie" subfield of the "cs-correlation" attribute. 564 When the SDP Offer/Answer exchange is completed, each endpoint has 565 become aware of the value that will be used in the User-User 566 Information Element of the call setup message of the PSTN protocol. 567 The endpoint that initiates the call setup attempt includes this 568 value in the User-User Information Element. The recipient of the 569 call setup attempt can extract the User-User Information Element and 570 correlate it with the value previously received in the SDP. If both 571 values match, then the call setup attempt corresponds to that 572 indicated in the SDP. 574 According to ITU-T Q.931 [ITU.Q931.1998], the User-User Information 575 Element (UUIE) identifier is composed of a first octet identifying 576 this as a User-User Information Element, a second octet containing 577 the Length of the user-user contents, a third octet containing a 578 Protocol Discriminator, and a value of up to 32 or 128 octets 579 (depending on network settings) containing the actual User 580 Information (see Figure 4-36 in ITU-T Q.931). The first two octets 581 of the UUIE MUST NOT be used for correlation, only the octets 582 carrying the Protocol Discriminator and the User Information value 583 are input to the creation of the value of the "uuie" subfield in the 584 "cs-correlation" attribute. Therefore, the value of the "uuie" 585 subfield in the "cs-correlation" attribute MUST start with the 586 Protocol Discriminator octet, followed by the User Information 587 octets. The value of the Protocol Discriminator octet is not 588 specified in this document; it is expected that organizations using 589 this technology will allocate a suitable value for the Protocol 590 Discriminator. 592 Once the binary value of the "uuie" subfield in the "cs-correlation" 593 attribute is created, it MUST be base 16 (also known as "hex") 594 encoded before it is inserted in SDP. Please refer to RFC 4648 595 [RFC4648] for a detailed description of base 16 encoding. The 596 resulting encoded value needs to have an even number of hexadecimal 597 digits, and MUST be considered invalid if it has an odd number. 599 Note that the encoding of the "uuie" subfield of the "cs- 600 correlation" attribute is largely inspired by the encoding of the 601 same value in the User-to-User header field in SIP, according to 602 the document "A Mechanism for Transporting User to User Call 603 Control Information in SIP" [I-D.ietf-cuss-sip-uui]. 605 As an example, an endpoint willing to send a UUIE containing a 606 protocol discriminator with the hexadecimal value of %x56 and an 607 hexadecimal User Information value of %xA390F3D2B7310023 would 608 include a "cs-correlation" attribute line as follows: 610 a=cs-correlation:uuie:56A390F3D2B7310023 612 Note that, for correlation purposes, the value of the User-User 613 Information Element is considered as an opaque string and only used 614 for correlation purposes. Typically call signaling protocols impose 615 requirements on the creation of User-User Information Element for 616 end-user protocol exchange. The details regarding the generation of 617 the User-User Information Element are outside the scope of this 618 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) tones 636 over the circuit-switched bearer once this bearer is established. If 637 the DTMF digit sequence received through the circuit-switched bearer 638 matches the digit string negotiated in the SDP, the circuit-switched 639 bearer is correlated with the session described in the SDP. The 640 mechanism is similar to many voice conferencing systems which require 641 the user to enter a PIN code using DTMF tones in order to be accepted 642 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 699 In order to avoid a situation where both endpoints attempt to 700 initiate a connection simultaneously, the direction in which the 701 circuit-switched bearer is set up MUST be negotiated during the Offer 702 /Answer exchange. 704 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 705 agree which endpoint acts as the active endpoint when initiating a 706 TCP connection. While RFC 4145 [RFC4145] was originally designed for 707 establishing TCP connections, it can be easily extrapolated to the 708 connection establishment of circuit-switched bearers. This 709 specification uses the concepts specified in RFC 4145 [RFC4145] for 710 agreeing on the direction of establishment of a circuit-switched 711 bearer. 713 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 714 "connection". The "setup" attribute indicates which of the endpoints 715 should initiate the connection establishment of the PSTN circuit- 716 switched bearer. Four values are defined in Section 4 of RFC 4145 717 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 718 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 719 attribute. 721 The "connection" attribute indicates whether a new connection is 722 needed or an existing connection is reused. The attribute can take 723 the values "new" or "existing". Please refer to Section 5 of RFC 724 4145 [RFC4145] for a detailed description of this attribute. 726 Implementations according to this specification MUST support the 727 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 728 but applied to circuit-switched bearers in the PSTN. 730 We define the active party as the one that initiates the circuit- 731 switched bearer after the Offer/Answer exchange. The passive party 732 is the one receiving the circuit-switched bearer. Either party may 733 indicate its desire to become the active or passive party during the 734 Offer/Answer exchange using the procedures described in Section 5.6. 