idnits 2.17.1 draft-ietf-mmusic-sdp-cs-17.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The document has examples using IPv4 documentation addresses according to RFC6890, but does not use any IPv6 documentation addresses. Maybe there should be IPv6 examples, too? Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 14, 2013) is 4118 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'RFCxxxx' is mentioned on line 1509, but not defined ** 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reference: RFC 4566 (Obsoleted by RFC 8866) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) == Outdated reference: A later version (-17) exists of draft-ietf-cuss-sip-uui-08 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--).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: July 18, 2013 Nokia 6 January 14, 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-17 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 July 18, 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 . . . . . . . . . . . . . . . . . . . . . . . . . 5 55 2. Conventions Used in This Document . . . . . . . . . . . . . . 6 56 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 6 57 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7 58 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 7 59 5. Protocol Description . . . . . . . . . . . . . . . . . . . . . 9 60 5.1. Level of Compliance . . . . . . . . . . . . . . . . . . . 9 61 5.2. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 9 62 5.2.1. Connection Data . . . . . . . . . . . . . . . . . . . 9 63 5.2.2. Media Descriptions . . . . . . . . . . . . . . . . . . 11 64 5.2.3. Correlating the PSTN Circuit-Switched Bearer with 65 SDP . . . . . . . . . . . . . . . . . . . . . . . . . 12 66 5.2.3.1. The "cs-correlation" attribute . . . . . . . . . . 13 67 5.2.3.2. Caller-ID Correlation Mechanism . . . . . . . . . 13 68 5.2.3.3. User-User Information Element Correlation 69 Mechanism . . . . . . . . . . . . . . . . . . . . 14 70 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . . 16 71 5.2.3.5. Extensions to correlation mechanisms . . . . . . . 17 72 5.3. Negotiating the correlation mechanisms . . . . . . . . . . 17 73 5.3.1. Determining the Direction of the Circuit-Switched 74 Bearer Setup . . . . . . . . . . . . . . . . . . . . . 17 75 5.3.2. Populating the cs-correlation attribute . . . . . . . 18 76 5.3.3. Considerations on correlations . . . . . . . . . . . . 19 77 5.4. Considerations for Usage of Existing SDP . . . . . . . . . 19 78 5.4.1. Originator of the Session . . . . . . . . . . . . . . 19 79 5.4.2. Contact information . . . . . . . . . . . . . . . . . 20 80 5.5. Considerations for Usage of Third Party Call Control 81 (3PCC) . . . . . . . . . . . . . . . . . . . . . . . . . . 20 82 5.6. Offer/Answer mode extensions . . . . . . . . . . . . . . . 21 83 5.6.1. Generating the Initial Offer . . . . . . . . . . . . . 21 84 5.6.2. Generating the Answer . . . . . . . . . . . . . . . . 23 85 5.6.3. Offerer processing the Answer . . . . . . . . . . . . 26 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 Video 91 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" values . . . . . . . . . . 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 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 104 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 192 bearer MUST be available. 194 REQ-2: The mechanism MUST allow the endpoints to combine circuit- 195 switched audio or video media streams with other 196 complementary 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 200 endpoint is active when initiating the circuit-switched 201 bearer. 203 REQ-4: The mechanism MUST be independent of the type of the circuit- 204 switched access (e.g., Integrated Services Digital Network 205 (ISDN), Global System for Mobile Communication (GSM), etc.) 207 REQ-5: There MUST be a mechanism that helps an endpoint to correlate 208 an incoming circuit-switched bearer with the one negotiated 209 in SDP, as opposed to another incoming call that is not 210 related to that. 212 REQ-6: It MUST be possible for endpoints to advertise different 213 lists of audio or video codecs in the circuit-switched audio 214 or video stream from those used in a packet-switched audio or 215 video stream. 217 REQ-7: It MUST be possible for endpoints to not advertise the list 218 of available codecs for circuit-switched audio or video 219 streams. 221 4. Overview of Operation 223 The mechanism defined in this memo extends SDP and allows describing 224 an audio or video media stream established over a circuit-switched 225 bearer. A new network type ("PSTN") and a new protocol type ("PSTN") 226 are defined for the "c=" and "m=" lines to be able to describe a 227 media stream over a circuit-switched bearer. These SDP extensions 228 are described in Section 5.2. Since circuit-switched bearers are 229 connection-oriented media streams, the mechanism re-uses the 230 connection-oriented extensions defined in RFC 4145 [RFC4145] to 231 negotiate the active and passive sides of a connection setup. This 232 is further described in Section 5.3.1. 234 4.1. Example Call Flow 236 Consider the example presented in Figure 1. In this example, Alice 237 is located in an environment where she has access to both IP and 238 circuit-switched bearers for communicating with other endpoints. 239 Alice decides that the circuit-switched bearer offers a better 240 perceived quality of service for voice, and issues an SDP Offer 241 containing the description of an audio media stream over circuit- 242 switched bearer. 244 Alice Bob 245 | (1) SDP Offer (PSTN audio) | 246 |----------------------------------->| 247 | | 248 | (2) SDP Answer (PSTN audio) | 249 |<-----------------------------------| 250 | | 251 | PSTN call setup | 252 |<-----------------------------------| 253 | | 254 | | 255 |<===== media over PSTN bearer =====>| 256 | | 258 Figure 1: Example Flow 260 Bob receives the SDP offer and determines that he is located in an 261 environment where the IP based bearer is not suitable for real-time 262 audio media. However he also has PSTN circuit-switched bearer 263 available for audio. Bob generates an SDP answer containing a 264 description of the audio media stream over a circuit-switched bearer. 266 During the offer-answer exchange Alice and Bob also agree the 267 direction in which the circuit-switched bearer should be established. 268 In this example, Bob becomes the active party, in other words, he 269 establishes the circuit-switched call to the other endpoint. The 270 Offer/Answer exchange contains identifiers or references that can be 271 used on the circuit-switched network for addressing the other 272 endpoint, as well as information that is used to determine that the 273 incoming circuit-switched bearer establishment is related to the 274 ongoing session between Alice and Bob. 276 Bob establishes a circuit-switched bearer towards Alice using 277 whatever mechanisms are defined for the network type in question. 278 When receiving the incoming circuit-switched connection attempt, 279 Alice is able to determine that the attempt is related to the session 280 she is just establishing with Bob. 282 Alice accepts the circuit-switched connection; the circuit-switched 283 bearer setup is completed. Bob and Alice can now use the circuit- 284 switched connection for two-way audio media. 286 If, for some reason, Bob would like to reject the offered stream, he 287 would set the port number of the specific stream to zero, as 288 specified in RFC3264 [RFC3264]. Also, if Bob does not understand 289 some of the SDP attributes specified in this document, he would 290 ignore them, as specified in RFC4566 [RFC4566]. 292 5. Protocol Description 294 5.1. Level of Compliance 296 Implementations according to this specification MUST implement the 297 SDP extensions described in Section 5.2, and MUST implement the 298 considerations discussed in Section 5.3, Section 5.4 and Section 5.6. 