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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (October 13, 2011) is 4571 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 1240, but not defined ** Obsolete normative reference: RFC 2434 (Obsoleted by RFC 5226) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) Summary: 3 errors (**), 0 flaws (~~), 4 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: April 15, 2012 Nokia 6 October 13, 2011 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-08 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 April 15, 2012. 37 Copyright Notice 39 Copyright (c) 2011 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 This document may contain material from IETF Documents or IETF 53 Contributions published or made publicly available before November 54 10, 2008. The person(s) controlling the copyright in some of this 55 material may not have granted the IETF Trust the right to allow 56 modifications of such material outside the IETF Standards Process. 57 Without obtaining an adequate license from the person(s) controlling 58 the copyright in such materials, this document may not be modified 59 outside the IETF Standards Process, and derivative works of it may 60 not be created outside the IETF Standards Process, except to format 61 it for publication as an RFC or to translate it into languages other 62 than English. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 2. Conventions Used in This Document . . . . . . . . . . . . . . 5 68 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6 70 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 6 71 5. Protocol Description . . . . . . . . . . . . . . . . . . . . . 8 72 5.1. Level of Compliance . . . . . . . . . . . . . . . . . . . 8 73 5.2. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 8 74 5.2.1. Connection Data . . . . . . . . . . . . . . . . . . . 8 75 5.2.2. Media Descriptions . . . . . . . . . . . . . . . . . . 9 76 5.2.3. Correlating the PSTN Circuit-Switched Bearer with 77 SDP . . . . . . . . . . . . . . . . . . . . . . . . . 10 78 5.2.3.1. The "cs-correlation" attribute . . . . . . . . . . 11 79 5.2.3.2. Caller-ID Correlation Mechanism . . . . . . . . . 11 80 5.2.3.3. User-User Information Element Correlation 81 Mechanism . . . . . . . . . . . . . . . . . . . . 12 82 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . . 13 83 5.3. Negotiating the correlation mechanisms . . . . . . . . . . 14 84 5.3.1. Determining the Direction of the Circuit-Switched 85 Bearer Setup . . . . . . . . . . . . . . . . . . . . . 14 86 5.3.2. Populating the cs-correlation attribute . . . . . . . 15 87 5.3.3. Considerations on successful correlation . . . . . . . 16 88 5.4. Considerations for Usage of Existing SDP . . . . . . . . . 16 89 5.4.1. Originator of the Session . . . . . . . . . . . . . . 16 90 5.4.2. Contact information . . . . . . . . . . . . . . . . . 17 91 5.5. Offer/Answer mode extensions . . . . . . . . . . . . . . . 17 92 5.5.1. Generating the Initial Offer . . . . . . . . . . . . . 17 93 5.5.2. Generating the Answer . . . . . . . . . . . . . . . . 19 94 5.5.3. Offerer processing the Answer . . . . . . . . . . . . 22 95 5.5.4. Modifying the session . . . . . . . . . . . . . . . . 23 96 5.6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 23 97 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 98 7. Security Considerations . . . . . . . . . . . . . . . . . . . 26 99 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 100 8.1. Registration of new cs-correlation SDP attribute . . . . . 27 101 8.2. Registration of a new "nettype" value . . . . . . . . . . 27 102 8.3. Registration of new "addrtype" values . . . . . . . . . . 28 103 8.4. Registration of a new "proto" value . . . . . . . . . . . 28 104 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 105 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 106 10.1. Normative References . . . . . . . . . . . . . . . . . . . 28 107 10.2. Informative References . . . . . . . . . . . . . . . . . . 29 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 110 1. Introduction 112 The Session Description Protocol (SDP) [RFC4566] is intended for 113 describing multimedia sessions for the purposes of session 114 announcement, session invitation, and other forms of multimedia 115 session initiation. SDP is most commonly used for describing media 116 streams that are transported over the Real-Time Transport Protocol 117 (RTP) [RFC3550], using the profiles for audio and video media defined 118 in RTP Profile for Audio and Video Conferences with Minimal Control 119 [RFC3551]. 121 However, SDP can be used to describe other transport protocols than 122 RTP. Previous work includes SDP conventions for describing ATM 123 bearer connections [RFC3108] and the Message Session Relay Protocol 124 [RFC4975]. 126 SDP is commonly carried in Session Initiation Protocol (SIP) 127 [RFC3261] messages in order to agree on a common media description 128 among the endpoints. An Offer/Answer Model with Session Description 129 Protocol (SDP) [RFC3264] defines a framework by which two endpoints 130 can exchange SDP media descriptions and come to an agreement as to 131 which media streams should be used, along with the media related 132 parameters. 