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'ITU.Q931.1998' ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) == Outdated reference: A later version (-17) exists of draft-ietf-cuss-sip-uui-11 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC WG M. Garcia-Martin 3 Internet-Draft Ericsson 4 Intended status: Standards Track S. Veikkolainen 5 Expires: August 4, 2014 Nokia 6 January 31, 2014 8 Session Description Protocol (SDP) Extension For Setting Audio and Video 9 Media Streams Over Circuit-Switched Bearers In The Public Switched 10 Telephone Network (PSTN) 11 draft-ietf-mmusic-sdp-cs-22 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 August 4, 2014. 37 Copyright Notice 39 Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 5 57 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5 58 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 6 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 . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . 12 67 5.2.3.3. User-User Information Element Correlation 68 Mechanism . . . . . . . . . . . . . . . . . . . . 13 69 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . 14 70 5.2.3.5. The external correlation mechanism . . . . . . . 15 71 5.2.3.6. Extensions to correlation mechanisms . . . . . . 16 72 5.3. Negotiating the correlation mechanisms . . . . . . . . . 16 73 5.3.1. Determining the Direction of the Circuit-Switched 74 Bearer Setup . . . . . . . . . . . . . . . . . . . . 16 75 5.3.2. Populating the cs-correlation attribute . . . . . . . 17 76 5.3.3. Considerations on correlations . . . . . . . . . . . 18 77 5.4. Considerations for Usage of Existing SDP . . . . . . . . 18 78 5.4.1. Originator of the Session . . . . . . . . . . . . . . 18 79 5.4.2. Contact information . . . . . . . . . . . . . . . . . 19 80 5.5. Considerations for Usage of Third Party Call Control 81 (3PCC) . . . . . . . . . . . . . . . . . . . . . . . . . 19 82 5.6. Offer/Answer mode extensions . . . . . . . . . . . . . . 20 83 5.6.1. Generating the Initial Offer . . . . . . . . . . . . 20 84 5.6.2. Generating the Answer . . . . . . . . . . . . . . . . 22 85 5.6.3. Offerer processing the Answer . . . . . . . . . . . . 25 86 5.6.4. Modifying the session . . . . . . . . . . . . . . . . 27 87 5.7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 27 88 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 28 89 6.1. Single PSTN audio stream . . . . . . . . . . . . . . . . 29 90 6.2. Advanced SDP example: Circuit-Switched Audio and 91 Video Streams . . . . . . . . . . . . . . . . . . . . . . 31 92 7. Security Considerations . . . . . . . . . . . . . . . . . . . 32 93 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 94 8.1. Registration of new cs-correlation SDP attribute . . . . 33 95 8.2. Registration of a new "nettype" value . . . . . . . . . . 34 96 8.3. Registration of new "addrtype" value . . . . . . . . . . 34 97 8.4. Registration of a new "proto" value . . . . . . . . . . . 34 98 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 99 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 100 10.1. Normative References . . . . . . . . . . . . . . . . . . 35 101 10.2. Informative References . . . . . . . . . . . . . . . . . 36 102 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 37 103 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 105 1. Introduction 107 The Session Description Protocol (SDP) [RFC4566] is intended for 108 describing multimedia sessions for the purposes of session 109 announcement, session invitation, and other forms of multimedia 110 session initiation. SDP is most commonly used for describing media 111 streams that are transported over the Real-Time Transport Protocol 112 (RTP) [RFC3550], using the profiles for audio and video media defined 113 in RTP Profile for Audio and Video Conferences with Minimal Control 114 [RFC3551]. 116 However, SDP can be used to describe other media transport protocols 117 than RTP. Previous work includes SDP conventions for describing ATM 118 bearer connections [RFC3108] and the Message Session Relay Protocol 119 [RFC4975]. 121 SDP is commonly carried in Session Initiation Protocol (SIP) 122 [RFC3261] messages in order to agree on a common media description 123 among the endpoints. An Offer/Answer Model with Session Description 124 Protocol (SDP) [RFC3264] defines a framework by which two endpoints 125 can exchange SDP media descriptions and come to an agreement as to 126 which media streams should be used, along with the media related 127 parameters. 129 In some scenarios it might be desirable to establish the media stream 130 over a circuit-switched bearer connection even if the signaling for 131 the session is carried over an IP bearer. An example of such a 132 scenario is illustrated with two mobile devices capable of both 133 circuit-switched and packet-switched communication over a low- 134 bandwidth radio bearer. The radio bearer may not be suitable for 135 carrying real-time audio or video media, and using a circuit-switched 136 bearer would offer a better perceived quality of service. So, 137 according to this scenario, SDP and its higher layer session control 138 protocol (e.g., the Session Initiation Protocol (SIP) [RFC3261]) are 139 used over regular IP connectivity, while the audio or video is 140 received through the classical circuit-switched bearer. 142 This document addresses only the use of circuit-switched bearers in 143 the PSTN, not a generic circuit-switched network. The mechanisms 144 presented below require a call signaling protocol of the PSTN to be 145 used (such as ITU-T Q.931 [ITU.Q931.1998] or 3GPP TS 24.008 146 [TS.24.008]). 148 Setting up a signaling relationship in the IP domain instead of just 149 setting up a circuit-switched call offers also the possibility of 150 negotiating in the same session other IP based media that is not 151 sensitive to jitter and delay, for example, text messaging or 152 presence information. 154 At a later point in time the mobile device might move to an area 155 where a high-bandwidth packet-switched bearer, for example a Wireless 156 Local Area Network (WLAN) connection, is available. At this point 157 the mobile device may perform a handover and move the audio or video 158 media streams over to the high-speed bearer. This implies a new 159 exchange of SDP Offer/Answer that leads to a re-negotiation of the 160 media streams. 162 Other use cases exist. For example, an endpoint might have at its 163 disposal circuit-switched and packet-switched connectivity, but the 164 same audio or video codecs are not feasible for both access networks. 165 For example, the circuit-switched audio or video stream supports 166 narrow-bandwidth codecs, while the packet-switched access allows any 167 other audio or video codec implemented in the endpoint. In this 168 case, it might be beneficial for the endpoint to describe different 169 codecs for each access type and get an agreement on the bearer 170 together with the remote endpoint. 172 There are additional use cases related to third party call control 173 where the session setup time is improved when the circuit-switched 174 bearer in the PSTN is described together with one or more codecs. 176 The rest of the document is structured as follows: Section 2 provides 177 the document conventions, Section 3 introduces the requirements, 178 Section 4 presents an overview of the proposed solutions, and 179 Section 5 contains the protocol description. Section 6 provides an 180 example of descriptions of circuit-switched audio or video streams in 181 SDP. Section 7 and Section 8 contain the Security and IANA 182 considerations, respectively. 184 2. Conventions Used in This Document 186 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 187 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 188 "OPTIONAL" in this document are to be interpreted as described in BCP 189 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant 190 implementations. 192 3. Requirements 194 This section presents the general requirements that are specific for 195 the audio or video media streams over circuit-switched bearers. 197 REQ-1: A mechanism for endpoints to negotiate and agree on an audio 198 or video media stream established over a circuit-switched 199 bearer MUST be available. 201 REQ-2: The mechanism MUST allow the endpoints to combine circuit- 202 switched audio or video media streams with other 203 complementary media streams, for example, text messaging. 205 REQ-3: The mechanism MUST allow the endpoint to negotiate the 206 direction of the circuit-switched bearer, i.e., which 207 endpoint is active when initiating the circuit-switched 208 bearer. 210 REQ-4: The mechanism MUST be independent of the type of the circuit- 211 switched access (e.g., Integrated Services Digital Network 212 (ISDN), Global System for Mobile Communication (GSM), etc.) 214 REQ-5: There MUST be a mechanism that helps an endpoint to correlate 215 an incoming circuit-switched bearer with the one negotiated 216 in SDP, as opposed to another incoming call that is not 217 related to that. In case correlation by programmatic means 218 is not possible, correlation may also be performed by the 219 human user. 221 REQ-6: It MUST be possible for endpoints to advertise different 222 lists of audio or video codecs in the circuit-switched audio 223 or video stream from those used in a packet-switched audio or 224 video stream. 226 REQ-7: It MUST be possible for endpoints to not advertise the list 227 of available codecs for circuit-switched audio or video 228 streams. 