300 5.2. Extensions to SDP 302 This section provides the syntax and semantics of the extensions 303 required for providing a description of audio or video media streams 304 over circuit-switched bearers in SDP. 306 5.2.1. Connection Data 308 According to SDP [RFC4566], the connection data line in SDP has the 309 following syntax: 311 c= 313 where indicates the network type, indicates the 314 address type, and the is the connection address, 315 which is dependent on the address type. 317 At the moment, the only network type defined is "IN", which indicates 318 Internet network type. The address types "IP4" and "IP6" indicate 319 the type of IP addresses. 321 This memo defines a new network type for describing a circuit- 322 switched bearer network type in the PSTN. The mnemonic "PSTN" is 323 used for this network type. 325 For the address type, we initially consider the possibility of 326 describing E.164 telephone numbers. We define a new "E164" address 327 type to be used within the context of a "PSTN" network type. The 328 "E164" address type indicates that the connection address contains an 329 E.164 number represented according to the ITU-T E.164 [ITU.E164.1991] 330 recommendation. 332 It is a common convention that an international E.164 number contains 333 a leading '+' sign. For consistency's sake, we also require the 334 E.164 telephone is prepended with a '+', even if that is not 335 necessary for routing of the call in the PSTN network. 337 There are cases, though, when the endpoint is merely aware of a 338 circuit-switched bearer, without having further information about the 339 address type or the E.164 number allocated to it. In these cases a 340 dash ("-") is used to indicate an unknown address type or connection 341 address. This makes the connection data line be according to the SDP 342 syntax. 344 Please note that these "E164" and "-" address types defined in this 345 memo are exclusively defined to be used in conjunction with the 346 "PSTN" network type in accordance with [RFC4566]. Usage of "E164" or 347 "-" address types in conjunction with other network types may be 348 defined elsewhere. 350 This memo exclusively uses the international representation of E.164 351 numbers, i.e., those including a country code and, as described above 352 prepended with a '+' sign. Implementations conforming to this 353 specification and using the "E164" address type together with the 354 "PSTN" network type MUST use the 'global-number-digits' construction 355 specified in RFC 3966 [RFC3966] for representing international E.164 356 numbers. This representation requires the presence of the '+' sign, 357 and additionally allows for the presence of one or more 'visual- 358 separator' constructions for easier human readability (see 359 Section 5.7). 361 Note that and/or MUST NOT be omitted 362 when unknown since this would violate basic syntax of SDP [RFC4566]. 363 In such cases, they MUST be set to a "-". 365 The following are examples of the extension to the connection data 366 line: 368 c=PSTN E164 +441134960123 370 c=PSTN - - 372 When the is PSTN, the connection address is defined as 373 follows: 375 o an international E.164 number 377 When the is "-", the connection address is defined as 378 follows: 380 o the value "-", signifying that the address is unknown 382 o any syntactically valid value, which is to be ignored 384 5.2.2. Media Descriptions 386 According to SDP [RFC4566], the media description line in SDP has the 387 following syntax: 389 m= ... 391 The subfield carries the media type. For establishing an 392 audio bearer, the existing "audio" media type is used. For 393 establishing a video bearer, the existing "video" media type is used. 395 The subfield is the transport port to which the media stream 396 is sent. Circuit-switched access lacks the concept of a port number, 397 and therefore the subfield does not carry any meaningful 398 value. In order to be compliant with SDP syntax, implementations 399 SHOULD set the subfield to the discard port value "9" and MUST 400 ignore it on reception. 402 According to RFC 3264 [RFC3264], a port number of zero in the offer 403 of a unicast stream indicates that the stream is offered but must not 404 be used. If a port number of zero is present in the answer of a 405 unicast stream, it indicates that the stream is rejected. These 406 rules are still valid when the media line in SDP represents a 407 circuit-switched bearer. 409 The subfield is the transport protocol. The circuit-switched 410 bearer uses whatever transport protocol it has available. This 411 subfield SHOULD be set to the mnemonic "PSTN" to be syntactically 412 correct with SDP [RFC4566] and to indicate the usage of circuit- 413 switched protocols in the PSTN. 415 The subfield is the media format description. In the classical 416 usage of SDP to describe RTP-based media streams, when the 417 subfield is set to "RTP/AVP" or "RTP/SAVP", the subfield 418 contains the payload types as defined in the RTP audio profile 419 [RFC3551]. 421 When "RTP/AVP" is used in the field, the subfield 422 contains the RTP payload type numbers. We use the subfield to 423 indicate the list of available codecs over the circuit-switched 424 bearer, by re-using the conventions and payload type numbers defined 425 for RTP/AVP. The RTP audio and video media types, which, when 426 applied to PSTN circuit-switched bearers, represent merely an audio 427 or video codec. The endpoint SHOULD only use those payload type 428 whose corresponding codecs is available for PSTN media streams. 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. 481 Implementations MAY use any of these mechanisms and MAY use two or 482 more mechanisms simultaneously. 484 5.2.3.1. The "cs-correlation" attribute 486 In order to provide support for the correlation mechanisms, we define 487 a new media-level SDP attribute called "cs-correlation". This "cs- 488 correlation" attribute can include any of the "callerid", "uuie", or 489 "dtmf" subfields, which specify additional information required by 490 the Caller-ID, User to User Information, or DTMF correlation 491 mechanisms, respectively. The list of correlation mechanisms may be 492 extended by other specifications, see Section 5.2.3.5 for more 493 details. There MUST be at most one "cs-correlation" attribute per 494 media description. 496 The following sections provide more detailed information of these 497 subfields. Section 5.7> defined the formal syntax. 499 The values "callerid", "uuie" and "dtmf" refer to the correlation 500 mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and 501 Section 5.2.3.4, respectively. The formal Augmented Backus-Naur 502 Format (ABNF) syntax of the "cs-correlation" attribute is presented 503 in Section 5.7. 505 5.2.3.2. Caller-ID Correlation Mechanism 507 The Caller-ID correlation mechanisms consists of an exchange of the 508 calling party number as an international E.164 number in SDP, 509 followed by the availability of the Calling Party Number information 510 element in the call setup signaling of the circuit switched 511 connection. If both pieces of information match, the circuit- 512 switched bearer is correlated to the session described in SDP. 514 Example of inclusion of an international E.164 number in the "cs- 515 correlation" attribute is: 517 a=cs-correlation:callerid:+441134960123 519 The presence of the "callerid" subfield indicates that the endpoint 520 supports use of the calling party number as a means of correlating a 521 PSTN call with the session being negotiated. The "callerid" subfield 522 MAY be accompanied by the international E.164 number of the party 523 inserting the parameter. 525 Note that there are no guarantees that this correlation mechanism 526 works or is even available, due a number of problems: 528 o The endpoint might not be aware of its own E.164 number, in which 529 case it cannot populate the SDP appropriately. 531 o The Calling Party Number information element in the circuit- 532 switched signaling might not be available, e.g., due to policy 533 restrictions of the network operator or caller restriction due to 534 privacy. 