134 In some scenarios it might be desirable to establish the media stream 135 over a circuit-switched bearer connection even if the signaling for 136 the session is carried over an IP bearer. An example of such a 137 scenario is illustrated with two mobile devices capable of both 138 circuit-switched and packet-switched communication over a low- 139 bandwidth radio bearer. The radio bearer may not be suitable for 140 carrying real-time audio or video media, and using a circuit-switched 141 bearer would offer a better perceived quality of service. So, 142 according to this scenario, SDP and its higher layer session control 143 protocol (e.g., the Session Initiation Protocol (SIP) [RFC3261]) are 144 used over regular IP connectivity, while the audio or video is 145 received through the classical circuit-switched bearer. 147 Setting up a signaling relationship in the IP domain instead of just 148 setting up a circuit-switched call offers also the possibility of 149 negotiating in the same session other IP based media that is not 150 sensitive to jitter and delay, for example, text messaging or 151 presence information. 153 At a later point in time the mobile device might move to an area 154 where a high-bandwidth packet-switched bearer, for example a Wireless 155 Local Area Network (WLAN) connection, is available. At this point 156 the mobile device may perform a handover and move the audio or video 157 media streams over to the high-speed bearer. This implies a new 158 exchange of SDP Offer/Answer that lead to a re-negotiation of the 159 media streams. 161 Other use cases exist. For example, and endpoint might have at its 162 disposal circuit-switched and packet-switched connectivity, but the 163 same audio or video codecs are not feasible for both access networks. 164 For example, the circuit-switched audio or video stream supports 165 narrow-bandwidth codecs, while the packet-switched access allows any 166 other audio or video codec implemented in the endpoint. In this 167 case, it might be beneficial for the endpoint to describe different 168 codecs for each access type and get an agreement on the bearer 169 together with the remote endpoint. 171 There are additional use cases related to third party call control 172 where the session setup time is improved when the circuit-switched 173 bearer in the PSTN is described together with one or more codecs. 175 The rest of the document is structured as follows: Section 2 provides 176 the document conventions, Section 3 introduces the requirements, 177 Section 4 presents an overview of the proposed solutions, and 178 Section 5 contains the protocol description. Section 6 provides an 179 example of descriptions of circuit-switched audio or video streams in 180 SDP. Section 8 and Section 7 contain the IANA and Security 181 considerations, respectively. 183 2. Conventions Used in This Document 185 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 186 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 187 document are to be interpreted as described in BCP 14, RFC 2119 188 [RFC2119] and indicate requirement levels for compliant 189 implementations. 191 3. Requirements 193 This section presents the general requirements that are specific for 194 the audio or video media streams over circuit-switched bearers. 196 REQ-1: A mechanism for endpoints to negotiate and agree on an audio 197 or video media stream established over a circuit-switched 198 bearer MUST be available. 200 REQ-2: The mechanism MUST allow the endpoints to combine circuit- 201 switched audio or video media streams with other 202 complementary media streams, for example, text messaging. 204 REQ-3: The mechanism MUST allow the endpoint to negotiate the 205 direction of the circuit-switched bearer, i.e., which 206 endpoint is active when initiating the circuit-switched 207 bearer. 209 REQ-4: The mechanism MUST be independent of the type of the circuit- 210 switched access (e.g., Integrated Services Digital Network 211 (ISDN), Global System for Mobile Communication (GSM), etc.) 213 REQ-5: There MUST be a mechanism that helps an endpoint to correlate 214 an incoming circuit-switched bearer with the one negotiated 215 in SDP, as opposed to another incoming call that is not 216 related to that. 218 REQ-6: It MUST be possible for endpoints to advertise different list 219 of audio or video codecs in the circuit-switched audio or 220 video stream from those used in a packet-switched audio or 221 video stream. 223 REQ-7: It MUST be possible for endpoints to not advertise the list 224 of available codecs for circuit-switched audio or video 225 streams. 227 4. Overview of Operation 229 The mechanism defined in this memo extends SDP and allows describing 230 an audio or video media stream established over a circuit-switched 231 bearer. New tokens are registered in the "c=" and "m=" lines to be 232 able to describe a media stream over a circuit-switched bearer. 233 These SDP extensions are described in Section 5.2. Since circuit- 234 switched bearers are connection-oriented media streams, the mechanism 235 re-uses the connection-oriented extensions defined in RFC 4145 236 [RFC4145] to negotiate the active and passive sides of a connection 237 setup. This is further described in Section 5.3.1. 