736 5.3.2. Populating the cs-correlation attribute 738 By defining values for the subfields in the "a=cs-correlation" 739 attribute, the endpoint indicates that it is willing to become the 740 active party, and that it can use those values in the Calling party 741 number, User-User Information Element, or as DTMF tones during the 742 circuit-switched bearer setup. 744 Thus, the following rules apply: 746 An endpoint that can only become the active party in the circuit- 747 switched bearer setup MUST include all correlation mechanisms it 748 supports in the "a=cs-correlation" attribute, and MUST also 749 specify values for the subfields. 751 An endpoint that can only become the passive party in the circuit- 752 switched bearer setup MUST include all correlation mechanisms it 753 supports in the "a=cs-correlation" attribute, but MUST NOT specify 754 values for the subfields. 756 An endpoint that is willing to become either the active or passive 757 party (by including the "a=setup:actpass" attribute in the Offer), 758 MUST include all correlation mechanisms it supports in the "a=cs- 759 correlation" attribute, and MUST also specify values for the 760 subfields. 762 5.3.3. Considerations on correlations 764 Passive endpoints should expect an incoming CS call for setting up 765 the audio bearer. Passive endpoints MAY suppress the incoming CS 766 alert during a certain time periods. Additional restrictions can be 767 applied, such as the passive endpoint not alerting incoming calls 768 originated from the number that was observed during the offer/answer 769 negotiation. 771 Note that it cannot be guaranteed that any given correlation 772 mechanism will succeed even if the usage of those was agreed 773 beforehand. This is due to the fact that the correlation mechanisms 774 require support from the circuit-switched bearer technology used. 776 Therefore, even a single positive indication using any of these 777 mechanisms SHOULD be interpreted by the passive endpoint so that the 778 circuit-switched bearer establishment is related to the ongoing 779 session, even if the other correlation mechanisms fail. 781 If, after negotiating one or more correlation mechanisms in the SDP 782 offer/answer exchange, an endpoint receives a circuit-switched bearer 783 with no correlation information present, the endpoint has two 784 choices: it can either treat the call as unrelated, or treat the call 785 as related to the ongoing session in the IP domain. 787 An endpoint may for example specify a time window after SDP offer/ 788 answer exchange during which received calls are treated as correlated 789 even if the signaling in the circuit-switched domain does not carry 790 any correlation information. In this case, there is a chance that 791 the call is erroneously treated as related to the ongoing session. 793 An endpoint may also choose to always treat an incoming call as 794 unrelated if the signaling in the circuit-switched domain does not 795 carry any correlation information. In this case, there is a chance 796 that the call is erroneously treated as unrelated. 798 Since, in these cases, no correlation information can be deduced from 799 the signaling, it is up to the implementation to decide how to 800 behave. One option is also to let the user decide whether to accept 801 the call as related, or to treat the call as unrelated. 803 5.4. Considerations for Usage of Existing SDP 805 5.4.1. Originator of the Session 807 According to SDP [RFC4566], the origin line in SDP has the following 808 syntax: 810 o= 811 813 Of interest here are the and fields, which 814 indicate the type of network and type of address, respectively. 815 Typically, this field carries the IP address of the originator of the 816 session. Even if the SDP was used to negotiate an audio or video 817 media stream transported over a circuit-switched bearer, the 818 originator is using SDP over an IP bearer. Therefore, and 819 fields in the "o=" line should be populated with the IP 820 address identifying the source of the signaling. 822 5.4.2. Contact information 824 SDP [RFC4566] defines the "p=" line which may include the phone 825 number of the person responsible for the conference. Even though 826 this line can carry a phone number, it is not suited for the purpose 827 of defining a connection address for the media. Therefore, we have 828 selected to define the PSTN specific connection addresses in the "c=" 829 line. 831 5.5. Considerations for Usage of Third Party Call Control (3PCC) 833 Best Current Practices for Third Party Call Control (3pcc) in the 834 Session Initiation Protocol (SIP) [RFC3725] outlines several flows 835 which are possible in third party call control scenarios and 836 recommends some flows for specific situations. 838 One of the assumptions in [RFC3725] is that an SDP Offer may include 839 a "black hole" connection address, which has the property that 840 packets sent to it will never leave the host which sent them. For 841 IPv4, this "black hole" connection address is 0.0.0.0, or a domain 842 name within the .invalid DNS top level domain. 844 When using an E.164 address scheme in the context of third-party call 845 control, when the User Agent needs to indicate an unknown phone 846 number, it MUST populate the of the SDP "c=" line with a 847 "-" string. 