536 o The Calling Party Number information element in the circuit- 537 switched signaling might be available, but the digit 538 representation of the E.164 number might differ from the one 539 expressed in the SDP, due to, e.g., lack of of country code. To 540 mitigate this problem implementations should consider only some of 541 the rightmost digits from the E.164 number for correlation. For 542 example, the numbers +44-113-496-0123 and 0113-496-0123 could be 543 considered as the same number. This is also the behavior of some 544 cellular phones, which correlate the incoming calling party with a 545 number stored in the phone book, for the purpose of displaying the 546 caller's name. Please refer to ITU-T E.164 reccommendation 547 [ITU.E164.1991] for consideration of the relevant number of digits 548 to consider. 550 5.2.3.3. User-User Information Element Correlation Mechanism 552 A second correlation mechanism is based on including in SDP a string 553 that represents the User-User Information Element that is part of the 554 call setup signaling of the circuit-switched bearer. The User-User 555 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 556 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 557 Element has a maximum size of 35 or 131 octets, depending on the 558 actual message of the PSTN protocol where it is included and the 559 network settings. 561 The mechanism works as follows: An endpoint creates a User-User 562 Information Element, according to the requirements of the call setup 563 signaling protocol. The same value is included in the SDP offer or 564 SDP answer, in the "uuie" subfield of the "cs-correlation" attribute. 565 When the SDP Offer/Answer exchange is completed, each endpoint has 566 become aware of the value that will be used in the User-User 567 Information Element of the call setup message of the PSTN protocol. 568 The endpoint that initiates the call setup attempt includes this 569 value in the User-User Information Element. The recipient of the 570 call setup attempt can extract the User-User Information Element and 571 correlate it with the value previously received in the SDP. If both 572 values match, then the call setup attempt corresponds to that 573 indicated in the SDP. 575 According to ITU-T Q.931 [ITU.Q931.1998], the User-User Information 576 Element (UUIE) identifier is composed of a first octet identifying 577 this as a User-User Information Element, a second octet containing 578 the Length of the user-user contents, a third octet containing a 579 Protocol Discriminator, and a value of up to 32 or 128 octets 580 (depending on network settings) containing the actual User 581 Information (see Figure 4-36 in ITU-T Q.931). The first two octets 582 of the UUIE MUST NOT be used for correlation, only the octets 583 carrying the Protocol Discriminator and the User Information value 584 are input to the creation of the value of the "uuie" subfield in the 585 "cs-correlation" attribute. Therefore, the value of the "uuie" 586 subfield in the "cs-correlation" attribute MUST start with the 587 Protocol Discriminator octet, followed by the User Information 588 octets. The value of the Protocol Discriminator octet is not 589 specified in this document; it is expected that organizations using 590 this technology will allocate a suitable value for the Protocol 591 Discriminator. 593 Once the binary value of the "uuie" subfield in the "cs-correlation" 594 attribute is created, it MUST be base 16 (also known as "hex") 595 encoded before it is inserted in SDP. Please refer to RFC 4648 596 [RFC4648] for a detailed description of base 16 encoding. The 597 resulting encoded value needs to have an even number of hexadecimal 598 digits, and MUST be considered invalid if it has an odd number. 600 Note that the encoding of the "uuie" subfield of the "cs- 601 correlation" attribute is largely inspired by the encoding of the 602 same value in the User-to-User header field in SIP, according to 603 the document "A Mechanism for Transporting User to User Call 604 Control Information in SIP" [I-D.ietf-cuss-sip-uui]. 606 As an example, an endpoint willing to send a UUIE containing a 607 protocol discriminator with the hexadecimal value of %x56 and an 608 hexadecimal User Information value of %xA390F3D2B7310023 would 609 include a "cs-correlation" attribute line as follows: 611 a=cs-correlation:uuie:56A390F3D2B7310023 613 Note that, for correlation purposes, the value of the User-User 614 Information Element is considered as an opaque string and only used 615 for correlation purposes. Typically call signaling protocols impose 616 requirements on the creation of User-User Information Element for 617 end-user protocol exchange. The details regarding the generation of 618 the User-User Information Element are outside the scope of this 619 specification. 621 Please note that there are no guarantees that this correlation 622 mechanism works. On one side, policy restrictions might not make the 623 User-User information available end to end in the PSTN. On the other 624 hand, the generation of the User-User Information Element is 625 controlled by the PSTN circuit-switched call protocol, which might 626 not offer enough freedom for generating different values from one 627 endpoint to another one, or from one call to another in the same 628 endpoint. This might result in the same value of the User-User 629 Information Element for all calls. 631 5.2.3.4. DTMF Correlation Mechanism 633 We introduce a third mechanism for correlating the circuit-switched 634 bearer with the session described with SDP. This is based on 635 agreeing on a sequence of digits that are negotiated in the SDP 636 Offer/Answer exchange and sent as Dual Tone Multifrequency (DTMF) 637 tones over the circuit-switched bearer once this bearer is 638 established. If the DTMF digit sequence received through the 639 circuit-switched bearer matches the digit string negotiated in the 640 SDP, the circuit-switched bearer is correlated with the session 641 described in the SDP. The mechanism is similar to many voice 642 conferencing systems which require the user to enter a PIN code using 643 DTMF tones in order to be accepted in a voice conference. 645 The mechanism works as follows: An endpoint selects a DTMF digit 646 sequence. The same sequence is included in the SDP offer or SDP 647 answer, in a "dtmf" subfield of the "cs-correlation" attribute. When 648 the SDP Offer/Answer exchange is completed, each endpoint has become 649 aware of the DTMF sequence that will be sent right after the circuit- 650 switched bearer is set up. The endpoint that initiates the call 651 setup attempt sends the DTMF digits according to the procedures 652 defined for the circuit-switched bearer technology used. The 653 recipient (passive side of the bearer setup) of the call setup 654 attempt collects the digits and compares them with the value 655 previously received in the SDP. If the digits match, then the call 656 setup attempt corresponds to that indicated in the SDP. 658 Implementations are advised to select a number of DTMF digits that 659 provide enough assurance that the call is related, but on the 660 other hand do not prolong the bearer setup time unnecessarily. A 661 number of 5 to 10 digits is a good compromise. 663 As an example, an endpoint willing to send DTMF tone sequence "14D*3" 664 would include a "cs-correlation" attribute line as follows: 666 a=cs-correlation:dtmf:14D*3 668 If the endpoints successfully agree on the usage of the DTMF digit 669 correlation mechanism, but the passive side does not receive any DTMF 670 digits after successful circuit-switched bearer setup, or receives a 671 set of DTMF digits that do not match the value of the "dtmf" 672 attribute (including receiving too many digits), the passive side 673 SHOULD consider that this DTMF mechanism has failed to correlate the 674 incoming call. 676 5.2.3.5. Extensions to correlation mechanisms 678 New values for the "cs-correlation" attribute may be specified. The 679 registration policy for new values is "Specification Required", see 680 Section 8. Any such specification MUST include a description of how 681 SDP Offer/Answer mechanism is used to negotiate the use of the new 682 values, taking into account how endpoints determine which side will 683 become active or passive (see Section 5.3 for more details). 685 If, during the Offer/Answer negotiation, either endpoint encounters 686 an unknown value in the "cs-correlation" attribute, it MUST consider 687 that mechanism as unsupported, and MUST NOT include that value in 688 subsequent Offer/Answer negotiation. 