230 4. Overview of Operation 232 The mechanism defined in this memo extends SDP and allows describing 233 an audio or video media stream established over a circuit-switched 234 bearer. A new network type ("PSTN") and a new protocol type ("PSTN") 235 are defined for the "c=" and "m=" lines to be able to describe a 236 media stream over a circuit-switched bearer. These SDP extensions 237 are described in Section 5.2. Since circuit-switched bearers are 238 connection-oriented media streams, the mechanism re-uses the 239 connection-oriented extensions defined in RFC 4145 [RFC4145] to 240 negotiate the active and passive sides of a connection setup. This 241 is further described in Section 5.3.1. 243 4.1. Example Call Flow 245 Consider the example presented in Figure 1. In this example, 246 Endpoint A is located in an environment where it has access to both 247 IP and circuit-switched bearers for communicating with other 248 endpoints. Endpoint A decides that the circuit-switched bearer 249 offers a better perceived quality of service for voice, and issues an 250 SDP Offer containing the description of an audio media stream over 251 circuit-switched bearer. 253 Endpoint A Endpoint B 254 | (1) SDP Offer (PSTN audio) | 255 |----------------------------------->| 256 | | 257 | (2) SDP Answer (PSTN audio) | 258 |<-----------------------------------| 259 | | 260 | PSTN call setup | 261 |<-----------------------------------| 262 | | 263 | | 264 |<===== media over PSTN bearer =====>| 265 | | 267 Figure 1: Example Flow 269 Endpoint B receives the SDP offer and determines that it is located 270 in an environment where the IP based bearer is not suitable for real- 271 time audio media. However, Endpoint B also has PSTN circuit-switched 272 bearer available for audio. Endpoint B generates an SDP answer 273 containing a description of the audio media stream over a circuit- 274 switched bearer. 276 During the offer-answer exchange Endpoints A and B also agree the 277 direction in which the circuit-switched bearer should be established. 278 In this example, Endpoint B becomes the active party, in other words, 279 it 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 the two endpoints. 286 Endpoint B establishes a circuit-switched bearer towards Endpoint A 287 using whatever mechanisms are defined for the network type in 288 question. When receiving the incoming circuit-switched connection 289 attempt, Endpoint A is able to determine that the attempt is related 290 to the session it is just establishing with B. 292 Endpoint A accepts the circuit-switched connection; the circuit- 293 switched bearer setup is completed. The two endpoints can now use 294 the circuit-switched connection for two-way audio media. 296 If, for some reason, Endpoint B would like to reject the offered 297 stream, it would set the port number of the specific stream to zero, 298 as specified in RFC3264 [RFC3264]. Also, if B does not understand 299 some of the SDP attributes specified in this document, it 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 to be used within the context of a "PSTN" network type. The 338 "E164" address type indicates that the connection address contains an 339 E.164 number represented according to the ITU-T E.164 [ITU.E164.1991] 340 recommendation. 342 It is a common convention that an international E.164 number contains 343 a leading '+' sign. For consistency's sake, we also require the 344 E.164 telephone is prepended with a '+', even if that is not 345 necessary for routing of the call in the PSTN network. 347 There are cases, though, when the endpoint is merely aware of a 348 circuit-switched bearer, without having further information about the 349 E.164 number allocated to it. In these cases a dash ("-") is used to 350 indicate an unknown connection address. This makes the connection 351 data line be according to the SDP syntax. 353 Please note that the "E164" address type defined in this memo is 354 exclusively defined to be used in conjunction with the "PSTN" network 355 type in accordance with [RFC4566]. Usage of "E164" address type in 356 conjunction with other network types may be defined elsewhere. 358 This memo exclusively uses the international representation of E.164 359 numbers, i.e., those including a country code and, as described above 360 prepended with a '+' sign. Implementations conforming to this 361 specification and using the "E164" address type together with the 362 "PSTN" network type MUST use the 'global-number-digits' construction 363 specified in RFC 3966 [RFC3966] for representing international E.164 364 numbers. This representation requires the presence of the '+' sign, 365 and additionally allows for the presence of one or more 'visual- 366 separator' constructions for easier human readability (see 367 Section 5.7). 369 Note that MUST NOT be omitted when unknown since 370 this would violate basic syntax of SDP [RFC4566]. In such cases, it 371 MUST be set to a "-". 373 The following are examples of the extension to the connection data 374 line: 376 c=PSTN E164 +441134960123 378 c=PSTN E164 - 380 When the is E164, the connection address is defined as 381 follows: 383 o an international E.164 number (prepended with a '+' sign) 385 o the value "-", signifying that the address is unknown 387 o any other value resulting from the production rule of connection- 388 address in RFC4566 [RFC4566], but in all cases any value 389 encountered will be ignored. 391 5.2.2. Media Descriptions 393 According to SDP [RFC4566], the media description line in SDP has the 394 following syntax: 396 m= ... 398 The subfield carries the media type. For establishing an 399 audio bearer, the existing "audio" media type is used. For 400 establishing a video bearer, the existing "video" media type is used. 402 The subfield is the transport port to which the media stream 403 is sent. Circuit-switched access lacks the concept of a port number, 404 and therefore the subfield does not carry any meaningful 405 value. In order to be compliant with SDP syntax, implementations 406 SHOULD set the subfield to the discard port value "9" and MUST 407 ignore it on reception. 409 According to RFC 3264 [RFC3264], a port number of zero in the offer 410 of a unicast stream indicates that the stream is offered but must not 411 be used. If a port number of zero is present in the answer of a 412 unicast stream, it indicates that the stream is rejected. These 413 rules are still valid when the media line in SDP represents a 414 circuit-switched bearer. 416 The subfield is the transport protocol. The circuit-switched 417 bearer uses whatever transport protocol it has available. This 418 subfield SHOULD be set to the mnemonic "PSTN" to be syntactically 419 correct with SDP [RFC4566] and to indicate the usage of circuit- 420 switched protocols in the PSTN. 422 The subfield is the media format description. In the classical 423 usage of SDP to describe RTP-based media streams, when the 424 subfield is set to "RTP/AVP" or "RTP/SAVP", the subfield 425 contains the payload types as defined in the RTP audio profile 426 [RFC3551]. 428 When "RTP/AVP" is used in the field, the subfield 429 contains the RTP payload type numbers. We use the subfield to 430 indicate the list of available codecs over the circuit-switched 431 bearer, by re-using the conventions and payload type numbers defined 432 for RTP/AVP. The RTP audio and video media types, when applied to 433 PSTN circuit-switched bearers, represent merely an audio or video 434 codec. If the endpoint is able to determine the list of available 435 codecs for circuit-switched media streams, it MUST use the 436 corresponding payload type numbers in the subfield. 438 In some cases, the endpoint is not able to determine the list of 439 available codecs for circuit-switched media streams. In this case, 440 in order to be syntactically compliant with SDP [RFC4566], the 441 endpoint MUST include a single dash ("-") in the subfield. 443 As per RFC 4566 [RFC4566], the media format descriptions are listed 444 in priority order. 446 Examples of media descriptions for circuit-switched audio streams 447 are: 449 m=audio 9 PSTN 3 0 8 451 m=audio 9 PSTN - 453 Similarly, an example of a media description for circuit-switched 454 video stream is: 456 m=video 9 PSTN 34 458 m=video 9 PSTN - 460 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 462 The endpoints should be able to correlate the circuit-switched bearer 463 with the session negotiated with SDP in order to avoid ringing for an 464 incoming circuit-switched bearer that is related to the session 465 controlled with SDP (and SIP). 467 Several alternatives exist for performing this correlation. This 468 memo provides three mutually non-exclusive correlation mechanisms. 469 Additionally, we define a fourth mechanism where correlation may be 470 performed by external means, typically by the human user, in case 471 using other correlation mechanisms is not possible or does not 472 succeed. Other correlation mechanisms may exist, and their usage 473 will be specified when need arises. 475 All mechanisms share the same principle: some unique information is 476 sent in the SDP and in the circuit-switched signaling protocol. If 477 these pieces of information match, then the circuit-switched bearer 478 is part of the session described in the SDP exchange. Otherwise, 479 there is no guarantee that the circuit-switched bearer is related to 480 such session. 482 The first mechanism is based on the exchange of PSTN caller-ID 483 between the endpoints. The caller-ID is also available as the 484 Calling Party ID in the circuit-switched signaling. 