239 4.1. Example Call Flow 241 Consider the example presented in Figure 1. In this example, Alice 242 is located in an environment where she has access to both IP and 243 circuit-switched bearers for communicating with other endpoints. 244 Alice decides that the circuit-switched bearer offers a better 245 perceived quality of service for voice, and issues an SDP Offer 246 containing the description of an audio media stream over circuit- 247 switched bearer. 249 Alice Bob 250 | (1) SDP Offer (PSTN audio) | 251 |----------------------------------->| 252 | | 253 | (2) SDP Answer (PSTN audio) | 254 |<-----------------------------------| 255 | | 256 | PSTN call setup | 257 |<-----------------------------------| 258 | | 259 | | 260 |<===== media over PSTN bearer =====>| 261 | | 263 Figure 1: Example Flow 265 Bob receives the SDP offer and determines that he is located in an 266 environment where the IP based bearer is not suitable for real-time 267 audio media. However he also has PSTN circuit-switched bearer 268 available for audio. Bob generates an SDP answer containing a 269 description of the audio media stream over a circuit-switched bearer. 271 During the offer-answer exchange Alice and Bob also agree the 272 direction in which the circuit-switched bearer should be established. 273 In this example, Bob becomes the active party, in other words, he 274 establishes the circuit-switched call to the other endpoint. The 275 Offer/Answer exchange contains identifiers or references that can be 276 used on the circuit-switched network for addressing the other 277 endpoint, as well as information that is used to determine that the 278 incoming circuit-switched bearer establishment is related to the 279 ongoing session between Alice and Bob. 281 Bob establishes a circuit-switched bearer towards Alice using 282 whatever mechanisms are defined for the network type in question. 283 When receiving the incoming circuit-switched connection attempt, 284 Alice is able to determine that the attempt is related to the session 285 she is just establishing with Bob. 287 Alice accepts the circuit-switched connection; the circuit-switched 288 bearer setup is completed. Bob and Alice can now use the circuit- 289 switched connection for two-way audio media. 291 If, for some reason, Bob would like to reject the offered stream, he 292 would set the port number of the specific stream to zero, as 293 specified in RFC3264 [RFC3264]. Also, if Bob does not understand 294 some of the SDP attributes specified in this document, he would 295 ignore them, as specified in RFC4566 [RFC4566]. 297 5. Protocol Description 299 5.1. Level of Compliance 301 Implementations according to this specification MUST implement the 302 SDP extensions described in Section 5.2, and MUST implement the 303 considerations discussed in Section 5.3, Section 5.4 and Section 5.5. 305 5.2. Extensions to SDP 307 This section provides the syntax and semantics of the extensions 308 required for providing a description of audio or video media streams 309 over circuit-switched bearers in SDP. 311 5.2.1. Connection Data 313 According to SDP [RFC4566], the connection data line in SDP has the 314 following syntax: 316 c= 318 where indicates the network type, indicates the 319 address type, and the is the connection address, 320 which is dependent on the address type. 322 At the moment, the only network type defined is "IN", which indicates 323 Internet network type. The address types "IP4" and "IP6" indicate 324 the type of IP addresses. 326 This memo defines a new network type for describing a circuit- 327 switched bearer network type in the PSTN. The mnemonic "PSTN" is 328 used for this network type. 330 For the address type, we initially consider the possibility of 331 describing E.164 telephone numbers. We define a new "E164" address 332 type. When used, the "E164" address type indicates that the 333 connection address contains a telephone number represented according 334 to the ITU-T E.164 [ITU.E164.1991] recommendation. 336 There are cases, though, when the endpoint is merely aware of a 337 circuit-switched bearer, without having further information about the 338 address type or the E.164 number allocated to it. In these cases a 339 dash "-" is used to indicate an unknown address type or connection 340 address. This makes the connection data line be according to the SDP 341 syntax. 343 Note that and/or should not be 344 omitted without being set to a "-" since this would violate basic 345 syntax of SDP [RFC4566]. 347 The following are examples of the extension to the connection data 348 line: 350 c=PSTN E164 +15551234 352 c=PSTN - - 354 5.2.2. Media Descriptions 356 According to SDP [RFC4566], the media descriptions line in SDP has 357 the following syntax: 359 m= ... 361 The sub-field carries the media type. For establishing an 362 audio bearer, the existing "audio" media type is used. For 363 establishing a video bearer, the existing "video" media type is used. 365 The sub-field is the transport port to which the media stream 366 is sent. Circuit-switched access lacks the concept of a port number, 367 and therefore the sub-field is set to the discard port "9". 