690 5.3. Negotiating the correlation mechanisms 692 The three correlation mechanisms presented above (based on called 693 party number, User-User Information Element and DTMF digit sending) 694 are non-exclusive, and can be used independently of each other. In 695 order to know how to populate the "cs-correlation" attribute, the 696 endpoints need to agree which endpoint will become the active party, 697 i.e., the one that will set up the circuit-switched bearer. 699 5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup 701 In order to avoid a situation where both endpoints attempt to 702 initiate a connection simultaneously, the direction in which the 703 circuit-switched bearer is set up MUST be negotiated during the 704 Offer/Answer exchange. 706 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 707 agree which endpoint acts as the active endpoint when initiating a 708 TCP connection. While RFC 4145 [RFC4145] was originally designed for 709 establishing TCP connections, it can be easily extrapolated to the 710 connection establishment of circuit-switched bearers. This 711 specification uses the concepts specified in RFC 4145 [RFC4145] for 712 agreeing on the direction of establishment of a circuit-switched 713 bearer. 715 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 716 "connection". The "setup" attribute indicates which of the endpoints 717 should initiate the connection establishment of the PSTN circuit- 718 switched bearer. Four values are defined in Section 4 of RFC 4145 719 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 720 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 721 attribute. 723 The "connection" attribute indicates whether a new connection is 724 needed or an existing connection is reused. The attribute can take 725 the values "new" or "existing". Please refer to Section 5 of RFC 726 4145 [RFC4145] for a detailed description of this attribute. 728 Implementations according to this specification MUST support the 729 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 730 but applied to circuit-switched bearers in the PSTN. 732 We define the active party as the one that initiates the circuit- 733 switched bearer after the Offer/Answer exchange. The passive party 734 is the one receiving the circuit-switched bearer. Either party may 735 indicate its desire to become the active or passive party during the 736 Offer/Answer exchange using the procedures described in Section 5.6. 738 5.3.2. Populating the cs-correlation attribute 740 By defining values for the subfields in the "a=cs-correlation" 741 attribute, the endpoint indicates that it is willing to become the 742 active party, and that it can use those values in the Calling party 743 number, User-User Information Element, or as DTMF tones during the 744 circuit-switched bearer setup. 746 Thus, the following rules apply: 748 An endpoint that can only become the active party in the circuit- 749 switched bearer setup MUST include all correlation mechanisms it 750 supports in the "a=cs-correlation" attribute, and MUST also 751 specify values for the subfields. 753 An endpoint that can only become the passive party in the circuit- 754 switched bearer setup MUST include all correlation mechanisms it 755 supports in the "a=cs-correlation" attribute, but MUST NOT specify 756 values for the subfields. 758 An endpoint that is willing to become either the active or passive 759 party (by including the "a=setup:actpass" attribute in the Offer), 760 MUST include all correlation mechanisms it supports in the "a=cs- 761 correlation" attribute, and MUST also specify values for the 762 subfields. 764 5.3.3. Considerations on correlations 766 Passive endpoints should expect an incoming CS call for setting up 767 the audio bearer. Passive endpoints MAY suppress the incoming CS 768 alert during a certain time periods. Additional restrictions can be 769 applied, such as the passive endpoint not alerting incoming calls 770 originated from the number that was observed uduring the offer/answer 771 negotiation. 773 Note that it cannot be guaranteed that any given correlation 774 mechanism will succeed even if the usage of those was agreed 775 beforehand. This is due to the fact that the correlation mechanisms 776 require support from the circuit-switched bearer technology used. 778 Therefore, even a single positive indication using any of these 779 mechanisms SHOULD be interpreted by the passive endpoint so that the 780 circuit-switched bearer establishment is related to the ongoing 781 session, even if the other correlation mechanisms fail. 783 If, after negotiating one or more correlation mechanisms in the SDP 784 offer/answer exchange, an endpoint receives a circuit-switched bearer 785 with no correlation information present, the endpoint has two 786 choices: it can either treat the call as unrelated, or treat the call 787 as related to the ongoing session in the IP domain. 789 An endpoint may for example specify a time window after SDP offer/ 790 answer exchange during which received calls are treated as correlated 791 even if the signaling in the circuit-switched domain does not carry 792 any correlation information. In this case, there is a chance that 793 the call is erroneously treated as related to the ongoing session. 795 An endpoint may also choose to always treat an incoming call as 796 unrelated if the signaling in the circuit-switched domain does not 797 carry any correlation information. In this case, there is a chance 798 that the call is erroneously treated as unrelated. 800 Since, in these cases, no correlation information can be deduced from 801 the signaling, it is up to the implementation to decide how to 802 behave. One option is also to let the user decide whether to accept 803 the call as related, or to treat the call as unrelated. 805 5.4. Considerations for Usage of Existing SDP 807 5.4.1. Originator of the Session 809 According to SDP [RFC4566], the origin line in SDP has the following 810 syntax: 812 o= 813 815 Of interest here are the and fields, which 816 indicate the type of network and type of address, respectively. 817 Typically, this field carries the IP address of the originator of the 818 session. Even if the SDP was used to negotiate an audio or video 819 media stream transported over a circuit-switched bearer, the 820 originator is using SDP over an IP bearer. Therefore, and 821 fields in the "o=" line should be populated with the IP 822 address identifying the source of the signaling. 824 5.4.2. Contact information 826 SDP [RFC4566] defines the "p=" line which may include the phone 827 number of the person responsible for the conference. Even though 828 this line can carry a phone number, it is not suited for the purpose 829 of defining a connection address for the media. Therefore, we have 830 selected to define the PSTN specific connection addresses in the "c=" 831 line. 833 5.5. Considerations for Usage of Third Party Call Control (3PCC) 835 Best Current Practices for Third Party Call Control (3pcc) in the 836 Session Initiation Protocol (SIP) [RFC3725] outlines several flows 837 which are possible in third party call control scenarios and 838 recommends some flows for specific situations. 840 One of the assumptions in [RFC3725] is that an SDP Offer may include 841 a "black hole" connection address, which has the property that 842 packets sent to it will never leave the host which sent them. For 843 IPv4, this "black hole" connection address is 0.0.0.0, or a domain 844 name within the .invalid DNS top level domain. 846 When using an E.164 address scheme in the context of third-party call 847 control, when the User Agent needs to indicate an unknown phone 848 number, it MUST populate the of the SDP "c=" line with a 849 "-" string. 851 Note that this may result in the recipient of the initial offer 852 rejecting such offer if the recipient of the offer was not aware 853 of its own E.164 number. Consequently it will not be possible to 854 establish a circuit-switched bearer, since neither party is aware 855 of their E.164 number. 