486 The second mechanism is based on the inclusion in SDP of a value that 487 is also sent in the User-to-User Information Element that is part of 488 the bearer setup signaling in the PSTN. 490 The third mechanism is based on sending in SDP a string that 491 represents Dual Tone MultiFrequency (DTMF) digits that will be later 492 sent right after the circuit-switched bearer is established. 494 The fourth correlation mechanism declares support for cases where 495 correlation is done by external means. Typically this means that the 496 decision is left for the human user. This is the way how some 497 current conferencing systems operate: after logging on to the 498 conference, the system calls back to the user's phone number to 499 establish audio communiations, and it is up to the human user to 500 accept or reject the incoming call. By declaring explicit support 501 for this mechanism endpoints can use it only when such possibility 502 exist. 504 Endpoints may opt to implement any combination of the correlation 505 mechanisms specified in Section 5.2.3.2, Section 5.2.3.3, 506 Section 5.2.3.4, and Section 5.2.3.5, including an option of 507 implementing none at all. 509 5.2.3.1. The "cs-correlation" attribute 511 In order to provide support for the correlation mechanisms, we define 512 a new media-level SDP attribute called "cs-correlation". There MUST 513 be at most one "cs-correlation" attribute per media description. 515 This "cs-correlation" attribute MAY contain zero or more subfields, 516 either "callerid", "uuie", "dtmf", or "external" to specify 517 additional information required by the Caller-ID, User to User 518 Information, DTMF, or external correlation mechanisms, respectively. 519 The list of correlation mechanisms may be extended by other 520 specifications, see Section 5.2.3.6 for more details. 522 The following sections provide more detailed information of these 523 subfields. 525 The values "callerid", "uuie", "dtmf" and "external" refer to the 526 correlation mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, 527 Section 5.2.3.4 and, Section 5.2.3.5 respectively. The formal 528 Augmented Backus-Naur Format (ABNF) syntax of the "cs-correlation" 529 attribute is presented in Section 5.7. 531 5.2.3.2. Caller-ID Correlation Mechanism 533 The Caller-ID correlation mechanisms consists of an exchange of the 534 calling party number as an international E.164 number in SDP, 535 followed by the availability of the Calling Party Number information 536 element in the call setup signaling of the circuit switched 537 connection. If both pieces of information match, the circuit- 538 switched bearer is correlated to the session described in SDP. 540 Example of inclusion of an international E.164 number in the "cs- 541 correlation" attribute is: 543 a=cs-correlation:callerid:+441134960123 545 The presence of the "callerid" subfield indicates that the endpoint 546 supports use of the calling party number as a means of correlating a 547 PSTN call with the session being negotiated. The "callerid" subfield 548 MAY be accompanied by the international E.164 number of the party 549 inserting the parameter. 551 Note that there are no guarantees that this correlation mechanism 552 works or is even available, due a number of problems: 554 o The endpoint might not be aware of its own E.164 number, in which 555 case it cannot populate the SDP appropriately. 557 o The Calling Party Number information element in the circuit- 558 switched signaling might not be available, e.g., due to policy 559 restrictions of the network operator or caller restriction due to 560 privacy. 562 o The Calling Party Number information element in the circuit- 563 switched signaling might be available, but the digit 564 representation of the E.164 number might differ from the one 565 expressed in the SDP, due to, e.g., lack of of country code. To 566 mitigate this problem implementations should consider only some of 567 the rightmost digits from the E.164 number for correlation. For 568 example, the numbers +44-113-496-0123 and 0113-496-0123 could be 569 considered as the same number. This is also the behavior of some 570 cellular phones, which correlate the incoming calling party with a 571 number stored in the phone book, for the purpose of displaying the 572 caller's name. Please refer to ITU-T E.164 reccommendation 573 [ITU.E164.1991] for consideration of the relevant number of digits 574 to consider. 576 5.2.3.3. User-User Information Element Correlation Mechanism 578 A second correlation mechanism is based on including in SDP a string 579 that represents the User-User Information Element that is part of the 580 call setup signaling of the circuit-switched bearer. The User-User 581 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 582 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 583 Element has a maximum size of 35 or 131 octets, depending on the 584 actual message of the PSTN protocol where it is included and the 585 network settings. 587 The mechanism works as follows: An endpoint creates a User-User 588 Information Element, according to the requirements of the call setup 589 signaling protocol. The same value is included in the SDP offer or 590 SDP answer, in the "uuie" subfield of the "cs-correlation" attribute. 591 When the SDP Offer/Answer exchange is completed, each endpoint has 592 become aware of the value that will be used in the User-User 593 Information Element of the call setup message of the PSTN protocol. 594 The endpoint that initiates the call setup attempt includes this 595 value in the User-User Information Element. The recipient of the 596 call setup attempt can extract the User-User Information Element and 597 correlate it with the value previously received in the SDP. If both 598 values match, then the call setup attempt corresponds to that 599 indicated in the SDP. 601 According to ITU-T Q.931 [ITU.Q931.1998], the User-User Information 602 Element (UUIE) identifier is composed of a first octet identifying 603 this as a User-User Information Element, a second octet containing 604 the Length of the user-user contents, a third octet containing a 605 Protocol Discriminator, and a value of up to 32 or 128 octets 606 (depending on network settings) containing the actual User 607 Information (see Figure 4-36 in ITU-T Q.931). The first two octets 608 of the UUIE MUST NOT be used for correlation, only the octets 609 carrying the Protocol Discriminator and the User Information value 610 are input to the creation of the value of the "uuie" subfield in the 611 "cs-correlation" attribute. Therefore, the value of the "uuie" 612 subfield in the "cs-correlation" attribute MUST start with the 613 Protocol Discriminator octet, followed by the User Information 614 octets. The value of the Protocol Discriminator octet is not 615 specified in this document; it is expected that organizations using 616 this technology will allocate a suitable value for the Protocol 617 Discriminator. 619 Once the binary value of the "uuie" subfield in the "cs-correlation" 620 attribute is created, it MUST be base 16 (also known as "hex") 621 encoded before it is inserted in SDP. Please refer to RFC 4648 622 [RFC4648] for a detailed description of base 16 encoding. The 623 resulting encoded value needs to have an even number of hexadecimal 624 digits, and MUST be considered invalid if it has an odd number. 626 Note that the encoding of the "uuie" subfield of the "cs- 627 correlation" attribute is largely inspired by the encoding of the 628 same value in the User-to-User header field in SIP, according to 629 the document "A Mechanism for Transporting User to User Call 630 Control Information in SIP" [I-D.ietf-cuss-sip-uui]. 632 As an example, an endpoint willing to send a UUIE containing a 633 protocol discriminator with the hexadecimal value of %x56 and an 634 hexadecimal User Information value of %xA390F3D2B7310023 would 635 include a "cs-correlation" attribute line as follows: 637 a=cs-correlation:uuie:56A390F3D2B7310023 639 Note that the value of the User-User Information Element is 640 considered as an opaque string and only used for correlation 641 purposes. Typically call signaling protocols impose requirements on 642 the creation of User-User Information Element for end-user protocol 643 exchange. The details regarding the generation of the User-User 644 Information Element are outside the scope of this specification. 646 Please note that there are no guarantees that this correlation 647 mechanism works. On one side, policy restrictions might not make the 648 User-User information available end to end in the PSTN. On the other 649 hand, the generation of the User-User Information Element is 650 controlled by the PSTN circuit-switched call protocol, which might 651 not offer enough freedom for generating different values from one 652 endpoint to another one, or from one call to another in the same 653 endpoint. This might result in the same value of the User-User 654 Information Element for all calls. 656 5.2.3.4. DTMF Correlation Mechanism 658 We introduce a third mechanism for correlating the circuit-switched 659 bearer with the session described with SDP. This is based on 660 agreeing on a sequence of digits that are negotiated in the SDP Offer 661 /Answer exchange and sent as Dual Tone Multifrequency (DTMF) ITU-T 662 Recommendation Q.23 [ITU.Q23.1988] tones over the circuit-switched 663 bearer once this bearer is established. If the DTMF digit sequence 664 received through the circuit-switched bearer matches the digit string 665 negotiated in the SDP, the circuit-switched bearer is correlated with 666 the session described in the SDP. The mechanism is similar to many 667 voice conferencing systems which require the user to enter a PIN code 668 using DTMF tones in order to be accepted in a voice conference. 