369 According to RFC 3264 [RFC3264], a port number of zero in the offer 370 of a unicast stream indicates that the stream is offered but must not 371 be used. If a port number of zero is present in the answer of a 372 unicast stream, it indicates that the stream is rejected. These 373 rules are still valid when the media line in SDP represents a 374 circuit-switched bearer. 376 The sub-field is the transport protocol. The circuit- 377 switched bearer uses whatever transport protocol it has available. 378 This subfield SHOULD be set to the mnemonic "PSTN" to be 379 syntactically correct with SDP [RFC4566] and to indicate the usage of 380 circuit-switched protocols in the PSTN. 382 The sub-field is the media format description. In the 383 classical usage of SDP to describe RTP-based media streams, when the 384 sub-field is set to "RTP/AVP" or "RTP/SAVP", the sub- 385 field contains the payload types as defined in the RTP audio profile 386 [RFC3551]. 388 When "RTP/AVP" is used in the field, the sub-field 389 contains the RTP payload type numbers. We use the sub-field to 390 indicate the list of available codecs over the circuit-switched 391 bearer, by re-using the conventions and payload type numbers defined 392 for RTP/AVP. The RTP audio and video media types, which, when 393 applied to PSTN circuit-switched bearers, represent merely an audio 394 or video codec. 396 In some cases, the endpoint is not able to determine the list of 397 available codecs for circuit-switched media streams. In this case, 398 in order to be syntactically compliant with SDP [RFC4566], the 399 endpoint MUST include a single dash "-" in the sub-field. 401 As per RFC 4566 [RFC4566], the media format descriptions are listed 402 in priority order. 404 Examples of media descriptions for circuit-switched audio streams 405 are: 407 m=audio 9 PSTN 3 0 8 409 m=audio 9 PSTN - 411 Similarly, an example of a media description for circuit-switched 412 video stream is: 414 m=video 9 PSTN 34 416 m=video 9 PSTN - 418 5.2.3. Correlating the PSTN Circuit-Switched Bearer with SDP 420 The endpoints should be able to correlate the circuit-switched bearer 421 with the session negotiated with SDP in order to avoid ringing for an 422 incoming circuit-switched bearer that is related to the session 423 controlled with SDP (and SIP). 425 Several alternatives exist for performing this correlation. This 426 memo provides three mutually non-exclusive correlation mechanisms. 427 Other correlation mechanisms may exist, and their usage will be 428 specified when need arises. All mechanisms share the same principle: 429 some unique information is sent in the SDP and in the circuit- 430 switched signaling protocol. If these pieces of information match, 431 then the circuit-switched bearer is part of the session described in 432 the SDP exchange. Otherwise, there is no guarantee that the circuit- 433 switched bearer is related to such session. 435 The first mechanism is based on the exchange of PSTN caller-ID 436 between the endpoints. The caller-ID is also available as the 437 Calling Party ID in the circuit-switched signaling. 439 The second mechanism is based on the inclusion in SDP of a value that 440 is also sent in the User-to-User Information Element that is part of 441 the bearer setup signaling in the PSTN. 443 The third mechanism is based on sending in SDP a string that 444 represents Dual Tone MultiFrequency (DTMF) digits that will be later 445 sent right after the circuit-switched bearer is established. 446 Implementations MAY use any of these mechanisms and MAY use two or 447 more mechanisms simultaneously. 449 5.2.3.1. The "cs-correlation" attribute 451 In order to provide support for the correlation mechanisms, we define 452 a new SDP attribute called "cs-correlation". This "cs-correlation" 453 attribute can include any of the "callerid", "uuie", or "dtmf" sub- 454 fields, which specify additional information required by the 455 Caller-ID, User to User Information, or DTMF correlation mechanisms, 456 respectively. The list of correlation mechanisms may be extended by 457 other specifications. 459 The following sections provide more detailed information of these 460 subfields. The "cs-correlation" attribute has the following format: 462 a=cs-correlation: callerid: | 463 uuie: | 464 dtmf: | 465 *[extension-name:] 467 The values "callerid", "uuie" and "dtmf" refer to the correlation 468 mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and 469 Section 5.2.3.4, respectively. The formal Augmented Backus-Naur 470 Format (ABNF) syntax of the "cs-correlation" attribute is presented 471 in Section 5.6. 473 5.2.3.2. Caller-ID Correlation Mechanism 475 The Caller-ID correlation mechanisms consists of an exchange of the 476 calling party number in E.164 format in SDP, followed by the 477 availability of the Calling Party Number information element in the 478 call setup signaling of the circuit switched connection. If both 479 pieces of information match, the circuit-switched bearer is 480 correlated to the session described in SDP. 482 Example of inclusion of E.164 number in the "cs-correlation" 483 attribute is: 485 a=cs-correlation:callerid:+15551234 487 The presence of the "callerid" sub-field indicates that the endpoint 488 supports use of the calling party number as a means of correlating a 489 PSTN call with the session being negotiated. The "callerid" sub- 490 field MAY be accompanied by the E.164 number of the party inserting 491 the parameter. 