849 Note that this may result in the recipient of the initial offer 850 rejecting such offer if the recipient of the offer was not aware 851 of its own E.164 number. Consequently it will not be possible to 852 establish a circuit-switched bearer, since neither party is aware 853 of their E.164 number. 855 5.6. Offer/Answer mode extensions 857 In this section, we define extensions to the Offer/Answer model 858 defined in The Offer/Answer Model in SDP [RFC3264] to allow for PSTN 859 addresses to be used with the Offer/Answer model. 861 5.6.1. Generating the Initial Offer 863 The Offerer, wishing to use PSTN audio or video stream, MUST populate 864 the "c=" and "m=" lines as follows. 866 The endpoint MUST set the in the "c=" line to "PSTN", and 867 the to "E164". Furthermore, the endpoint SHOULD set the 868 field to its own international E.164 number 869 (with a leading "+"). If the endpoint is not aware of its own E.164 870 number, it MUST set the to "-". 872 In the "m=" line, the endpoint MUST set the subfield to 873 "audio" or "video", depending on the media type, and the 874 subfield to "PSTN". The subfield SHOULD be set to "9" (the 875 discard port). 877 The subfield carries the payload type number(s) the endpoint is 878 wishing to use. Payload type numbers in this case refer to the 879 codecs that the endpoint wishes to use on the PSTN media stream. For 880 example, if the endpoint wishes to use the GSM codec, it would add 881 payload type number 3 in the list of codecs. The list of payload 882 types MUST only contain those codecs the endpoint is able to use on 883 the PSTN bearer. In case the endpoint is not aware of the codecs 884 available for the circuit-switched media streams, it MUST include a 885 dash ("-") in the subfield. 887 For dynamic payload types, the endpoint MUST define the set of valid 888 encoding names and related parameters using the "a=rtpmap" attribute 889 line. See Section 6 of SDP [RFC4566] for details. 891 When generating the Offer, if the Offerer supports any of the 892 correlation mechanisms defined in this memo, it MUST include an 893 attribute line "a=cs-correlation" in the SDP offer. The Offerer MUST 894 NOT include more than one "cs-correlation" attribute per media 895 decription. The "a=cs-correlation" line contains an enumeration of 896 the correlation mechanisms supported by the Offerer, in the format of 897 subfields. 899 The current list of subfields include "callerid", "uuie" and "dtmf" 900 and they refer to the correlation mechanisms defined in 901 Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. 903 If the Offerer supports any of the correlation mechanisms defined in 904 this memo, and is willing to become the active party, the Offerer 905 MUST add the "callerid", "uuie", and/or "dtmf" subfields and MUST 906 specify values for those subfields. 908 o the international E.164 number as the value in the "callerid" 909 subfield, 911 o the contents of the User-User information element as the value of 912 the "uuie" subfield, and/or 914 o the DTMF tone string as the value of the "dtmf" subfield 916 If the Offerer is only able to become the passive party in the 917 circuit-switched bearer setup, it MUST add at least one of the 918 possible correlation mechanisms, but MUST NOT specify values for 919 those subfields. 921 For example, if the Offerer is willing to use the User-User 922 Information element and DTMF digit sending mechanisms, but can only 923 become the passive party, it includes the following lines in the SDP: 925 a=cs-correlation:uuie dtmf 927 a=setup:passive 929 If, on the other hand, the Offerer is willing to use the User-User 930 Information element and the DTMF correlation mechanisms, and is able 931 to become the active or passive side, it includes the following lines 932 in the SDP: 934 a=cs-correlation:uuie:56A390F3D2B7310023 dtmf:14D*3 935 a=setup:actpass 937 The negotiation of the value of the 'setup' attribute takes place as 938 defined in Section 4.1 of TCP-Based Media Transport in the SDP 939 [RFC4145]. 941 The Offerer states which role or roles it is willing to perform; and 942 the Answerer, taking the Offerer's willingness into consideration, 943 chooses which roles both endpoints will actually perform during the 944 circuit-switched bearer setup. 946 By 'active' endpoint, we refer to an endpoint that will establish the 947 circuit-switched bearer; and by 'passive' endpoint, we refer to an 948 endpoint that will receive a circuit-switched bearer. 950 If an Offerer does not know its international E.164 number, it MUST 951 set the 'a=setup' attribute to the value 'active'. If the Offerer 952 knows its international E.164 number, it SHOULD set the value to 953 either 'actpass' or 'passive'. 955 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 956 It indicates that the connection is not established for the time 957 being. 959 The Offerer uses the "a=connection" attribute to decide whether a new 960 circuit-switched bearer is to be established or not. For the initial 961 Offer, the Offerer MUST use value 'new'. 963 5.6.2. Generating the Answer 965 If the Offer contained a circuit-switched audio or video stream, the 966 Answerer first determines whether it is able to accept and use such 967 streams. If the Answerer is not willing to use circuit-switched 968 media for the session, it MUST construct an Answer where the port 969 number for such media stream(s) is set to zero, according to 970 Section 6 of An Offer/Answer Model with the Session Description 971 Protocol (SDP) [RFC3264]. If the Answerer is willing to use circuit- 972 switched media for the session, it MUST ignore the received port 973 number (unless the port number is set to zero). 