670 The mechanism works as follows: An endpoint selects a DTMF digit 671 sequence. The same sequence is included in the SDP offer or SDP 672 answer, in a "dtmf" subfield of the "cs-correlation" attribute. When 673 the SDP Offer/Answer exchange is completed, each endpoint has become 674 aware of the DTMF sequence that will be sent right after the circuit- 675 switched bearer is set up. The endpoint that initiates the call 676 setup attempt sends the DTMF digits according to the procedures 677 defined for the circuit-switched bearer technology used. The 678 recipient (passive side of the bearer setup) of the call setup 679 attempt collects the digits and compares them with the value 680 previously received in the SDP. If the digits match, then the call 681 setup attempt corresponds to that indicated in the SDP. 683 Implementations are advised to select a number of DTMF digits that 684 provide enough assurance that the call is related, but on the 685 other hand do not prolong the bearer setup time unnecessarily. A 686 number of 5 to 10 digits is a good compromise. 688 As an example, an endpoint willing to send DTMF tone sequence "14D*3" 689 would include a "cs-correlation" attribute line as follows: 691 a=cs-correlation:dtmf:14D*3 693 If the endpoints successfully agree on the usage of the DTMF digit 694 correlation mechanism, but the passive side does not receive any DTMF 695 digits after successful circuit-switched bearer setup, or receives a 696 set of DTMF digits that do not match the value of the "dtmf" 697 attribute (including receiving too many digits), the passive side 698 SHOULD consider that this DTMF mechanism has failed to correlate the 699 incoming call. 701 5.2.3.5. The external correlation mechanism 703 The fourth correlation mechanism relies on external means for 704 correlating the incoming call to the session. Since endpoints can 705 select which correlation mechanisms they support, it may happen that 706 no other common correlation mechanism is found, or that the selected 707 correlation mechanism does not succeed due to the required feature 708 not being supported by the underlying PSTN network. In these cases, 709 the human user can do the decision of accepting or rejecting the 710 incoming call, thus "correlating" the call with the session. Since 711 not all endpoints are operated by a human user, or if there is no 712 other external means implemented by the endpoint for the correlation 713 funtion, we explicitly define support for such external correlation 714 mechanism. 716 Endpoints wishing to use this external correlation mechanism would 717 use a subfield "external" in the "a=cs-correlation" attribute. 719 Unlike the three other correlation mechansism, the "external" 720 subfield does not accept a value. An example of a "a=cs-correlation" 721 attribute line would look like this: 723 a=cs-correlation:external 725 5.2.3.6. Extensions to correlation mechanisms 727 New values for the "cs-correlation" attribute may be specified. The 728 registration policy for new values is "Specification Required", see 729 Section 8. Any such specification MUST include a description of how 730 SDP Offer/Answer mechanism is used to negotiate the use of the new 731 values, taking into account how endpoints determine which side will 732 become active or passive (see Section 5.3 for more details). 734 If, during the Offer/Answer negotiation, either endpoint encounters 735 an unknown value in the "cs-correlation" attribute, it MUST consider 736 that mechanism as unsupported, and MUST NOT include that value in 737 subsequent Offer/Answer negotiation. 739 5.3. Negotiating the correlation mechanisms 741 The four correlation mechanisms presented above (based on called 742 party number, User-User Information Element, DTMF digit sending, and 743 external) are non-exclusive, and can be used independently of each 744 other. In order to know how to populate the "cs-correlation" 745 attribute, the endpoints need to agree which endpoint will become the 746 active party, i.e., the one that will set up the circuit-switched 747 bearer. 749 5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup 751 In order to avoid a situation where both endpoints attempt to 752 initiate a connection simultaneously, the direction in which the 753 circuit-switched bearer is set up MUST be negotiated during the Offer 754 /Answer exchange. 756 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 757 agree which endpoint acts as the active endpoint when initiating a 758 TCP connection. While RFC 4145 [RFC4145] was originally designed for 759 establishing TCP connections, it can be easily extrapolated to the 760 connection establishment of circuit-switched bearers. This 761 specification uses the concepts specified in RFC 4145 [RFC4145] for 762 agreeing on the direction of establishment of a circuit-switched 763 bearer. 765 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 766 "connection". The "setup" attribute indicates which of the endpoints 767 should initiate the connection establishment of the PSTN circuit- 768 switched bearer. Four values are defined in Section 4 of RFC 4145 769 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 770 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 771 attribute. 773 The "connection" attribute indicates whether a new connection is 774 needed or an existing connection is reused. The attribute can take 775 the values "new" or "existing". Please refer to Section 5 of RFC 776 4145 [RFC4145] for a detailed description of this attribute. 778 Implementations according to this specification MUST support the 779 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 780 but applied to circuit-switched bearers in the PSTN. 782 We define the active party as the one that initiates the circuit- 783 switched bearer after the Offer/Answer exchange. The passive party 784 is the one receiving the circuit-switched bearer. Either party may 785 indicate its desire to become the active or passive party during the 786 Offer/Answer exchange using the procedures described in Section 5.6. 788 5.3.2. Populating the cs-correlation attribute 790 By defining values for the subfields in the "a=cs-correlation" 791 attribute, the endpoint indicates that it is willing to become the 792 active party, and that it can use those values in the Calling party 793 number, User-User Information Element, or as DTMF tones during the 794 circuit-switched bearer setup. 796 Thus, the following rules apply: 798 An endpoint that can only become the active party in the circuit- 799 switched bearer setup MUST include all correlation mechanisms it 800 supports in the "a=cs-correlation" attribute, and MUST also 801 specify values for the "callerid", "uuie" and "dtmf" subfields. 802 Notice that the "external" subfield does not accept a value. 804 An endpoint that can only become the passive party in the circuit- 805 switched bearer setup MUST include all correlation mechanisms it 806 supports in the "a=cs-correlation" attribute, but MUST NOT specify 807 values for the subfields. 809 An endpoint that is willing to become either the active or passive 810 party (by including the "a=setup:actpass" attribute in the Offer), 811 MUST include all correlation mechanisms it supports in the "a=cs- 812 correlation" attribute, and MUST also specify values for the 813 "callerid", "uuie" and "dtmf" subfields. Notice that the 814 "external" subfield does not accept a value. 816 5.3.3. Considerations on correlations 818 Passive endpoints should expect an incoming CS call for setting up 819 the audio bearer. Passive endpoints MAY suppress the incoming CS 820 alert during a certain time periods. Additional restrictions can be 821 applied, such as the passive endpoint not alerting incoming calls 822 originated from the number that was observed during the offer/answer 823 negotiation. 825 There may be cases when an endpoint is not willing to include one or 826 more correlation mechanisms in the "a=cs-correlation" attribute line 827 even if it supports it. For example, some correlation mechanisms can 828 be omitted if the endpoint is certain that the PSTN network does not 829 support carrying the correlation identifier. Also, since using the 830 DTMF based correlation mechanism requires the call to be accepted 831 before DTMF tones cane be sent, some endpoints may enforce a policy 832 restricting this due to for example cost associated with received 833 calls, making the DTMF based mechanism unusable. 835 Note that it cannot be guaranteed that the correlation mechanisms 836 relying on caller identification, User-User Information Element and 837 DTMF sending will succeed even if the usage of those was agreed 838 beforehand. This is due to the fact that the correlation mechanisms 839 require support from the circuit-switched bearer technology used. 841 Therefore, even a single positive indication using any of these 842 mechanisms SHOULD be interpreted by the passive endpoint so that the 843 circuit-switched bearer establishment is related to the ongoing 844 session, even if the other correlation mechanisms fail. 846 If, after successfully negotiating any of the "callerid", "uuie" or 847 "dtmf" correlation mechanisms in the SDP offer/answer exchange, an 848 endpoint receives an incoming establishment of a circuit-switched 849 bearer with no correlation information present, the endpoint first 850 checks whether the offer/answer exchange was used to successfully 851 negotiate also the "external" correlation mechanism. If it was, the 852 endpoint should leave the decision to be made by this external means, 853 typically the human user. If the "external" correlation mechanism 854 was not succesfully negotiated, the endpoint should treat the call as 855 unrelated to the ongoing session in the IP domain. 