493 Note that there are no warranties that this correlation mechanism 494 works or is even available, due a number of problems: 496 o The endpoint might not be aware of its own E.164 number, in which 497 case it cannot populate the SDP appropriately. 499 o The Calling Party Number information element in the circuit- 500 switched signaling might not be available, e.g., due to policy 501 restrictions of the network operator or caller restriction due to 502 privacy. 504 o The Calling Party Number information element in the circuit- 505 switched signaling might be available, but the digit 506 representation of the E.164 number might differ from the one 507 expressed in the SDP. For example, one can be represented in 508 international format and the other might only contain the 509 significant national digits. To mitigate this problem 510 implementations should consider only some of the rightmost digits 511 from the E.164 number for correlation. For example, the numbers 512 +358-1-555-12345 and 01-555-12345 could be considered as the same 513 number. This is also the behavior of some cellular phones, which 514 correlate the incoming calling party with a number stored in the 515 phone book, for the purpose of displaying the caller's name. 517 5.2.3.3. User-User Information Element Correlation Mechanism 519 A second correlation mechanism is based on including in SDP a string 520 that represents the User-User Information Element that is part of the 521 call setup signaling of the circuit-switched bearer. The User-User 522 Information Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 523 3GPP TS 24.008 [TS.24.008], among others. The User-User Information 524 Element has a maximum size of 35 or 131 octets, depending on the 525 actual message of the PSTN protocol where it is included. 527 The mechanism works as follows: An endpoint creates a User-User 528 Information Element, according to the requirements of the call setup 529 signaling protocol. The same value is included in the SDP offer or 530 SDP answer, in a "cs-correlation:uuie" attribute. When the SDP 531 Offer/Answer exchange is completed, each endpoint has become aware of 532 the value that will be used in the User-User Information Element of 533 the call setup message of the PSTN protocol. The endpoint that 534 initiates the call setup attempt includes this value in the User-User 535 Information Element. The recipient of the call setup attempt can 536 extract the User-User Information Element and correlate it with the 537 value previously received in the SDP. If both values match, then the 538 call setup attempt corresponds to that indicated in the SDP. 540 Note that, for correlation purposes, the value of the User-User 541 Information Element is considered as a opaque string and only used 542 for correlation purposes. Typically call signaling protocols impose 543 requirements on the creation of User-User Information Element for 544 end-user protocol exchange. The details regarding the generation of 545 the User-User Information Element are outside the scope of this 546 specification. 548 Please note that there are no warranties that this correlation 549 mechanism works. On one side, policy restrictions might not make the 550 User-User information available end to end in the PSTN. On the other 551 hand, the generation of the User-User Information Element is 552 controlled by the PSTN circuit-switched call protocol, which might 553 not offer enough freedom for generating different values from one 554 endpoint to another one, or from one call to another in the same 555 endpoint. This might result in the same value of the User-User 556 Information Element for all calls. 558 5.2.3.4. DTMF Correlation Mechanism 560 We introduce a third mechanism for correlating the circuit-switched 561 bearer with the session described with SDP. This is based on 562 agreeing on a sequence of digits that are negotiated in the SDP 563 Offer/Answer exchange and sent as Dual Tone Multifrequency (DTMF) 564 tones over the circuit-switched bearer once this bearer is 565 established. If the DTMF digit sequence received through the 566 circuit-switched bearer matches the digit string negotiated in the 567 SDP, the circuit-switched bearer is correlated with the session 568 described in the SDP. The mechanism is similar to many voice 569 conferencing systems which require the user to enter a PIN code using 570 DTMF tones in order to be accepted in a voice conference. 572 The mechanism works as follows: An endpoint selects a DTMF digit 573 sequence. The same sequence is included in the SDP offer or SDP 574 answer, in a "cs-correlation:dtmf" attribute. When the SDP Offer/ 575 Answer exchange is completed, each endpoint has become aware of the 576 DTMF sequence that will be sent right after the circuit-switched 577 bearer is set up. The endpoint that initiates the call setup attempt 578 sends the DTMF digits according to the procedures defined for the 579 circuit-switched bearer technology used. The recipient (passive side 580 of the bearer setup) of the call setup attempt collects the digits 581 and compares them with the value previously received in the SDP. If 582 the digits match, then the call setup attempt corresponds to that 583 indicated in the SDP. 585 Implementations are advised to select a number of DTMF digits that 586 provide enough assurance that the call is related, but on the 587 other hand do not prolong the bearer setup time unnecessarily. 