857 5.6. Offer/Answer mode extensions 859 In this section, we define extensions to the Offer/Answer model 860 defined in The Offer/Answer Model in SDP [RFC3264] to allow for PSTN 861 addresses to be used with the Offer/Answer model. 863 5.6.1. Generating the Initial Offer 865 The Offerer, wishing to use PSTN audio or video stream, MUST populate 866 the "c=" and "m=" lines as follows. 868 The endpoint MUST set the in the "c=" line to "PSTN", and 869 the to "E164". Furthermore, the endpoint SHOULD set the 870 field to its own international E.164 number 871 (with a leading "+"). If the endpoint is not aware of its own E.164 872 number, it MUST set the to "-". 874 In the "m=" line, the endpoint MUST set the subfield to 875 "audio" or "video", depending on the media type, and the 876 subfield to "PSTN". The subfield SHOULD be set to "9" (the 877 discard port). 879 The subfield carries the payload type number(s) the endpoint is 880 wishing to use. Payload type numbers in this case refer to the 881 codecs that the endpoint wishes to use on the PSTN media stream. For 882 example, if the endpoint wishes to use the GSM codec, it would add 883 payload type number 3 in the list of codecs. The list of payload 884 types SHOULD only contain those codecs the endpoint is able to use on 885 the PSTN bearer. In case the endpoint is not aware of the codecs 886 available for the circuit-switched media streams, it MUST include a 887 dash ("-") in the subfield. 889 For dynamic payload types, the endpoint MUST define the set of valid 890 encoding names and related parameters using the "a=rtpmap" attribute 891 line. See Section 6 of SDP [RFC4566] for details. 893 When generating the Offer, if the Offerer supports any of the 894 correlation mechanisms defined in this memo, it MUST include an 895 attribute line "a=cs-correlation" in the SDP offer. The Offerer MUST 896 NOT include more than one "cs-correlation" attribute per media 897 decription. The "a=cs-correlation" line contains an enumeration of 898 the correlation mechanisms supported by the Offerer, in the format of 899 subfields. 901 The current list of subfields include "callerid", "uuie" and "dtmf" 902 and they refer to the correlation mechanisms defined in 903 Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. 905 If the Offerer supports any of the correlation mechanisms defined in 906 this memo, and is willing to become the active party, the Offerer 907 MUST add the "callerid", "uuie", and/or "dtmf" subfields and MUST 908 specify values for those subfields. 910 o the international E.164 number as the value in the "callerid" 911 subfield, 913 o the contents of the User-User information element as the value of 914 the "uuie" subfield, and/or 916 o the DTMF tone string as the value of the "dtmf" subfield 918 If the Offerer is only able to become the passive party in the 919 circuit-switched bearer setup, it MUST add the "callerid", "uuie" 920 and/or "dtmf" subfields, but MUST NOT specify values for those 921 subfields. 923 For example, if the Offerer is willing to use the User-User 924 Information element and DTMF digit sending mechanisms, but can only 925 become the passive party, it includes the following lines in the SDP: 927 a=cs-correlation:uuie dtmf 929 a=setup:passive 931 If, on the other hand, the Offerer is willing to use the User-User 932 Information element and the DTMF correlation mechanisms, and is able 933 to become the active or passive side, it includes the following lines 934 in the SDP: 936 a=cs-correlation:uuie:56A390F3D2B7310023 dtmf:14D*3 938 a=setup:actpass 940 The negotiation of the value of the 'setup' attribute takes place as 941 defined in Section 4.1 of TCP-Based Media Transport in the SDP 942 [RFC4145]. 944 The Offerer states which role or roles it is willing to perform; and 945 the Answerer, taking the Offerer's willingness into consideration, 946 chooses which roles both endpoints will actually perform during the 947 circuit-switched bearer setup. 949 By 'active' endpoint, we refer to an endpoint that will establish the 950 circuit-switched bearer; and by 'passive' endpoint, we refer to an 951 endpoint that will receive a circuit-switched bearer. 953 If an Offerer does not know its international E.164 number, it MUST 954 set the 'a=setup' attribute to the value 'active'. If the Offerer 955 knows its international E.164 number, it SHOULD set the value to 956 either 'actpass' or 'passive'. 958 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 959 It indicates that the connection is not established for the time 960 being. 962 The Offerer uses the "a=connection" attribute to decide whether a new 963 circuit-switched bearer is to be established or not. For the initial 964 Offer, the Offerer MUST use value 'new'. 966 5.6.2. Generating the Answer 968 If the Offer contained a circuit-switched audio or video stream, the 969 Answerer first determines whether it is able to accept and use such 970 streams. If the Answerer is not willing to use circuit-switched 971 media for the session, it MUST construct an Answer where the port 972 number for such media stream(s) is set to zero, according to Section 973 6 of An Offer/Answer Model with the Session Description Protocol 974 (SDP) [RFC3264]. If the Answerer is willing to use circuit-switched 975 media for the session, it MUST ignore the received port number 976 (unless the port number is set to zero). 978 If the Offer included a "-" as the payload type number, it indicates 979 that the Offerer is not willing or able to define any specific 980 payload type. Most often, a "-" is expected to be used instead of 981 the payload type when the endpoint is not aware of or not willing to 982 define the codecs which will eventually be used on the circuit- 983 switched bearer. The circuit-switched signaling protocols have their 984 own means of negotiating or indicating the codecs, therefore an 985 Answerer SHOULD accept such Offers, and SHOULD set the payload type 986 to "-" also in the Answer. 988 If the Answerer explicitly wants to specify a codec for the circuit- 989 switched media, it MAY set the respective payload numbers in the 990 subfield in the answer. This behavior, however, is NOT 991 RECOMMENDED. 993 When receiving the Offer, the Answerer MUST determine whether it 994 becomes the active or passive party. 996 If the SDP in the Offer indicates that the Offerer is only able to 997 become the active party, the Answerer needs to determine whether it 998 is able to become the passive party. If this is not possible e.g. 999 due to the Answerer not knowing its international E.164 number, the 1000 Answerer MUST reject the circuit-switched media by setting the port 1001 number to zero on the Answer. If the Answerer is aware of its 1002 international E.164 number, it MUST include the "a=setup" attribute 1003 in the Answer and set it to value "passive" or "holdconn". The 1004 Answerer MUST also include its E.164 number on the "c=" line. 1006 If the SDP in the Offer indicates that the Offerer is only able to 1007 become the passive party, the Answerer MUST verify that the Offerer's 1008 E.164 number is included in the "c=" line of the Offer. If the 1009 number is included, the Answerer MUST include the "a=setup" attribute 1010 in the Answer and set it to value "active" or "holdconn". If the 1011 number is not included, call establishment is not possible, and the 1012 Answerer MUST reject the circuit-switched media by setting the port 1013 number to zero in the Answer. 1015 If the SDP in the Offer indicates that the Offerer is able to become 1016 either the active or passive party, the Answerer needs to determine 1017 which role it would like to take. If the Offer includes an 1018 international E.164 number in the "c=" line, the Answerer SHOULD 1019 become the active party. If the Offer does not include an E.164 1020 number, and if the Answerer is aware of its international E.164 1021 number, it MUST become the passive party. If the Offer does not 1022 include an E.164 number in the "c=" line and the Answerer is not 1023 aware of its E.164 number, it MUST reject the circuit-switched media 1024 by setting the port number to zero in the Answer. 