857 5.4. Considerations for Usage of Existing SDP 859 5.4.1. Originator of the Session 861 According to SDP [RFC4566], the origin line in SDP has the following 862 syntax: 864 o= 865 867 Of interest here are the and fields, which 868 indicate the type of network and type of address, respectively. 869 Typically, this field carries the IP address of the originator of the 870 session. Even if the SDP was used to negotiate an audio or video 871 media stream transported over a circuit-switched bearer, the 872 originator is using SDP over an IP bearer. Therefore, and 873 fields in the "o=" line should be populated with the IP 874 address identifying the source of the signaling. 876 5.4.2. Contact information 878 SDP [RFC4566] defines the "p=" line which may include the phone 879 number of the person responsible for the conference. Even though 880 this line can carry a phone number, it is not suited for the purpose 881 of defining a connection address for the media. Therefore, we have 882 selected to define the PSTN specific connection addresses in the "c=" 883 line. 885 5.5. Considerations for Usage of Third Party Call Control (3PCC) 887 Best Current Practices for Third Party Call Control (3pcc) in the 888 Session Initiation Protocol (SIP) [RFC3725] outlines several flows 889 which are possible in third party call control scenarios and 890 recommends some flows for specific situations. 892 One of the assumptions in [RFC3725] is that an SDP Offer may include 893 a "black hole" connection address, which has the property that 894 packets sent to it will never leave the host which sent them. For 895 IPv4, this "black hole" connection address is 0.0.0.0, or a domain 896 name within the .invalid DNS top level domain. 898 When using an E.164 address scheme in the context of third-party call 899 control, when the User Agent needs to indicate an unknown phone 900 number, it MUST populate the of the SDP "c=" line with a 901 "-" string. 903 Note that this may result in the recipient of the initial offer 904 rejecting such offer if the recipient of the offer was not aware 905 of its own E.164 number. Consequently it will not be possible to 906 establish a circuit-switched bearer, since neither party is aware 907 of their E.164 number. 909 5.6. Offer/Answer mode extensions 911 In this section, we define extensions to the Offer/Answer model 912 defined in The Offer/Answer Model in SDP [RFC3264] to allow for PSTN 913 addresses to be used with the Offer/Answer model. 915 5.6.1. Generating the Initial Offer 917 The Offerer, wishing to use PSTN audio or video stream, MUST populate 918 the "c=" and "m=" lines as follows. 920 The endpoint MUST set the in the "c=" line to "PSTN", and 921 the to "E164". Furthermore, the endpoint SHOULD set the 922 field to its own international E.164 number 923 (with a leading "+"). If the endpoint is not aware of its own E.164 924 number, it MUST set the to "-". 926 In the "m=" line, the endpoint MUST set the subfield to 927 "audio" or "video", depending on the media type, and the 928 subfield to "PSTN". The subfield SHOULD be set to "9" (the 929 discard port). The values "audio" or "video" in the subfield 930 MUST NOT be set by the endpoint unless it has knowledge that these 931 bearer types are available on the circuit-switched network. 933 The subfield carries the payload type number(s) the endpoint is 934 wishing to use. Payload type numbers in this case refer to the 935 codecs that the endpoint wishes to use on the PSTN media stream. For 936 example, if the endpoint wishes to use the GSM codec, it would add 937 payload type number 3 in the list of codecs. The list of payload 938 types MUST only contain those codecs the endpoint is able to use on 939 the PSTN bearer. In case the endpoint is not aware of the codecs 940 available for the circuit-switched media streams, it MUST include a 941 dash ("-") in the subfield. 943 The mapping table of static payload types numbers to payload types is 944 initally specified in [RFC3551] and maintained by IANA. For dynamic 945 payload types, the endpoint MUST define the set of valid encoding 946 names and related parameters using the "a=rtpmap" attribute line. 947 See Section 6 of RFC 4566 [RFC4566] for details. 949 When generating the Offer, the Offerer MUST include an attribute line 950 "a=cs-correlation" in the SDP offer. The Offerer MUST NOT include 951 more than one "cs-correlation" attribute per media decription. The 952 "a=cs-correlation" line SHOULD contain an enumeration of all the 953 correlation mechanisms supported by the Offerer, in the format of 954 subfields. See Section 5.3.3 for more information on usage of the 955 correlation mechanisms. 957 The current list of subfields include "callerid", "uuie", "dtmf", and 958 "extenal", and they refer to the correlation mechanisms defined in 959 Section 5.2.3.2, Section 5.2.3.3, Section 5.2.3.4, and 960 Section 5.2.3.5 respectively. 962 If the Offerer supports any of the correlation mechanisms defined in 963 this memo, and is willing to become the active party, the Offerer 964 MUST add the "callerid", "uuie", "dtmf" and/or "extern" subfields and 965 MUST specify values for them as follows: 967 o the international E.164 number as the value in the "callerid" 968 subfield, 970 o the contents of the User-User information element as the value of 971 the "uuie" subfield, and/or 973 o the DTMF tone string as the value of the "dtmf" subfield 975 o for the "external" subfield, the endpoint MUST NOT specify any 976 value. 978 If the Offerer is only able to become the passive party in the 979 circuit-switched bearer setup, it MUST add at least one of the 980 possible correlation mechanisms, but MUST NOT specify values for 981 those subfields. 983 For example, if the Offerer is willing to use the User-User 984 Information element and DTMF digit sending mechanisms but can only 985 become the passive party, and is also able to let the human user 986 decide whether the correlation should be done or not, it includes the 987 following lines in the SDP: 989 a=cs-correlation:uuie dtmf external 991 a=setup:passive 993 If, on the other hand, the Offerer is willing to use the User-User 994 Information element and the DTMF correlation mechanisms and is able 995 to become the active or passive side, and is also able to let the 996 human user decide whether the correlation should be done or no, it 997 includes the following lines in the SDP: 999 a=cs-correlation:uuie:56A390F3D2B7310023 dtmf:14D*3 external 1001 a=setup:actpass 1003 The negotiation of the value of the 'setup' attribute takes place as 1004 defined in Section 4.1 of RFC4145 [RFC4145]. 1006 The Offerer states which role or roles it is willing to perform; and 1007 the Answerer, taking the Offerer's willingness into consideration, 1008 chooses which roles both endpoints will actually perform during the 1009 circuit-switched bearer setup. 1011 By 'active' endpoint, we refer to an endpoint that will establish the 1012 circuit-switched bearer; and by 'passive' endpoint, we refer to an 1013 endpoint that will receive a circuit-switched bearer. 1015 If an Offerer does not know its international E.164 number, it MUST 1016 set the 'a=setup' attribute to the value 'active'. If the Offerer 1017 knows its international E.164 number, it SHOULD set the value to 1018 either 'actpass' or 'passive'. 1020 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 1021 It indicates that the connection should not be established for the 1022 time being. 1024 The Offerer uses the "a=connection" attribute to decide whether a new 1025 circuit-switched bearer is to be established or not. For the initial 1026 Offer, the Offerer MUST use value 'new'. 1028 5.6.2. Generating the Answer 1030 If the Offer contained a circuit-switched audio or video stream, the 1031 Answerer first determines whether it is able to accept and use such 1032 streams on the circuit-switched network. If the Answerer does not 1033 support or is not willing to use circuit-switched media for the 1034 session, it MUST construct an Answer where the port number for such 1035 media stream(s) is set to zero, according to Section 6 of [RFC3264]. 1036 If the Answerer is willing to use circuit-switched media for the 1037 session, it MUST ignore the received port number (unless the port 1038 number is set to zero). 1040 If the Offer included a "-" as the payload type number, it indicates 1041 that the Offerer is not willing or able to define any specific 1042 payload type. Most often, a "-" is expected to be used instead of 1043 the payload type when the endpoint is not aware of or not willing to 1044 define the codecs which will eventually be used on the circuit- 1045 switched bearer. The circuit-switched signaling protocols have their 1046 own means of negotiating or indicating the codecs, therefore an 1047 Answerer SHOULD accept such Offers, and SHOULD set the payload type 1048 to "-" also in the Answer. 1050 If the Answerer explicitly wants to specify a codec for the circuit- 1051 switched media, it MAY set the respective payload numbers in the 1052 subfield in the answer. This behavior, however, is NOT 1053 RECOMMENDED. 1055 When receiving the Offer, the Answerer MUST determine whether it 1056 becomes the active or passive party. 