589 As an example, an endpoint willing to send DTMF tone sequence "14D*3" 590 would include a "cs-correlation" attribute line as follows: 592 a=cs-correlation:dtmf:14D*3 594 If the endpoints successfully agree on the usage of the DTMF digit 595 correlation mechanism, but the passive side does not receive any DTMF 596 digits after successful circuit-switched bearer setup, or receives a 597 set of DTMF digits that do not match the value of the "dtmf" 598 attribute (including receiving too many digits), the passive side 599 SHOULD treat the circuit-switched bearer as not correlated to the 600 ongoing session. 602 DTMF digits can only be sent once the circuit-switched bearer is 603 set up. In order to suppress alerting for an incoming circuit- 604 switched call, implementations may choose various mechanisms. For 605 example, alerting may be suppressed for a certain time period for 606 incoming call attempts that originate from the number that was 607 observed during the offer/answer negotiation. 609 5.3. Negotiating the correlation mechanisms 611 The three correlation mechanisms presented above (based on called 612 party number, User-User Information Element and DTMF digit sending) 613 are non-exclusive, and can be used independently of each other. In 614 order to know how to populate the "a=cs-correlation" attribute, the 615 endpoints need to agree which endpoint will become the active party, 616 i.e. the one that will set up the circuit-switched bearer. 618 5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup 620 In order to avoid a situation where both endpoints attempt to 621 initiate a connection simultaneously, the direction in which the 622 circuit-switched bearer is set up should be negotiated during the 623 Offer/Answer exchange. 625 The framework defined in RFC 4145 [RFC4145] allows the endpoints to 626 agree which endpoint acts as the active endpoint when initiating a 627 TCP connection. While RFC 4145 [RFC4145] was originally designed for 628 establishing TCP connections, it can be easily extrapolated to the 629 connection establishment of circuit-switched bearers. This 630 specification uses the concepts specified in RFC 4145 [RFC4145] for 631 agreeing on the direction of establishment of a circuit-switched 632 bearer. 634 RFC 4145 [RFC4145] defines two new attributes in SDP: "setup" and 635 "connection". The "setup" attribute indicates which of the endpoints 636 should initiate the connection establishment of the PSTN circuit- 637 switched bearer. Four values are defined in Section 4 of RFC 4145 638 [RFC4145]: "active", "passive", "actpass", "holdconn". Please refer 639 to Section 4 of RFC 4145 [RFC4145] for a detailed description of this 640 attribute. 642 The "connection" attribute indicates whether a new connection is 643 needed or an existing connection is reused. The attribute can take 644 the values "new" or "existing". Please refer to Section 5 of RFC 645 4145 [RFC4145] for a detailed description of this attribute. 647 Implementations according to this specification MUST support the 648 "setup" and "connection" attributes specified in RFC 4145 [RFC4145], 649 but applied to circuit-switched bearers in the PSTN. 651 We define the active party as the one that initiates the circuit- 652 switched bearer after the Offer/Answer process. The passive party is 653 the one receiving the circuit-switched bearer. Either party may 654 indicate its desire to become the active or passive party during the 655 Offer/Answer exchange using the procedures described in Section 5.5. 657 5.3.2. Populating the cs-correlation attribute 659 By defining values for the sub-fields in the "a=cs-correlation" 660 attribute, the endpoint indicates that it is willing to become the 661 active party, and that it can use those values in the Calling party 662 number, User-User Information Element, or as DTMF tones during the 663 circuit-switched bearer setup. 665 Thus, the following rules apply: 667 An endpoint that can only become the active party in the circuit- 668 switched bearer setup MUST include all correlation mechanisms it 669 supports in the "a=cs-correlation" attribute, and MUST also 670 specify values for the sub-fields. 672 An endpoint that can only become the passive party in the circuit- 673 switched bearer setup MUST include all correlation mechanisms it 674 supports in the "a=cs-correlation" attribute, but MUST NOT specify 675 values for the sub-fields. 677 An endpoint that is willing to become either the active or passive 678 party (by including the "a=setup:actpass" attribute in the Offer), 679 MUST include all correlation mechanisms it supports in the "a=cs- 680 correlation" attribute, and MUST also specify values for the sub- 681 fields. 