975 If the Offer included a "-" as the payload type number, it indicates 976 that the Offerer is not willing or able to define any specific 977 payload type. Most often, a "-" is expected to be used instead of 978 the payload type when the endpoint is not aware of or not willing to 979 define the codecs which will eventually be used on the circuit- 980 switched bearer. The circuit-switched signaling protocols have their 981 own means of negotiating or indicating the codecs, therefore an 982 Answerer SHOULD accept such Offers, and SHOULD set the payload type 983 to "-" also in the Answer. 985 If the Answerer explicitly wants to specify a codec for the circuit- 986 switched media, it MAY set the respective payload numbers in the 987 subfield in the answer. This behavior, however, is NOT 988 RECOMMENDED. 990 When receiving the Offer, the Answerer MUST determine whether it 991 becomes the active or passive party. 993 If the SDP in the Offer indicates that the Offerer is only able to 994 become the active party, the Answerer needs to determine whether it 995 is able to become the passive party. If this is not possible e.g. 996 due to the Answerer not knowing its international E.164 number, the 997 Answerer MUST reject the circuit-switched media by setting the port 998 number to zero on the Answer. If the Answerer is aware of its 999 international E.164 number, it MUST include the "a=setup" attribute 1000 in the Answer and set it to value "passive" or "holdconn". The 1001 Answerer MUST also include its E.164 number on the "c=" line. 1003 If the SDP in the Offer indicates that the Offerer is only able to 1004 become the passive party, the Answerer MUST verify that the Offerer's 1005 E.164 number is included in the "c=" line of the Offer. If the 1006 number is included, the Answerer MUST include the "a=setup" attribute 1007 in the Answer and set it to value "active" or "holdconn". If the 1008 number is not included, call establishment is not possible, and the 1009 Answerer MUST reject the circuit-switched media by setting the port 1010 number to zero in the Answer. 1012 If the SDP in the Offer indicates that the Offerer is able to become 1013 either the active or passive party, the Answerer needs to determine 1014 which role it would like to take. If the Offer includes an 1015 international E.164 number in the "c=" line, the Answerer SHOULD 1016 become the active party. If the Offer does not include an E.164 1017 number, and if the Answerer is aware of its international E.164 1018 number, it MUST become the passive party. If the Offer does not 1019 include an E.164 number in the "c=" line and the Answerer is not 1020 aware of its E.164 number, it MUST reject the circuit-switched media 1021 by setting the port number to zero in the Answer. 1023 For each media description where the Offer includes a "a=cs- 1024 correlation" attribute, the Answerer MUST select from the Offer those 1025 correlation mechanisms it supports, and include in the Answer one "a 1026 =cs-correlation" attribute line containing those mechanisms it is 1027 willing to use. The Answerer MUST only add one "a=cs-correlation" 1028 attribute in those media descriptions where also the Offer included a 1029 "a=cs-correlation" attribute. The Answerer MUST NOT add any 1030 mechanisms which were not included in the offer. If there are more 1031 than one "cs-correlation" attributes per media description in the 1032 Offer, the Answerer MUST discard all but the first for any media 1033 description. Also, the Answerer MUST discard all unknown "cs- 1034 correlation" attribute values. 1036 If the Answerer becomes the active party, it MUST add a value to any 1037 of the possible subfields. 1039 If the Answerer becomes the passive party, it MUST NOT add any values 1040 to the subfields in the "cs-correlation" attribute. 1042 After generating and sending the Answer, if the Answerer became the 1043 active party, it 1045 o MUST extract the E.164 number from the "c=" line of the Offer and 1046 MUST establish a circuit-switched bearer to that address. 1048 o if the SDP Answer contained a value for the "callerid" subfield, 1049 MUST set the Calling Party Number Information Element to that 1050 number, 1052 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1053 send the User-User Information element according to the rules 1054 defined for the circuit-switched technology used, and set the 1055 value of the Information Element to that received in the SDP 1056 Offer, 1058 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1059 send those DTMF digits according to the circuit-switched 1060 technology used. 1062 If, on the other hand, the Answerer became the passive party, it 1064 o MUST be prepared to receive a circuit-switched bearer, 1066 o if the Offer contained a value for the "callerid" subfield, MUST 1067 compare that value to the Calling Party Number Information Element 1068 of the circuit-switched bearer, 1070 o if the Offer contained a value for the "dtmf" subfield, MUST be 1071 prepared to receive and collect DTMF digits once the circuit- 1072 switched bearer is set up. The Answerer MUST compare the received 1073 DTMF digits to the value of the "dtmf" subfield. If the received 1074 DTMF digits match the value of the "dtmf" subfield in the "cs- 1075 correlation" attribute, the call SHOULD be treated as correlated 1076 to the ongoing session. 