1026 For each media description where the Offer includes a "a=cs- 1027 correlation" attribute, the Answerer MUST select from the Offer those 1028 correlation mechanisms it supports, and include in the Answer one 1029 "a=cs-correlation" attribute line containing those mechanisms it is 1030 willing to use. The Answerer MUST only add one "a=cs-correlation" 1031 attribute in those media descriptions where also the Offer included a 1032 "a=cs-correlation" attribute. The Answerer MUST NOT add any 1033 mechanisms which were not included in the offer. If there are more 1034 than one "cs-correlation" attributes per media description in the 1035 Offer, the Answerer MUST discard all but the first for any media 1036 description. Also, the Answerer MUST discard all unknown "cs- 1037 correlation" attribute values. 1039 If the Answerer becomes the active party, it MUST add any of the 1040 "callerid", "uuie" or "dtmf" subfield values. 1042 If the Answerer becomes the passive party, it MUST NOT add values to 1043 the "callerid", "uuie" and/or "dtmf" subfields. 1045 After generating and sending the Answer, if the Answerer became the 1046 active party, it 1047 o MUST extract the E.164 number from the "c=" line of the Offer and 1048 MUST establish a circuit-switched bearer to that address. 1050 o if the SDP Answer contained a value for the "callerid" subfield, 1051 MUST set the Calling Party Number Information Element to that 1052 number, 1054 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1055 send the User-User Information element according to the rules 1056 defined for the circuit-switched technology used, and set the 1057 value of the Information Element to that received in the SDP 1058 Offer, 1060 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1061 send those DTMF digits according to the circuit-switched 1062 technology used. 1064 If, on the other hand, the Answerer became the passive party, it 1066 o MUST be prepared to receive a circuit-switched bearer, 1068 o if the Offer contained a value for the "callerid" subfield, MUST 1069 compare that value to the Calling Party Number Information Element 1070 of the circuit-switched bearer, 1072 o if the Offer contained a value for the "dtmf" subfield, MUST be 1073 prepared to receive and collect DTMF digits once the circuit- 1074 switched bearer is set up. The Answerer MUST compare the received 1075 DTMF digits to the value of the "dtmf" subfield. If the received 1076 DTMF digits match the value of the "dtmf" subfield in the "cs- 1077 correlation" attribute, the call SHOULD be treated as correlated 1078 to the ongoing session. 1080 o if the Offer contained a value for the "uuie" subfield, MUST be 1081 prepared to receive a User-User Information element once the 1082 circuit-switched bearer is set up. The Answerer MUST compare the 1083 received UUI to the value of the "uuie" subfield. If the value of 1084 the received UUI matches the value of the "uuie" subfield, the 1085 call SHOULD be treated as correlated to the ongoing session. 1087 If the Answerer becomes the active party, generates an SDP answer, 1088 and then it finds out that the circuit-switched call cannot be 1089 established, then the Answerer MUST create a new SDP offer where 1090 circuit-switched stream is removed from the session (actually, by 1091 setting the corresponding port in the m= line to zero) and send it to 1092 its counterpart. This is to synchronize both parties (and potential 1093 intermediaries) on the state of the session. 1095 5.6.3. Offerer processing the Answer 1097 When receiving the Answer, if the SDP does not contain "a=cs- 1098 correlation" attribute line, the Offerer should take that as an 1099 indication that the other party does not support or is not willing to 1100 use the procedures defined in the document for this session, and MUST 1101 revert to normal processing of SDP. 1103 When receiving the Answer, the Offerer MUST first determine whether 1104 it becomes the active or passive party, as described in 1105 Section 5.3.1. 1107 If the Offerer becomes the active party, it 1109 o MUST extract the E.164 number from the "c=" line and MUST 1110 establish a circuit-switched bearer to that address. 1112 o if the SDP Answer contained a value for the "uuie" subfield, MUST 1113 send the User-User Information element according to the rules 1114 defined for the circuit-switched technology used, and set the 1115 value of the Information Element to that received in the SDP 1116 Answer, 1118 o if the SDP Answer contained a value for the "dtmf" subfield, MUST 1119 send those DTMF digits according to the circuit-switched 1120 technology used. 1122 If the Offerer becomes the passive party, it 1124 o MUST be prepared to receive a circuit-switched bearer, 1126 o Note that if delivery of the Answer is delayed for some reason, 1127 the circuit-switched call attempt may arrive at the Offerer before 1128 the Answer has been processed. In this case, since the 1129 correlation mechanisms are negotiated as part of the Offer/Answer 1130 exchange, the Answerer cannot know whether or not the incoming 1131 circuit-switched call attempt is correlated with the session being 1132 negotiated, the Offerer SHOULD answer the circuit-switched call 1133 attempt only after it has received and processed the Answer. 1135 o If the Answer contained a value for the "dtmf" subfield, the 1136 Offerer MUST be prepared to receive and collect DTMF digits once 1137 the circuit-switched bearer is set up. The Offerer SHOULD compare 1138 the received DTMF digits to the value of the "dtmf" subfield. If 1139 the received DTMF digits match the value of the "dtmf" subfield in 1140 the "cs-correlation" attribute, the call SHOULD be treated as 1141 correlated to the ongoing session. 1143 o If the Answer contained a value for the "uuie" subfield, the 1144 Offerer MUST be prepared to receive a User-User Information 1145 element once the circuit-switched bearer is set up. The Offerer 1146 SHOULD compare the received UUI to the value of the "uuie" 1147 subfield. If the value of the received UUI matches the value of 1148 the "uuie" subfield, the call SHOULD be treated as correlated to 1149 the ongoing session. 1151 5.6.4. Modifying the session 1153 If, at a later time, one of the parties wishes to modify the session, 1154 e.g., by adding new media stream, or by changing properties used on 1155 an existing stream, it may do so via the mechanisms defined for An 1156 Offer/Answer Model with SDP [RFC3264]. 1158 If there is an existing circuit-switched bearer between the 1159 endpoints, and the Offerer wants to reuse that, the Offerer MUST set 1160 the value of the "a=connection" attribute to 'existing'. 1162 If either party removes the circuit-switched media from the session 1163 (by setting the port number to zero), it MUST terminate the circuit- 1164 switched bearer using whatever mechanism is appropriate for the 1165 technology in question. 1167 If either party wishes to drop and reestablish an existing call, that 1168 party MUST first remove the circuit-switched media from the session 1169 by setting the port number to zero, and then use another Offer/Answer 1170 exchange where it MUST set the "a=connection" attribute to 'new'". 1171 If the media types are different (for example, a different codec will 1172 be used for the circuit-switched bearer), the media descriptions for 1173 terminating the existing bearer and the new bearer can be in the same 1174 Offer. 1176 5.7. Formal Syntax 1178 The following is the formal Augmented Backus-Naur Form (ABNF) 1179 [RFC5234] syntax that supports the extensions defined in this 1180 specification. The syntax is built above the SDP [RFC4566] and the 1181 tel URI [RFC3966] grammars. Implementations according to this 1182 specification MUST be compliant with this syntax. 