683 5.3.3. Considerations on successful correlation 685 Note that, as stated above, it cannot be guaranteed that any given 686 correlation mechanism will succeed even if the usage of those was 687 agreed beforehand. This is due to the fact that the correlation 688 mechanisms require support from the circuit-switched bearer 689 technology used. 691 Therefore, even a single positive indication using any of these 692 mechanisms SHOULD be interpreted by the passive endpoint so that the 693 circuit-switched bearer establishment is related to the ongoing 694 session, even if the other correlation mechanisms fail. 696 If, after negotiating one or more correlation mechanisms in the SDP 697 offer/answer exchange, an endpoint receives a circuit-switched bearer 698 with no correlation information present, the endpoint has two 699 choices: it can either treat the call as unrelated, or treat the call 700 as related to the ongoing session in the IP domain. 702 An endpoint may for example specify a time window after SDP offer/ 703 answer exchange during which received calls are treated as correlated 704 even if the signaling in the circuit-switched domain does not carry 705 any correlation information. In this case, there is a chance that 706 the call is erroneously treated as related to the ongoing session. 708 An endpoint may also choose to always treat an incoming call as 709 unrelated if the signaling in the circuit-switched domain does not 710 carry any correlation information. In this case, there is a chance 711 that the call is erroneously treated as unrelated. 713 Since, in these cases, no correlation information can be deduced from 714 the signaling, it is up to the implementation to decide how to 715 behave. One option is also to let the user decide whether to accept 716 the call as related, or to treat the call as unrelated. 718 5.4. Considerations for Usage of Existing SDP 720 5.4.1. Originator of the Session 722 According to SDP [RFC4566], the origin line in SDP has the following 723 syntax: 725 o= 726 728 Of interest here are the and fields, which 729 indicate the type of network and type of address, respectively. 730 Typically, this field carries the IP address of the originator of the 731 session. Even if the SDP was used to negotiate an audio or video 732 media stream transported over a circuit-switched bearer, the 733 originator is using SDP over an IP bearer. Therefore, and 734 fields in the "o=" line should be populated with the IP 735 address identifying the source of the signaling. 737 5.4.2. Contact information 739 SDP [RFC4566] defines the "p=" line which may include the phone 740 number of the person responsible for the conference. Even though 741 this line can carry a phone number, it is not suited for the purpose 742 of defining a connection address for the media. Therefore, we have 743 selected to define the PSTN specific connection addresses in the "c=" 744 line. 746 5.5. Offer/Answer mode extensions 748 In this section, we define extensions to the Offer/Answer model 749 defined in The Offer/Answer Model in SDP [RFC3264] and extended in 750 the SDP Capability Negotiation [RFC5939] to allow for PSTN addresses 751 to be used with the Offer/Answer model. 753 5.5.1. Generating the Initial Offer 755 The Offerer, wishing to use PSTN audio or video stream, MUST populate 756 the "c=" and "m=" lines as follows. 758 The endpoint MUST set the in the "c=" line to "PSTN", and 759 the to "E164". Furthermore, the endpoint SHOULD set the 760 field to its own E.164 number in the 761 international format (beginning with a "+"). If the endpoint is not 762 aware of its own E.164 number, it MUST set the 763 to "-". 765 In the "m=" line, the endpoint MUST set the subfield to 766 "audio" or "video", depending on the media type, the to "9" 767 (the discard port), and the sub-field to "PSTN". 769 The sub-field carries the payload type number(s) the endpoint 770 is wishing to use. Payload type numbers in this case refer to the 771 codecs that the endpoint wishes to use. For example, if the endpoint 772 wishes to use the GSM codec, it would add payload type number 3 in 773 the list of codecs. 775 For dynamic payload types, the endpoint MUST define the set of valid 776 encoding names and related parameters using the "a=rtpmap" attribute 777 line. See Section 6 of SDP [RFC4566] for details. 779 When generating the Offer, if the Offerer supports any of the 780 correlation mechanisms defined in this memo, it MUST include an 781 attribute line "a=cs-correlation" in the SDP offer. The "a=cs- 782 correlation" line contains an enumeration of the correlation 783 mechanisms supported by the Offerer, in the format of sub-fields. 785 The current list of sub-fields include "callerid", "uuie" and "dtmf" 786 and they refer to the correlation mechanisms defined in 787 Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. 789 If the Offerer supports any of the correlation mechanisms defined in 790 this memo, and is willing to become the active party, the Offerer 791 MUST add the "callerid", "uuie", and/or "dtmf" sub-fields and MUST 792 specify values for those sub-fields. 794 o the E.164 number as the value in the "callerid" sub-field, 796 o the contents of the User-User information element as the value of 797 the "uuie" sub-field, and/or 799 o the DTMF tone string as the value of the "dtmf" sub-field 801 If the Offerer is only able to become the passive party in the 802 circuit-switched bearer setup, it MUST add the "callerid", "uuie" 803 and/or "dtmf" sub-fields, but MUST NOT specify values for those sub- 804 fields. 