1078 o if the Offer contained a value for the "uuie" subfield, MUST be 1079 prepared to receive a User-User Information element once the 1080 circuit-switched bearer is set up. The Answerer MUST compare the 1081 received UUI to the value of the "uuie" subfield. If the value of 1082 the received UUI matches the value of the "uuie" subfield, the 1083 call SHOULD be treated as correlated to the ongoing session. 1085 If the Answerer becomes the active party, generates an SDP answer, 1086 and then it finds out that the circuit-switched call cannot be 1087 established, then the Answerer MUST create a new SDP offer where 1088 circuit-switched stream is removed from the session (actually, by 1089 setting the corresponding port in the m= line to zero) and send it to 1090 its counterpart. This is to synchronize both parties (and potential 1091 intermediaries) on the state of the session. 1093 5.6.3. Offerer processing the Answer 1095 When receiving the Answer, if the SDP does not contain "a=cs- 1096 correlation" attribute line, the Offerer should take that as an 1097 indication that the other party does not support or is not willing to 1098 use the procedures defined in the document for this session, and MUST 1099 revert to normal processing of SDP. 1101 When receiving the Answer, the Offerer MUST first determine whether 1102 it becomes the active or passive party, as described in 1103 Section 5.3.1. 1105 If the Offerer becomes the active party, it 1107 o MUST extract the E.164 number from the "c=" line and MUST 1108 establish a circuit-switched bearer to that address. 1110 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1111 send the User-User Information element according to the rules 1112 defined for the circuit-switched technology used, and set the 1113 value of the Information Element to that received in the SDP 1114 Answer, 1116 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1117 send those DTMF digits according to the circuit-switched 1118 technology used. 1120 If the Offerer becomes the passive party, it 1122 o MUST be prepared to receive a circuit-switched bearer, 1124 o Note that if delivery of the Answer is delayed for some reason, 1125 the circuit-switched call attempt may arrive at the Offerer before 1126 the Answer has been processed. In this case, since the 1127 correlation mechanisms are negotiated as part of the Offer/Answer 1128 exchange, the Answerer cannot know whether or not the incoming 1129 circuit-switched call attempt is correlated with the session being 1130 negotiated, the Offerer SHOULD answer the circuit-switched call 1131 attempt only after it has received and processed the Answer. 1133 o If the Answer contained a value for the "dtmf" subfield, the 1134 Offerer MUST be prepared to receive and collect DTMF digits once 1135 the circuit-switched bearer is set up. The Offerer SHOULD compare 1136 the received DTMF digits to the value of the "dtmf" subfield. If 1137 the received DTMF digits match the value of the "dtmf" subfield in 1138 the "cs-correlation" attribute, the call SHOULD be treated as 1139 correlated to the ongoing session. 1141 o If the Answer contained a value for the "uuie" subfield, the 1142 Offerer MUST be prepared to receive a User-User Information 1143 element once the circuit-switched bearer is set up. The Offerer 1144 SHOULD compare the received UUI to the value of the "uuie" 1145 subfield. If the value of the received UUI matches the value of 1146 the "uuie" subfield, the call SHOULD be treated as correlated to 1147 the ongoing session. 1149 5.6.4. Modifying the session 1151 If, at a later time, one of the parties wishes to modify the session, 1152 e.g., by adding new media stream, or by changing properties used on 1153 an existing stream, it may do so via the mechanisms defined for An 1154 Offer/Answer Model with SDP [RFC3264]. 1156 If there is an existing circuit-switched bearer between the 1157 endpoints, and the Offerer wants to reuse that, the Offerer MUST set 1158 the value of the "a=connection" attribute to 'existing'. 1160 If either party removes the circuit-switched media from the session 1161 (by setting the port number to zero), it MUST terminate the circuit- 1162 switched bearer using whatever mechanism is appropriate for the 1163 technology in question. 1165 If either party wishes to drop and reestablish an existing call, that 1166 party MUST first remove the circuit-switched media from the session 1167 by setting the port number to zero, and then use another Offer/Answer 1168 exchange where it MUST set the "a=connection" attribute to 'new'". 1169 If the media types are different (for example, a different codec will 1170 be used for the circuit-switched bearer), the media descriptions for 1171 terminating the existing bearer and the new bearer can be in the same 1172 Offer. 1174 5.7. Formal Syntax 1176 The following is the formal Augmented Backus-Naur Form (ABNF) 1177 [RFC5234] syntax that supports the extensions defined in this 1178 specification. The syntax is built above the SDP [RFC4566] and the 1179 tel URI [RFC3966] grammars. Implementations according to this 1180 specification MUST be compliant with this syntax. 