1184 Figure 2 shows the formal syntax of the extensions defined in this 1185 memo. 1187 ; extension to the connection field originally specified 1188 ; in RFC 4566 1190 connection-field = [%x63 "=" nettype SP addrtype SP 1191 connection-address CRLF] 1192 ;nettype and addrtype are defined in RFC 4566 1194 connection-address /= global-number-digits / "-" 1195 ; global-number-digits specified in RFC 3966 1197 ;subrules for correlation attribute 1198 attribute /= cs-correlation-attr 1199 ; attribute defined in RFC 4566 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 uuie-mech = "uuie" [":" uuie-value] 1207 uuie-value = 1*65(HEXDIG HEXDIG) 1208 ;This represents up to 130 HEXDIG 1209 ; (65 octets) 1210 ;HEXDIG defined in RFC5234 1211 ;HEXDIG defined as 0-9, A-F 1212 dtmf-mech = "dtmf" [":" dtmf-value] 1213 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1214 ;0-9, A-D, '#' and '*' 1215 ext-mech = ext-mech-name [":" ext-mech-value] 1216 ext-mech-name = token 1217 ext-mech-value = token 1218 ; token is specified in RFC4566 1220 Figure 2: Syntax of the SDP extensions 1222 6. Examples 1224 In the examples below, where an SDP line is too long to be displayed 1225 as a single line, a breaking character "\" indicates continuation in 1226 the following line. Note that this character is included for display 1227 purposes only. Implementations MUST write a single line without 1228 breaks. 1230 6.1. Single PSTN audio stream 1232 Alice Bob 1233 | | 1234 | (1) SDP Offer (PSTN audio) | 1235 |--------------------------------->| 1236 | | 1237 | (2) SDP Answer (PSTN audio) | 1238 |<---------------------------------| 1239 | | 1240 | PSTN call setup | 1241 |<---------------------------------| 1242 | | 1243 |<==== media over PSTN bearer ====>| 1244 | | 1246 Figure 3: Basic flow 1248 Figure 3 shows a basic example that describes a single audio media 1249 stream over a circuit-switched bearer. Alice generates a SDP Offer 1250 which is shown in Figure 4. The Offer describes a PSTN circuit- 1251 switched bearer in the "m=" and "c=" line where it also indicates its 1252 international E.164 number format. Additionally, Alice expresses 1253 that she can initiate the circuit-switched bearer or be the recipient 1254 of it in the "a=setup" attribute line. The SDP Offer also includes 1255 correlation identifiers that this endpoint will insert in the Calling 1256 Party Number and/or User-User Information Element of the PSTN call 1257 setup if eventually this endpoint initiates the PSTN call. 1259 v=0 1260 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5 1261 s= 1262 t=0 0 1263 m=audio 9 PSTN - 1264 c=PSTN E164 +441134960123 1265 a=setup:actpass 1266 a=connection:new 1267 a=cs-correlation:callerid:+441134960123 \ 1268 uuie:56A390F3D2B7310023 1270 Figure 4: SDP offer (1) 1272 Bob generates a SDP Answer (Figure 5), describing a PSTN audio media 1273 on port 9 without information on the media sub-type on the "m=" line. 1274 The "c=" line contains Bob's international E.164 number. In the 1275 "a=setup" line Bob indicates that he is willing to become the active 1276 endpoint when establishing the PSTN call, and he also includes the 1277 "a=cs-correlation" attribute line containing the values he is going 1278 to include in the Calling Party Number and User-User IE of the PSTN 1279 call establishment. 1281 v=0 1282 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1283 s= 1284 t=0 0 1285 m=audio 9 PSTN - 1286 c=PSTN E164 +441134960124 1287 a=setup:active 1288 a=connection:new 1289 a=cs-correlation:callerid:+441134960124 \ 1290 uuie:56A390F3D2B7310023 1292 Figure 5: SDP Answer with circuit-switched media 1294 When Alice receives the Answer, she examines that Bob is willing to 1295 become the active endpoint when setting up the PSTN call. Alice 1296 temporarily stores Bob's E.164 number and the User-User IE value of 1297 the "cs-correlation" attribute, and waits for a circuit-switched 1298 bearer establishment. 1300 Bob initiates a circuit-switched bearer using whatever circuit- 1301 switched technology is available for him. The called party number is 1302 set to Alice's number, and calling party number is set to Bob's own 1303 number. Bob also sets the User-User Information Element value to the 1304 one contained in the SDP Answer. 1306 When Alice receives the circuit-switched bearer establishment, she 1307 examines the UUIE and the calling party number, and by comparing 1308 those received during O/A exchange determines that the call is 1309 related to the SDP session. 1311 It may also be that neither the UUIE nor the calling party number is 1312 received by the called party, or the format of the calling party 1313 number is changed by the PSTN. Implementations may still accept such 1314 call establishment attempts as being related to the session that was 1315 established in the IP network. As it cannot be guaranteed that the 1316 values used for correlation are always passed intact through the 1317 network, they should be treated as additional hints that the circuit- 1318 switched bearer is actually related to the session. 1320 6.2. Advanced SDP example: Circuit-Switched Audio and Video Streams 1322 Alice Bob 1323 | | 1324 | (1) SDP Offer (PSTN audio and video) | 1325 |------------------------------------------->| 1326 | | 1327 | (2) SDP Answer (PSTN audio) | 1328 |<-------------------------------------------| 1329 | | 1330 | PSTN call setup | 1331 |<-------------------------------------------| 1332 | | 1333 |<======== media over PSTN bearer ==========>| 1334 | | 1336 Figure 6: Circuit-Switched Audio and Video streams 1338 Figure 6 shows an example of negotiating audio and video media 1339 streams over circuit-switched bearers. 1341 v=0 1342 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5 1343 s= 1344 t=0 0 1345 a=setup:actpass 1346 a=connection:new 1347 c=PSTN E164 +441134960123 1348 m=audio 9 PSTN - 1349 a=cs-correlation:dtmf:1234536 1350 m=video 9 PSTN 34 1351 a=rtpmap:34 H263/90000 1352 a=cs-correlation:callerid:+441134960123 1354 Figure 7: SDP offer with circuit-switched audio and video (1) 1356 Upon receiving the SDP offer described in Figure 7, Bob rejects the 1357 video stream as his device does not currently support video, but 1358 accepts the circuit-switched audio stream. As Alice indicated that 1359 she is able to become either the active, or passive party, Bob gets 1360 to select which role he would like to take. Since the Offer 1361 contained the international E.164 number of Alice, Bob decides that 1362 he becomes the active party in setting up the circuit-switched 1363 bearer. Bob includes a new value in the "dtmf" subfield of the "cs- 1364 correlation" attribute, which he is going to send as DTMF tones once 1365 the bearer setup is complete. The Answer is described in Figure 8 1366 v=0 1367 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1368 s= 1369 t=0 0 1370 a=setup:active 1371 a=connection:new 1372 c=PSTN E164 +441134960124 1373 m=audio 9 PSTN - 1374 a=cs-correlation:dtmf:654321 1375 m=video 0 PSTN 34 1376 a=cs-correlation:callerid:+441134960124 1378 Figure 8: SDP answer with circuit-switched audio and video (2) 1380 7. Security Considerations 1382 This document provides an extension on top of RFC 4566 [RFC4566], and 1383 RFC 3264 [RFC3264]. As such, the security considerations of those 1384 documents apply. 1386 This memo provides mechanisms to agree on a correlation identifier or 1387 identifiers that are used to evaluate whether an incoming circuit- 1388 switched bearer is related to an ongoing session in the IP domain. 1389 If an attacker replicates the correlation identifier and establishes 1390 a call within the time window the receiving endpoint is expecting a 1391 call, the attacker may be able to hijack the circuit-switched bearer. 1392 These types of attacks are not specific to the mechanisms presented 1393 in this memo. For example, caller ID spoofing is a wellknown attack 1394 in the PSTN. Users are advised to use the same caution before 1395 revealing sensitive information as they would on any other phone 1396 call. Furthermore, users are advised that mechanisms that may be in 1397 use in the IP domain for securing the media, like Secure RTP (SRTP) 1398 [RFC3711], are not available in the CS domain. 1400 For the purposes of establishing a circuit-switched bearer, the 1401 active endpoint needs to know the passive endpoint's phone number. 1402 Phone numbers are sensitive information, and some people may choose 1403 not to reveal their phone numbers when calling using supplementary 1404 services like Calling Line Identification Restriction (CLIR) in GSM. 1405 Implementations should take the caller's preferences regarding 1406 calling line identification into account if possible, by restricting 1407 the inclusion of the phone number in SDP "c=" line if the caller has 1408 chosen to use CLIR. If this is not possible, implementations may 1409 present a prompt informing the user that their phone number may be 1410 transmitted to the other party. 