1058 If the SDP in the Offer indicates that the Offerer is only able to 1059 become the active party, the Answerer needs to determine whether it 1060 is able to become the passive party. If this is not possible e.g. 1061 due to the Answerer not knowing its international E.164 number, the 1062 Answerer MUST reject the circuit-switched media by setting the port 1063 number to zero on the Answer. If the Answerer is aware of its 1064 international E.164 number, it MUST include the "a=setup" attribute 1065 in the Answer and set it to value "passive" or "holdconn". The 1066 Answerer MUST also include its E.164 number in the "c=" line. 1068 If the SDP in the Offer indicates that the Offerer is only able to 1069 become the passive party, the Answerer MUST verify that the Offerer's 1070 E.164 number is included in the "c=" line of the Offer. If the 1071 number is included, the Answerer MUST include the "a=setup" attribute 1072 in the Answer and set it to value "active" or "holdconn". If the 1073 number is not included, or the recipient of the Offer is not willing 1074 to establish a connection the E.164 based on a priori knowledge of 1075 cost, or other reasons, call establishment is not possible, and the 1076 Answerer MUST reject the circuit-switched media by setting the port 1077 number to zero in the Answer. 1079 If the SDP in the Offer indicates that the Offerer is able to become 1080 either the active or passive party, the Answerer determines which 1081 role it will take. If the Offer includes an international E.164 1082 number in the "c=" line, the Answerer SHOULD become the active party. 1083 If the Answerer does not become the active party, and if the Answerer 1084 is aware of its E.164 number, it MUST become the passive party. If 1085 the Answerer does not become the active or the passive party, it MUST 1086 reject the circuit-switched media by setting the port number to zero 1087 in the Answer. 1089 For each media description where the Offer includes a "a=cs- 1090 correlation" attribute, the Answerer MUST select from the Offer those 1091 correlation mechanisms it supports, and include in the Answer one "a 1092 =cs-correlation" attribute line containing those mechanisms it is 1093 willing to use. The Answerer MUST only add one "a=cs-correlation" 1094 attribute in those media descriptions where also the Offer included a 1095 "a=cs-correlation" attribute. The Answerer MUST NOT add any 1096 mechanisms which were not included in the offer. If there are more 1097 than one "cs-correlation" attributes per media description in the 1098 Offer, the Answerer MUST discard all but the first for any media 1099 description. Also, the Answerer MUST discard all unknown "cs- 1100 correlation" attribute values. 1102 If the Answerer becomes the active party, it MUST add a value to any 1103 of the possible subfields. 1105 If the Answerer becomes the passive party, it MUST NOT add any values 1106 to the subfields in the "cs-correlation" attribute. 1108 After generating and sending the Answer, if the Answerer became the 1109 active party, it 1111 o MUST extract the E.164 number from the "c=" line of the Offer and 1112 MUST establish a circuit-switched bearer to that address. 1114 o if the SDP Answer contained a value for the "callerid" subfield, 1115 MUST set the Calling Party Number Information Element to that 1116 number, 1118 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1119 send the User-User Information element according to the rules 1120 defined for the circuit-switched technology used, and set the 1121 value of the Information Element to that received in the SDP 1122 Offer, 1124 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1125 send those DTMF digits according to the circuit-switched 1126 technology used. 1128 If, on the other hand, the Answerer became the passive party, it 1130 o MUST be prepared to receive a circuit-switched bearer, 1132 o if the Offer contained a value for the "callerid" subfield, MUST 1133 compare that value to the Calling Party Number Information Element 1134 of the circuit-switched bearer. If the received Calling Party 1135 Number Information Element matches the value of the "callerid" 1136 subfield, the call SHOULD be treated as correlated to the ongoing 1137 session. 1139 o if the Offer contained a value for the "dtmf" subfield, MUST be 1140 prepared to receive and collect DTMF digits once the circuit- 1141 switched bearer is set up. The Answerer MUST compare the received 1142 DTMF digits to the value of the "dtmf" subfield. If the received 1143 DTMF digits match the value of the "dtmf" subfield in the "cs- 1144 correlation" attribute, the call SHOULD be treated as correlated 1145 to the ongoing session. 1147 o if the Offer contained a value for the "uuie" subfield, MUST be 1148 prepared to receive a User-User Information element once the 1149 circuit-switched bearer is set up. The Answerer MUST compare the 1150 received UUI to the value of the "uuie" subfield. If the value of 1151 the received UUI matches the value of the "uuie" subfield, the 1152 call SHOULD be treated as correlated to the ongoing session. 1154 o if the Offer contained a "external" subfield, MUST be prepared to 1155 receive a circuit-switched call and use the external means 1156 (typically the human user) for accepting or rejecting the call. 1158 If the Answerer becomes the active party, generates an SDP answer, 1159 and then it finds out that the circuit-switched call cannot be 1160 established, then the Answerer MUST create a new SDP offer where 1161 circuit-switched stream is removed from the session (actually, by 1162 setting the corresponding port in the m= line to zero) and send it to 1163 its counterpart. This is to synchronize both parties (and potential 1164 intermediaries) on the state of the session. 1166 5.6.3. Offerer processing the Answer 1168 When receiving the Answer, if the SDP does not contain "a=cs- 1169 correlation" attribute line, the Offerer should take that as an 1170 indication that the other party does not support or is not willing to 1171 use the procedures defined in the document for this session, and MUST 1172 revert to normal processing of SDP. 1174 When receiving the Answer, the Offerer MUST first determine whether 1175 it becomes the active or passive party, as described in 1176 Section 5.3.1. 1178 If the Offerer becomes the active party, it 1180 o MUST extract the E.164 number from the "c=" line and MUST 1181 establish a circuit-switched bearer to that address. 1183 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1184 send the User-User Information element according to the rules 1185 defined for the circuit-switched technology used, and set the 1186 value of the Information Element to that received in the SDP 1187 Answer, 1189 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1190 send those DTMF digits according to the circuit-switched 1191 technology used. 1193 If the Offerer becomes the passive party, it 1195 o MUST be prepared to receive a circuit-switched bearer, 1196 o Note that if delivery of the Answer is delayed for some reason, 1197 the circuit-switched call attempt may arrive at the Offerer before 1198 the Answer has been processed. In this case, since the 1199 correlation mechanisms are negotiated as part of the Offer/Answer 1200 exchange, the Answerer cannot know whether or not the incoming 1201 circuit-switched call attempt is correlated with the session being 1202 negotiated, the Offerer SHOULD answer the circuit-switched call 1203 attempt only after it has received and processed the Answer. 1205 o If the Answer contained a value for the "dtmf" subfield, the 1206 Offerer MUST be prepared to receive and collect DTMF digits once 1207 the circuit-switched bearer is set up. The Offerer SHOULD compare 1208 the received DTMF digits to the value of the "dtmf" subfield. If 1209 the received DTMF digits match the value of the "dtmf" subfield in 1210 the "cs-correlation" attribute, the call SHOULD be treated as 1211 correlated to the ongoing session. 1213 o If the Answer contained a value for the "uuie" subfield, the 1214 Offerer MUST be prepared to receive a User-User Information 1215 element once the circuit-switched bearer is set up. The Offerer 1216 SHOULD compare the received UUI to the value of the "uuie" 1217 subfield. If the value of the received UUI matches the value of 1218 the "uuie" subfield, the call SHOULD be treated as correlated to 1219 the ongoing session. 1221 o If the Answer contained a "external" subfield, the Offerer MUST be 1222 prepared to receive a circuit-switched call and use the external 1223 means (typically the human user) for accepting or rejecting the 1224 call. 1226 According the Offer/Answer Model with SDP [RFC3264], the Offerer 1227 needs to be ready to receive media as soon as the Offer has been 1228 sent. It may happen that the Answerer, if it became the active 1229 party, will initiate a circuit-switched bearer setup which will 1230 arrive at the Offerer before the Answer has arrived. However, the 1231 Offerer needs to receive the Answer and examine the information about 1232 the correlation mechanisms in order to successfully perform 1233 correlation of the circuit-switched call to the session. Therefore, 1234 if the Offerer receives an incoming circuit-switched call, it MUST 1235 NOT accept the call before the Answer has been received. If no 1236 Answer is received during an implementation specific time, the 1237 Offerer MUST either modify the session according to [RFC3264] or 1238 terminate it according to the session signaling procedures in 1239 question (for terminating a SIP session, see Section 15 of 1240 [RFC3261]). 