1182 Figure 2 shows the formal syntax of the extensions defined in this 1183 memo. 1185 ; extension to the connection field originally specified 1186 ; in RFC4566 1188 connection-field = [%x63 "=" nettype SP addrtype SP 1189 connection-address CRLF] 1190 ; CRLF defined in RFC5234 1192 ;nettype and addrtype are defined in RFC 4566 1194 connection-address /= global-number-digits / "-" 1195 ; global-number-digits specified in RFC3966 1197 ;subrules for correlation attribute 1198 attribute /= cs-correlation-attr 1199 ; attribute defined in RFC4566 1200 cs-correlation-attr = "cs-correlation:" corr-mechanisms 1201 corr-mechanisms = corr-mech *(SP corr-mech) 1202 corr-mech = caller-id-mech / uuie-mech / 1203 dtmf-mech / ext-mech 1204 caller-id-mech = "callerid" [":" caller-id-value] 1205 caller-id-value = "+" 1*15DIGIT 1206 ; DIGIT defined in RFC5234 1207 uuie-mech = "uuie" [":" uuie-value] 1208 uuie-value = 1*65(HEXDIG HEXDIG) 1209 ;This represents up to 130 HEXDIG 1210 ; (65 octets) 1211 ;HEXDIG defined in RFC5234 1212 ;HEXDIG defined as 0-9, A-F 1214 dtmf-mech = "dtmf" [":" dtmf-value] 1215 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1216 ;0-9, A-D, '#' and '*' 1217 ext-mech = ext-mech-name [":" ext-mech-value] 1218 ext-mech-name = token 1219 ext-mech-value = token 1220 ; token is specified in RFC4566 1222 Figure 2: Syntax of the SDP extensions 1224 6. Examples 1226 In the examples below, where an SDP line is too long to be displayed 1227 as a single line, a breaking character "\" indicates continuation in 1228 the following line. Note that this character is included for display 1229 purposes only. Implementations MUST write a single line without 1230 breaks. 1232 6.1. Single PSTN audio stream 1234 Alice Bob 1235 | | 1236 | (1) SDP Offer (PSTN audio) | 1237 |--------------------------------->| 1238 | | 1239 | (2) SDP Answer (PSTN audio) | 1240 |<---------------------------------| 1241 | | 1242 | PSTN call setup | 1243 |<---------------------------------| 1244 | | 1245 |<==== media over PSTN bearer ====>| 1246 | | 1248 Figure 3: Basic flow 1250 Figure 3 shows a basic example that describes a single audio media 1251 stream over a circuit-switched bearer. Alice generates a SDP Offer 1252 which is shown in Figure 4. The Offer describes a PSTN circuit- 1253 switched bearer in the "m=" and "c=" line where it also indicates its 1254 international E.164 number format. Additionally, Alice expresses 1255 that she can initiate the circuit-switched bearer or be the recipient 1256 of it in the "a=setup" attribute line. The SDP Offer also includes 1257 correlation identifiers that this endpoint will insert in the Calling 1258 Party Number and/or User-User Information Element of the PSTN call 1259 setup if eventually this endpoint initiates the PSTN call. 1261 v=0 1262 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1263 s= 1264 t=0 0 1265 m=audio 9 PSTN - 1266 c=PSTN E164 +441134960123 1267 a=setup:actpass 1268 a=connection:new 1269 a=cs-correlation:callerid:+441134960123 \ 1270 uuie:56A390F3D2B7310023 1272 Figure 4: SDP offer (1) 1274 Bob generates a SDP Answer (Figure 5), describing a PSTN audio media 1275 on port 9 without information on the media sub-type on the "m=" line. 1276 The "c=" line contains Bob's international E.164 number. In the 1277 "a=setup" line Bob indicates that he is willing to become the active 1278 endpoint when establishing the PSTN call, and he also includes the "a 1279 =cs-correlation" attribute line containing the values he is going to 1280 include in the Calling Party Number and User-User IE of the PSTN call 1281 establishment. 1283 v=0 1284 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1285 s= 1286 t=0 0 1287 m=audio 9 PSTN - 1288 c=PSTN E164 +441134960124 1289 a=setup:active 1290 a=connection:new 1291 a=cs-correlation:callerid:+441134960124 \ 1292 uuie:56A390F3D2B7310023 1294 Figure 5: SDP Answer with circuit-switched media 1296 When Alice receives the Answer, she examines that Bob is willing to 1297 become the active endpoint when setting up the PSTN call. Alice 1298 temporarily stores Bob's E.164 number and the User-User IE value of 1299 the "cs-correlation" attribute, and waits for a circuit-switched 1300 bearer establishment. 1302 Bob initiates a circuit-switched bearer using whatever circuit- 1303 switched technology is available for him. The called party number is 1304 set to Alice's number, and calling party number is set to Bob's own 1305 number. Bob also sets the User-User Information Element value to the 1306 one contained in the SDP Answer. 1308 When Alice receives the circuit-switched bearer establishment, she 1309 examines the UUIE and the calling party number, and by comparing 1310 those received during O/A exchange determines that the call is 1311 related to the SDP session. 1313 It may also be that neither the UUIE nor the calling party number is 1314 received by the called party, or the format of the calling party 1315 number is changed by the PSTN. Implementations may still accept such 1316 call establishment attempts as being related to the session that was 1317 established in the IP network. As it cannot be guaranteed that the 1318 values used for correlation are always passed intact through the 1319 network, they should be treated as additional hints that the circuit- 1320 switched bearer is actually related to the session. 1322 6.2. Advanced SDP example: Circuit-Switched Audio and Video Streams 1324 Alice Bob 1325 | | 1326 | (1) SDP Offer (PSTN audio and video) | 1327 |------------------------------------------->| 1328 | | 1329 | (2) SDP Answer (PSTN audio) | 1330 |<-------------------------------------------| 1331 | | 1332 | PSTN call setup | 1333 |<-------------------------------------------| 1334 | | 1335 |<======== media over PSTN bearer ==========>| 1336 | | 1338 Figure 6: Circuit-Switched Audio and Video streams 1340 Figure 6 shows an example of negotiating audio and video media 1341 streams over circuit-switched bearers. 