1412 Similarly as with IP addresses, if there is a desire to protect the 1413 SDP containing phone numbers carried in SIP, implementers are advised 1414 to follow the security mechanisms defined in [RFC3261]. 1416 It is possible that an attacker creates a circuit-switched session 1417 whereby the attacked endpoint should dial a circuit-switched number, 1418 perhaps even a premium-rate telephone number. To mitigate the 1419 consequences of this attack, endpoints MUST authenticate and trust 1420 remote endpoints users who try to remain passive in the circuit- 1421 switched connection establishment. It is RECOMMENDED that endpoints 1422 have local policies precluding the active establishment of circuit 1423 switched connections to certain numbers (e.g., international, 1424 premium, long distance). Additionally, it is strongly RECOMMENDED 1425 that the end user is asked for consent prior to the endpoint 1426 initiating a circuit-switched connection. 1428 8. IANA Considerations 1430 This document instructs IANA to register a number of SDP tokens 1431 according to the following data. 1433 8.1. Registration of new cs-correlation SDP attribute 1435 Contact: Miguel Garcia 1437 Attribute name: cs-correlation 1439 Long-form attribute name: PSTN Correlation Identifier 1441 Type of attribute: media level only 1443 Subject to charset: No 1445 Description: This attribute provides the Correlation Identifier 1446 used in PSTN signaling 1448 Appropriate values:see Section 5.2.3.1 1450 Specification: RFC XXXX 1452 The IANA is requested to create a subregistry for 'cs-correlation' 1453 attribute under the Session Description Protocol (SDP) Parameters 1454 registry. The initial values for the subregistry are presented in 1455 the following, and IANA is requested to add them into its database: 1457 Value of 'cs-correlation' attribute Reference Description 1458 ----------------------------------- --------- ----------- 1459 callerid RFC XXXX Caller ID 1460 uuie RFC XXXX User-User 1461 Information Element 1462 dtmf RFC XXXX Dual-tone Multifrequency 1464 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1465 reference to the RFC number of this draft. 1467 As per the terminology in [RFC5226], the registration policy for new 1468 values of 'cs-correlation' parameter is 'Specification Required'. 1470 8.2. Registration of a new "nettype" value 1472 This memo provides instructions to IANA to register a new "nettype" 1473 in the Session Description Protocol Parameters registry [1]. The 1474 registration data, according to RFC 4566 [RFC4566] follows. 1476 Type SDP Name Reference 1477 ---- ------------------ --------- 1478 nettype PSTN [RFCxxxx] 1480 8.3. Registration of new "addrtype" values 1482 This memo provides instructions to IANA to register two new 1483 "addrtype" in the Session Description Protocol Parameters 1484 registry [1]. The registration data, according to RFC 4566 [RFC4566] 1485 follows. 1487 Type SDP Name Reference 1488 ---- ------------------ --------- 1489 addrtype E164 [RFCxxxx] 1490 - [RFCxxxx] 1492 Note: RFC XXXX defines the "E164" and "-" addrtypes in conjunction 1493 with the "PSTN" nettype only. Please refer to the relevant RFC for a 1494 description of that representation. 1496 Note to IANA: The current "addrtype" sub-registry structure does not 1497 capture the fact that a given addrtype is defined in the context of a 1498 particular "nettype". The sub-registry structure should be to 1499 correct that as part of this registration. 1501 8.4. Registration of a new "proto" value 1503 This memo provides instructions to IANA to register a new "proto" in 1504 the Session Description Protocol Parameters registry [1]. The 1505 registration data, according to RFC 4566 [RFC4566] follows. 1507 Type SDP Name Reference 1508 -------------- --------------------------- --------- 1509 proto PSTN [RFCxxxx] 1511 The related "fmt" namespace re-uses the conventions and payload type 1512 number defined for RTP/AVP. In this document, the RTP audio and 1513 video media types, when applied to PSTN circuit-switched bearers, 1514 represent merely an audio or video codec in its native format 1515 directly on top of a single PSTN bearer. 1517 In come cases, the endpoint is not able to determine the list of 1518 available codecs for circuit-switched media streams. In this case, 1519 in order to be syntactically compliant with SDP [RFC4566], the 1520 endpoint MUST include a single dash ("-") in the subfield. 1522 9. Acknowledgments 1524 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas 1525 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan 1526 Rosenberg, Ingemar Johansson, Christer Holmberg, Alf Heidermark, Tom 1527 Taylor, Thomas Belling, Keith Drage, and Andrew Allen for providing 1528 their insight and comments on this document. 1530 10. References 1532 10.1. Normative References 1534 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1535 Requirement Levels", BCP 14, RFC 2119, March 1997. 1537 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1538 with Session Description Protocol (SDP)", RFC 3264, 1539 June 2002. 1541 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", 1542 RFC 3966, December 2004. 1544 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1545 the Session Description Protocol (SDP)", RFC 4145, 1546 September 2005. 1548 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1549 Description Protocol", RFC 4566, July 2006. 1551 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1552 Encodings", RFC 4648, October 2006. 1554 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1555 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1556 May 2008. 1558 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1559 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1561 10.2. Informative References 1563 [I-D.ietf-cuss-sip-uui] 1564 Johnston, A. and J. Rafferty, "A Mechanism for 1565 Transporting User to User Call Control Information in 1566 SIP", draft-ietf-cuss-sip-uui-08 (work in progress), 1567 December 2012. 1569 [ITU.E164.1991] 1570 International Telecommunications Union, "The International 1571 Public Telecommunication Numbering Plan", ITU- 1572 T Recommendation E.164, 1991. 1574 [ITU.Q931.1998] 1575 "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN 1576 User - Network Interface Layer 3 Specification for Basic 1577 Call Control", ISO Standard 9594-1, May 1998. 1579 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1580 Session Description Protocol (SDP) for ATM Bearer 1581 Connections", RFC 3108, May 2001. 1583 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1584 A., Peterson, J., Sparks, R., Handley, M., and E. 1585 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1586 June 2002. 1588 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1589 Jacobson, "RTP: A Transport Protocol for Real-Time 1590 Applications", STD 64, RFC 3550, July 2003. 1592 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1593 Video Conferences with Minimal Control", STD 65, RFC 3551, 1594 July 2003. 1596 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1597 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1598 RFC 3711, March 2004. 1600 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1601 Camarillo, "Best Current Practices for Third Party Call 1602 Control (3pcc) in the Session Initiation Protocol (SIP)", 1603 BCP 85, RFC 3725, April 2004. 1605 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1606 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1608 [TS.24.008] 1609 3GPP, "Mobile radio interface Layer 3 specification; Core 1610 network protocols; Stage 3", 3GPP TS 24.008 3.20.0, 1611 December 2005. 1613 URIs 1615 [1] 1617 Authors' Addresses 1619 Miguel A. Garcia-Martin 1620 Ericsson 1621 Calle Via de los Poblados 13 1622 Madrid, ES 28033 1623 Spain 1625 Email: miguel.a.garcia@ericsson.com 1627 Simo Veikkolainen 1628 Nokia 1629 P.O. Box 226 1630 NOKIA GROUP, FI 00045 1631 Finland 1633 Phone: +358 50 486 4463 1634 Email: simo.veikkolainen@nokia.com