1242 5.6.4. Modifying the session 1244 If, at a later time, one of the parties wishes to modify the session, 1245 e.g., by adding new media stream, or by changing properties used on 1246 an existing stream, it may do so via the mechanisms defined for An 1247 Offer/Answer Model with SDP [RFC3264]. 1249 If there is an existing circuit-switched bearer between the 1250 endpoints, and the Offerer wants to reuse that, the Offerer MUST set 1251 the value of the "a=connection" attribute to 'existing'. 1253 If either party removes the circuit-switched media from the session 1254 (by setting the port number to zero), it MUST terminate the circuit- 1255 switched bearer using whatever mechanism is appropriate for the 1256 technology in question. 1258 If either party wishes to drop and reestablish an existing call, that 1259 party MUST first remove the circuit-switched media from the session 1260 by setting the port number to zero, and then use another Offer/Answer 1261 exchange where it MUST set the "a=connection" attribute to 'new'". 1262 If the media types are different (for example, a different codec will 1263 be used for the circuit-switched bearer), the media descriptions for 1264 terminating the existing bearer and the new bearer can be in the same 1265 Offer. 1267 5.7. Formal Syntax 1269 The following is the formal Augmented Backus-Naur Form (ABNF) 1270 [RFC5234] syntax that supports the extensions defined in this 1271 specification. The syntax is built above the SDP [RFC4566] and the 1272 tel URI [RFC3966] grammars. Implementations according to this 1273 specification MUST be compliant with this syntax. 1275 Figure 2 shows the formal syntax of the extensions defined in this 1276 memo. 1278 ; extension to the connection field originally specified 1279 ; in RFC4566 1281 connection-field = [%x63 "=" nettype SP addrtype SP 1282 connection-address CRLF] 1283 ; CRLF defined in RFC5234 1285 ;nettype and addrtype are defined in RFC 4566 1287 connection-address /= global-number-digits / "-" 1288 ; global-number-digits specified in RFC3966 1290 ;subrules for correlation attribute 1291 attribute /= cs-correlation-attr 1292 ; attribute defined in RFC4566 1293 cs-correlation-attr = "cs-correlation:" corr-mechanisms 1294 corr-mechanisms = corr-mech *(SP corr-mech) 1295 corr-mech = caller-id-mech / uuie-mech / 1296 dtmf-mech / external-mech / 1297 ext-mech 1298 caller-id-mech = "callerid" [":" caller-id-value] 1299 caller-id-value = "+" 1*15DIGIT 1300 ; DIGIT defined in RFC5234 1301 uuie-mech = "uuie" [":" uuie-value] 1302 uuie-value = 1*65(HEXDIG HEXDIG) 1303 ;This represents up to 130 HEXDIG 1304 ; (65 octets) 1305 ;HEXDIG defined in RFC5234 1306 ;HEXDIG defined as 0-9, A-F 1307 dtmf-mech = "dtmf" [":" dtmf-value] 1308 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1309 ;0-9, A-D, '#' and '*' 1310 external-mech = "external" 1311 ext-mech = ext-mech-name [":" ext-mech-value] 1312 ext-mech-name = token 1313 ext-mech-value = token 1314 ; token is specified in RFC4566 1316 Figure 2: Syntax of the SDP extensions 1318 6. Examples 1320 In the examples below, where an SDP line is too long to be displayed 1321 as a single line, a breaking character "\" indicates continuation in 1322 the following line. Note that this character is included for display 1323 purposes only. Implementations MUST write a single line without 1324 breaks. 1326 6.1. Single PSTN audio stream 1328 Endpoint A Endpoint B 1329 | | 1330 | (1) SDP Offer (PSTN audio) | 1331 |--------------------------------->| 1332 | | 1333 | (2) SDP Answer (PSTN audio) | 1334 |<---------------------------------| 1335 | | 1336 | PSTN call setup | 1337 |<---------------------------------| 1338 | | 1339 |<==== media over PSTN bearer ====>| 1340 | | 1342 Figure 3: Basic flow 1344 Figure 3 shows a basic example that describes a single audio media 1345 stream over a circuit-switched bearer. Endpoint A generates a SDP 1346 Offer which is shown in Figure 4. The Offer describes a PSTN 1347 circuit-switched bearer in the "m=" and "c=" line where it also 1348 indicates its international E.164 number format. Additionally, 1349 Endpoint A expresses that it can initiate the circuit-switched bearer 1350 or be the recipient of it in the "a=setup" attribute line. The SDP 1351 Offer also includes correlation identifiers that this endpoint will 1352 insert in the Calling Party Number and/or User-User Information 1353 Element of the PSTN call setup if eventually this endpoint initiates 1354 the PSTN call. Endpoint A also includes the "external" as one 1355 correlation mechanism indicating that it can use the human user to 1356 perform correlation in case other mechanisms fail. 1358 v=0 1359 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1360 s= 1361 t=0 0 1362 m=audio 9 PSTN - 1363 c=PSTN E164 +441134960123 1364 a=setup:actpass 1365 a=connection:new 1366 a=cs-correlation:callerid:+441134960123 \ 1367 uuie:56A390F3D2B7310023 external 1369 Figure 4: SDP offer (1) 1371 Endpoint B generates a SDP Answer (Figure 5), describing a PSTN audio 1372 media on port 9 without information on the media sub-type on the "m=" 1373 line. The "c=" line contains B's international E.164 number. In the 1374 "a=setup" line Endpoint B indicates that it is willing to become the 1375 active endpoint when establishing the PSTN call, and it also includes 1376 the "a=cs-correlation" attribute line containing the values it is 1377 going to include in the Calling Party Number and User-User IE of the 1378 PSTN call establishment. Endpoint B is also able to perform 1379 correlation by external means, in case other correlation mechanisms 1380 fail. 1382 v=0 1383 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1384 s= 1385 t=0 0 1386 m=audio 9 PSTN - 1387 c=PSTN E164 +441134960124 1388 a=setup:active 1389 a=connection:new 1390 a=cs-correlation:callerid:+441134960124 \ 1391 uuie:74B9027A869D7966A2 external 1393 Figure 5: SDP Answer with circuit-switched media 1395 When Endoint A receives the Answer, it examines that B is willing to 1396 become the active endpoint when setting up the PSTN call. Endoint A 1397 temporarily stores B's E.164 number and the User-User IE value of the 1398 "cs-correlation" attribute, and waits for a circuit-switched bearer 1399 establishment. 1401 Endpoint B initiates a circuit-switched bearer using whatever 1402 circuit-switched technology is available for it. The called party 1403 number is set to A's number, and calling party number is set to B's 1404 own number. Endpoint B also sets the User-User Information Element 1405 value to the one contained in the SDP Answer. 1407 When Endpoint A receives the circuit-switched bearer establishment, 1408 it examines the UUIE and the calling party number, and by comparing 1409 those received during O/A exchange determines that the call is 1410 related to the SDP session. 1412 It may also be that neither the UUIE nor the calling party number is 1413 received by the called party, or the format of the calling party 1414 number is changed by the PSTN. Implementations may still accept such 1415 call establishment attempts as being related to the session that was 1416 established in the IP network. As it cannot be guaranteed that the 1417 values used for correlation are always passed intact through the 1418 network, they should be treated as additional hints that the circuit- 1419 switched bearer is actually related to the session. 1421 6.2. Advanced SDP example: Circuit-Switched Audio and Video Streams 1423 Endpoint A Endpoint B 1424 | | 1425 | (1) SDP Offer (PSTN audio and video) | 1426 |------------------------------------------->| 1427 | | 1428 | (2) SDP Answer (PSTN audio) | 1429 |<-------------------------------------------| 1430 | | 1431 | PSTN call setup | 1432 |<-------------------------------------------| 1433 | | 1434 |<======== media over PSTN bearer ==========>| 1435 | | 1437 Figure 6: Circuit-Switched Audio and Video streams 1439 Figure 6 shows an example of negotiating audio and video media 1440 streams over circuit-switched bearers. 1442 v=0 1443 o=alice 2890844526 2890842807 IN IP4 192.0.2.5 1444 s= 1445 t=0 0 1446 a=setup:actpass 1447 a=connection:new 1448 c=PSTN E164 +441134960123 1449 m=audio 9 PSTN - 1450 a=cs-correlation:dtmf:1234536 1451 m=video 9 PSTN 34 1452 a=rtpmap:34 H263/90000 1453 a=cs-correlation:callerid:+441134960123 1455 Figure 7: SDP offer with circuit-switched audio and video (1) 1457 Upon receiving the SDP offer described in Figure 7, Endpoint B 1458 rejects the video stream as the device does not currently support 1459 video, but accepts the circuit-switched audio stream. As Endoint A 1460 indicated that it is able to become either the active or passive 1461 party, Endpoint B gets to select which role it would like to take. 1462 Since the Offer contained the international E.164 number of Endpoint 1463 A, Endpoint B decides that it becomes the active party in setting up 1464 the circuit-switched bearer. B includes a new value in the "dtmf" 1465 subfield of the "cs-correlation" attribute, which it is going to send 1466 as DTMF tones once the bearer setup is complete. The Answer is 1467 described in Figure 8 1468 v=0 1469 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1470 s= 1471 t=0 0 1472 a=setup:active 1473 a=connection:new 1474 c=PSTN E164 +441134960124 1475 m=audio 9 PSTN - 1476 a=cs-correlation:dtmf:654321 1477 m=video 0 PSTN 34 1478 a=cs-correlation:callerid:+441134960124 1480 Figure 8: SDP answer with circuit-switched audio and video (2) 1482 7. Security Considerations 1484 This document provides an extension on top of RFC 4566 [RFC4566], and 1485 RFC 3264 [RFC3264]. As such, the security considerations of those 1486 documents apply. 