1343 v=0 1344 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1345 s= 1346 t=0 0 1347 a=setup:actpass 1348 a=connection:new 1349 c=PSTN E164 +441134960123 1350 m=audio 9 PSTN - 1351 a=cs-correlation:dtmf:1234536 1352 m=video 9 PSTN 34 1353 a=rtpmap:34 H263/90000 1354 a=cs-correlation:callerid:+441134960123 1355 Figure 7: SDP offer with circuit-switched audio and video (1) 1357 Upon receiving the SDP offer described in Figure 7, Bob rejects the 1358 video stream as his device does not currently support video, but 1359 accepts the circuit-switched audio stream. As Alice indicated that 1360 she is able to become either the active, or passive party, Bob gets 1361 to select which role he would like to take. Since the Offer 1362 contained the international E.164 number of Alice, Bob decides that 1363 he becomes the active party in setting up the circuit-switched 1364 bearer. Bob includes a new value in the "dtmf" subfield of the "cs- 1365 correlation" attribute, which he is going to send as DTMF tones once 1366 the bearer setup is complete. The Answer is described in Figure 8 1368 v=0 1369 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1370 s= 1371 t=0 0 1372 a=setup:active 1373 a=connection:new 1374 c=PSTN E164 +441134960124 1375 m=audio 9 PSTN - 1376 a=cs-correlation:dtmf:654321 1377 m=video 0 PSTN 34 1378 a=cs-correlation:callerid:+441134960124 1380 Figure 8: SDP answer with circuit-switched audio and video (2) 1382 7. Security Considerations 1384 This document provides an extension on top of RFC 4566 [RFC4566], and 1385 RFC 3264 [RFC3264]. As such, the security considerations of those 1386 documents apply. 1388 This memo provides mechanisms to agree on a correlation identifier or 1389 identifiers that are used to evaluate whether an incoming circuit- 1390 switched bearer is related to an ongoing session in the IP domain. 1391 If an attacker replicates the correlation identifier and establishes 1392 a call within the time window the receiving endpoint is expecting a 1393 call, the attacker may be able to hijack the circuit-switched bearer. 1394 These types of attacks are not specific to the mechanisms presented 1395 in this memo. For example, caller ID spoofing is a well known attack 1396 in the PSTN. Users are advised to use the same caution before 1397 revealing sensitive information as they would on any other phone 1398 call. Furthermore, users are advised that mechanisms that may be in 1399 use in the IP domain for securing the media, like Secure RTP (SRTP) 1400 [RFC3711], are not available in the CS domain. 1402 For the purposes of establishing a circuit-switched bearer, the 1403 active endpoint needs to know the passive endpoint's phone number. 1404 Phone numbers are sensitive information, and some people may choose 1405 not to reveal their phone numbers when calling using supplementary 1406 services like Calling Line Identification Restriction (CLIR) in GSM. 1407 Implementations should take the caller's preferences regarding 1408 calling line identification into account if possible, by restricting 1409 the inclusion of the phone number in SDP "c=" line if the caller has 1410 chosen to use CLIR. If this is not possible, implementations may 1411 present a prompt informing the user that their phone number may be 1412 transmitted to the other party. 1414 Similarly as with IP addresses, if there is a desire to protect the 1415 SDP containing phone numbers carried in SIP, implementers are advised 1416 to follow the security mechanisms defined in [RFC3261]. 1418 It is possible that an attacker creates a circuit-switched session 1419 whereby the attacked endpoint should dial a circuit-switched number, 1420 perhaps even a premium-rate telephone number. To mitigate the 1421 consequences of this attack, endpoints MUST authenticate and trust 1422 remote endpoints users who try to remain passive in the circuit- 1423 switched connection establishment. It is RECOMMENDED that endpoints 1424 have local policies precluding the active establishment of circuit 1425 switched connections to certain numbers (e.g., international, 1426 premium, long distance). Additionally, it is strongly RECOMMENDED 1427 that the end user is asked for consent prior to the endpoint 1428 initiating a circuit-switched connection. 1430 8. IANA Considerations 1432 This document instructs IANA to register a number of SDP tokens 1433 according to the following data. 1435 8.1. Registration of new cs-correlation SDP attribute 1437 Contact: Miguel Garcia 1439 Attribute name: cs-correlation 1441 Long-form attribute name: PSTN Correlation Identifier 1443 Type of attribute: media level only 1445 Subject to charset: No 1447 Description: This attribute provides the Correlation Identifier 1448 used in PSTN signaling 1449 Appropriate values:see Section 5.2.3.1 1451 Specification: RFC XXXX 1453 The IANA is requested to create a subregistry for 'cs-correlation' 1454 attribute under the Session Description Protocol (SDP) Parameters 1455 registry. The initial values for the subregistry are presented in 1456 the following, and IANA is requested to add them into its database: 1458 Value of 'cs-correlation' attribute Reference Description 1459 ----------------------------------- --------- ----------- 1460 callerid RFC XXXX Caller ID 1461 uuie RFC XXXX User-User 1462 Information Element 1463 dtmf RFC XXXX Dual-tone Multifrequency 1465 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1466 reference to the RFC number of this draft. 1468 As per the terminology in [RFC5226], the registration policy for new 1469 values of 'cs-correlation' parameter is 'Specification Required'. 1471 8.2. Registration of a new "nettype" value 1473 This memo provides instructions to IANA to register a new "nettype" 1474 in the Session Description Protocol Parameters registry [1]. The 1475 registration data, according to RFC 4566 [RFC4566] follows. 1477 Type SDP Name Reference 1478 ---- ------------------ --------- 1479 nettype PSTN [RFCxxxx] 1481 8.3. Registration of new "addrtype" values 1483 This memo provides instructions to IANA to register two new 1484 "addrtype" in the Session Description Protocol Parameters registry 1485 [2]. The registration data, according to RFC 4566 [RFC4566] follows. 1487 Type SDP Name Reference 1488 ---- ------------------ --------- 1489 addrtype E164 [RFCxxxx] 1490 addrtype - [RFCxxxx] 1491 Note: RFC XXXX defines the "E164" and "-" addrtypes in the context of 1492 the "PSTN" nettype only. Please refer to the relevant RFC for a 1493 description of that representation. 1495 8.4. Registration of a new "proto" value 1497 This memo provides instructions to IANA to register a new "proto" in 1498 the Session Description Protocol Parameters registry [3]. The 1499 registration data, according to RFC 4566 [RFC4566] follows. 1501 Type SDP Name Reference 1502 -------------- --------------------------- --------- 1503 proto PSTN [RFCxxxx] 1505 The related "fmt" namespace re-uses the conventions and payload type 1506 number defined for RTP/AVP. In RFC XXXX, the RTP audio and video 1507 media types, when applied to PSTN circuit-switched bearers, represent 1508 merely an audio or video codec in its native format directly on top 1509 of a single PSTN bearer. 1511 In come cases, the endpoint is not able to determine the list of 1512 available codecs for circuit-switched media streams. In this case, 1513 in order to be syntactically compliant with SDP [RFC4566], the 1514 endpoint MUST include a single dash ("-") in the subfield. 1516 9. Acknowledgments 1518 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas 1519 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan 1520 Rosenberg, Ingemar Johansson, Christer Holmberg, Alf Heidermark, Tom 1521 Taylor, Thomas Belling, Keith Drage, and Andrew Allen for providing 1522 their insight and comments on this document. 1524 10. References 1526 10.1. Normative References 1528 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1529 Requirement Levels", BCP 14, RFC 2119, March 1997. 1531 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1532 with Session Description Protocol (SDP)", RFC 3264, June 1533 2002. 1535 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 1536 3966, December 2004. 1538 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1539 the Session Description Protocol (SDP)", RFC 4145, 1540 September 2005. 1542 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1543 Description Protocol", RFC 4566, July 2006. 1545 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1546 Encodings", RFC 4648, October 2006. 1548 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1549 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1550 May 2008. 1552 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1553 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1555 10.2. Informative References 1557 [I-D.ietf-cuss-sip-uui] 1558 Johnston, A. and J. Rafferty, "A Mechanism for 1559 Transporting User to User Call Control Information in 1560 SIP", draft-ietf-cuss-sip-uui-10 (work in progress), April 1561 2013. 1563 [ITU.E164.1991] 1564 International Telecommunications Union, "The International 1565 Public Telecommunication Numbering Plan", ITU-T 1566 Recommendation E.164, 1991. 1568 [ITU.Q931.1998] 1569 , "Digital Subscriber Signalling System No. 1 (DSS 1) - 1570 ISDN User - Network Interface Layer 3 Specification for 1571 Basic Call Control", ISO Standard 9594-1, May 1998. 1573 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1574 Session Description Protocol (SDP) for ATM Bearer 1575 Connections", RFC 3108, May 2001. 1577 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1578 A., Peterson, J., Sparks, R., Handley, M., and E. 1579 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1580 June 2002. 1582 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1583 Jacobson, "RTP: A Transport Protocol for Real-Time 1584 Applications", STD 64, RFC 3550, July 2003. 1586 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1587 Video Conferences with Minimal Control", STD 65, RFC 3551, 1588 July 2003. 1590 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1591 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1592 RFC 3711, March 2004. 1594 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1595 Camarillo, "Best Current Practices for Third Party Call 1596 Control (3pcc) in the Session Initiation Protocol (SIP)", 1597 BCP 85, RFC 3725, April 2004. 1599 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1600 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1602 [TS.24.008] 1603 3GPP , "Mobile radio interface Layer 3 specification; Core 1604 network protocols; Stage 3 ", 3GPP TS 24.008 3.20.0, 1605 December 2005. 1607 Authors' Addresses 1609 Miguel A. Garcia-Martin 1610 Ericsson 1611 Calle Via de los Poblados 13 1612 Madrid, ES 28033 1613 Spain 1615 Email: miguel.a.garcia@ericsson.com 1617 Simo Veikkolainen 1618 Nokia 1619 P.O. Box 226 1620 NOKIA GROUP, FI 00045 1621 Finland 1623 Phone: +358 50 486 4463 1624 Email: simo.veikkolainen@nokia.com