1488 This memo provides mechanisms to agree on a correlation identifier or 1489 identifiers that are used to evaluate whether an incoming circuit- 1490 switched bearer is related to an ongoing session in the IP domain. 1491 If an attacker replicates the correlation identifier and establishes 1492 a call within the time window the receiving endpoint is expecting a 1493 call, the attacker may be able to hijack the circuit-switched bearer. 1494 These types of attacks are not specific to the mechanisms presented 1495 in this memo. For example, caller ID spoofing is a well known attack 1496 in the PSTN. Users are advised to use the same caution before 1497 revealing sensitive information as they would on any other phone 1498 call. Furthermore, users are advised that mechanisms that may be in 1499 use in the IP domain for securing the media, like Secure RTP (SRTP) 1500 [RFC3711], are not available in the CS domain. 1502 For the purposes of establishing a circuit-switched bearer, the 1503 active endpoint needs to know the passive endpoint's phone number. 1504 Phone numbers are sensitive information, and some people may choose 1505 not to reveal their phone numbers when calling using supplementary 1506 services like Calling Line Identification Restriction (CLIR) in GSM. 1507 Implementations should take the caller's preferences regarding 1508 calling line identification into account if possible, by restricting 1509 the inclusion of the phone number in SDP "c=" line if the caller has 1510 chosen to use CLIR. If this is not possible, implementations may 1511 present a prompt informing the user that their phone number may be 1512 transmitted to the other party. 1514 Similarly as with IP addresses, if there is a desire to protect the 1515 SDP containing phone numbers carried in SIP, implementers are advised 1516 to follow the security mechanisms defined in [RFC3261]. 1518 It is possible that an attacker creates a circuit-switched session 1519 whereby the attacked endpoint should dial a circuit-switched number, 1520 perhaps even a premium-rate telephone number. To mitigate the 1521 consequences of this attack, endpoints MUST authenticate and trust 1522 remote endpoints users who try to remain passive in the circuit- 1523 switched connection establishment. It is RECOMMENDED that endpoints 1524 have local policies precluding the active establishment of circuit 1525 switched connections to certain numbers (e.g., international, 1526 premium, long distance). Additionally, it is strongly RECOMMENDED 1527 that the end user is asked for consent prior to the endpoint 1528 initiating a circuit-switched connection. 1530 8. IANA Considerations 1532 This document instructs IANA to register a number of SDP tokens 1533 according to the following data. 1535 8.1. Registration of new cs-correlation SDP attribute 1537 Contact: Miguel Garcia 1539 Attribute name: cs-correlation 1541 Long-form attribute name: PSTN Correlation Identifier 1543 Type of attribute: media level only 1545 Subject to charset: No 1547 Description: This attribute provides the Correlation Identifier 1548 used in PSTN signaling 1550 Appropriate values:see Section 5.2.3.1 1552 Specification: RFC XXXX 1554 The IANA is requested to create a subregistry for 'cs-correlation' 1555 attribute under the Session Description Protocol (SDP) Parameters 1556 registry. The initial values for the subregistry are presented in 1557 the following, and IANA is requested to add them into its database: 1559 Value of 'cs-correlation' attribute Reference Description 1560 ----------------------------------- --------- ----------- 1561 callerid RFC XXXX Caller ID 1562 uuie RFC XXXX User-User 1563 Information Element 1564 dtmf RFC XXXX Dual-tone 1565 Multifrequency 1566 external RFC XXXX External 1568 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1569 reference to the RFC number of this draft. 1571 As per the terminology in [RFC5226], the registration policy for new 1572 values of 'cs-correlation' parameter is 'Specification Required'. 1574 8.2. Registration of a new "nettype" value 1576 This memo provides instructions to IANA to register a new "nettype" 1577 in the Session Description Protocol Parameters registry [1]. The 1578 registration data, according to RFC 4566 [RFC4566] follows. 1580 Type SDP Name Reference 1581 ---- ------------------ --------- 1582 nettype PSTN [RFCxxxx] 1584 8.3. Registration of new "addrtype" value 1586 This memo provides instructions to IANA to register a new "addrtype" 1587 in the Session Description Protocol Parameters registry [2]. The 1588 registration data, according to RFC 4566 [RFC4566] follows. 1590 Type SDP Name Reference 1591 ---- ------------------ --------- 1592 addrtype E164 [RFCxxxx] 1594 Note: RFC XXXX defines the "E164" addrtype in the context of the 1595 "PSTN" nettype only. Please refer to the relevant RFC for a 1596 description of that representation. 1598 8.4. Registration of a new "proto" value 1600 This memo provides instructions to IANA to register a new "proto" in 1601 the Session Description Protocol Parameters registry [3]. The 1602 registration data, according to RFC 4566 [RFC4566] follows. 1604 Type SDP Name Reference 1605 -------------- --------------------------- --------- 1606 proto PSTN [RFCxxxx] 1608 The related "fmt" namespace re-uses the conventions and payload type 1609 number defined for RTP/AVP. In RFC XXXX, the RTP audio and video 1610 media types, when applied to PSTN circuit-switched bearers, represent 1611 merely an audio or video codec in its native format directly on top 1612 of a single PSTN bearer. 1614 In come cases, the endpoint is not able to determine the list of 1615 available codecs for circuit-switched media streams. In this case, 1616 in order to be syntactically compliant with SDP [RFC4566], the 1617 endpoint MUST include a single dash ("-") in the subfield. 1619 9. Acknowledgments 1621 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas 1622 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan 1623 Rosenberg, Ingemar Johansson, Christer Holmberg, Alf Heidermark, Tom 1624 Taylor, Thomas Belling, Keith Drage, and Andrew Allen for providing 1625 their insight and comments on this document. 1627 10. References 1629 10.1. Normative References 1631 [ITU.Q931.1998] 1632 "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN 1633 User - Network Interface Layer 3 Specification for Basic 1634 Call Control", ISO Standard 9594-1, May 1998. 1636 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1637 Requirement Levels", BCP 14, RFC 2119, March 1997. 1639 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1640 with Session Description Protocol (SDP)", RFC 3264, June 1641 2002. 1643 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 1644 3966, December 2004. 1646 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1647 the Session Description Protocol (SDP)", RFC 4145, 1648 September 2005. 1650 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1651 Description Protocol", RFC 4566, July 2006. 1653 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1654 Encodings", RFC 4648, October 2006. 1656 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1657 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1658 May 2008. 1660 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1661 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1663 10.2. Informative References 1665 [I-D.ietf-cuss-sip-uui] 1666 Johnston, A. and J. Rafferty, "A Mechanism for 1667 Transporting User to User Call Control Information in 1668 SIP", draft-ietf-cuss-sip-uui-11 (work in progress), 1669 October 2013. 1671 [ITU.E164.1991] 1672 International Telecommunications Union, "The International 1673 Public Telecommunication Numbering Plan", ITU-T 1674 Recommendation E.164, 1991. 1676 [ITU.Q23.1988] 1677 International Telecommunications Union, "Technical 1678 features of push-button telephone sets", ITU-T Technical 1679 Recommendation Q.23, 1988. 1681 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1682 Session Description Protocol (SDP) for ATM Bearer 1683 Connections", RFC 3108, May 2001. 1685 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1686 A., Peterson, J., Sparks, R., Handley, M., and E. 1687 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1688 June 2002. 1690 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1691 Jacobson, "RTP: A Transport Protocol for Real-Time 1692 Applications", STD 64, RFC 3550, July 2003. 1694 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1695 Video Conferences with Minimal Control", STD 65, RFC 3551, 1696 July 2003. 1698 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1699 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1700 RFC 3711, March 2004. 1702 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1703 Camarillo, "Best Current Practices for Third Party Call 1704 Control (3pcc) in the Session Initiation Protocol (SIP)", 1705 BCP 85, RFC 3725, April 2004. 1707 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1708 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1710 [TS.24.008] 1711 3GPP, "Mobile radio interface Layer 3 specification; Core 1712 network protocols; Stage 3", 3GPP TS 24.008 3.20.0, 1713 December 2005. 1715 10.3. URIs 1717 [1] http://www.iana.org/assignments/sdp-parameters 1719 [2] http://www.iana.org/assignments/sdp-parameters 1721 [3] http://www.iana.org/assignments/sdp-parameters 1723 Authors' Addresses 1725 Miguel A. Garcia-Martin 1726 Ericsson 1727 Calle Via de los Poblados 13 1728 Madrid, ES 28033 1729 Spain 1731 Email: miguel.a.garcia@ericsson.com 1733 Simo Veikkolainen 1734 Nokia 1735 P.O. Box 226 1736 NOKIA GROUP, FI 00045 1737 Finland 1739 Phone: +358 50 486 4463 1740 Email: simo.veikkolainen@nokia.com