806 For example, if the Offerer is willing to use the User-User 807 Information element and DTMF digit sending mechanisms, but can only 808 become the passive party, it includes the following lines to the SDP: 810 a=cs-correlation:uuie dtmf 812 a=setup:passive 814 If, on the other hand, the Offerer is willing to use the User-User 815 Information element and the DTMF correlation mechanisms, and is able 816 to become the active or passive side, it includes to following lines 817 to the SDP: 819 a=cs-correlation:uuie:2890W284hAT452612908awudfjang908 dtmf:14D*3 821 a=setup:actpass 823 The negotiation of the value of the 'setup' attribute takes place as 824 defined in Section 4.1 of TCP-Based Media Transport in the SDP 825 [RFC4145]. 827 The Offerer states which role or roles it is willing to perform; and 828 the Answerer, taking the Offerer's willingness into consideration, 829 chooses which roles both endpoints will actually perform during the 830 circuit-switched bearer setup. 832 By 'active' endpoint, we refer to an endpoint that will establish the 833 circuit-switched bearer; and by 'passive' endpoint, we refer to an 834 endpoint that will receive a circuit-switched bearer. 836 If an Offerer does not know its E.164 address, it MUST set the 837 'a=setup' attribute to the value 'active'. If the Offerer knows its 838 E.164 address, it MUST set the value to either 'actpass' or 839 'passive'. 841 Also 'holdconn' is a permissible value in the 'a=setup' attribute. 842 It indicates that the connection is not established for the time 843 being. 845 The Offerer uses the "a=connection" attribute to decide whether a new 846 circuit-switched bearer is to be established or not. If there is no 847 circuit-switched bearer established between the endpoints, the 848 Offerer MUST use a value of 'new' in the "a=connection" attribute. 849 If there is an existing circuit-switched bearer between the 850 endpoints, and the Offerer wants to reuse that (for example, in the 851 case of an updated Offer), the Offerer MUST set the value of the 852 "a=connection" attribute to "existing". 854 5.5.2. Generating the Answer 856 If the Offer contained a circuit-switched audio or video stream, the 857 Answerer first determines whether it is able to accept and use such 858 streams. If the Answerer is not willing to use circuit-switched 859 media for the session, it MUST construct an Answer where the port 860 number for such media stream(s) is set to zero, according to Section 861 6 of An Offer/Answer Model with the Session Description Protocol 862 (SDP) [RFC3264]. 864 If the Offer included a "-" as the payload type number, it indicates 865 that the Offerer is not willing or able to define any specific 866 payload type. Most often, a "-" is expected to be used instead of 867 the payload type when the endpoint is not aware of or not willing to 868 define the codecs which will eventually be used on the circuit- 869 switched bearer. The circuit-switched signaling protocols have their 870 own means of negotiating or indicating the codecs, therefore an 871 Answerer SHOULD accept such Offers, and SHOULD set the payload type 872 to "-" also in the Answer. 874 If the Answerer explicitly wants to specify a codec for the circuit- 875 switched media, it MAY set the respective payload numbers in the 876 sub-field in the answer. This behavior, however, is NOT 877 RECOMMENDED. 879 When receiving the Offer, the Answerer MUST determine whether it 880 becomes the active or passive party. 882 If the SDP in the Offer indicates that the Offerer is only able to 883 become the active party, the Answerer needs to determine whether it 884 is able to become the passive party. If this is not possible e.g. 885 due to the Answerer not knowing its E.164 address, the Answerer MUST 886 reject the circuit-switched media by setting the port number to zero 887 on the Answer. If the Answerer is aware of its E.164 number, it MUST 888 include the "a=setup" attribute in the Answer and set it to value 889 "passive" or "holdconn". 891 If the SDP in the Offer indicates that the Offerer is only able to 892 become the passive party, the Answerer MUST verify that the Offerer's 893 E.164 number is included in the "c=" line of the Offer. If the 894 number is included, the Answerer MUST include the "a=setup" attribute 895 in the Answer and set it to value "active" or "holdconn". If the 896 number is not included, call establishment is not possible, and the 897 Answerer MUST reject the circuit-switched media by setting the port 898 number to zero in the Answer. 900 If the SDP in the Offer indicates that the Offerer is able to become 901 either the active or passive party, the Answerer needs to determine 902 which role it would like to take. If the Offer includes an E.164 903 address in the "c=" line, the Answerer SHOULD become the active 904 party. If the Offer does not include an E.164 number, and if the 905 Answerer is aware of its E.164 number, it MUST become the passive 906 party. If the Offer does not include an E.164 number in the "c=" 907 line and the Answerer is not aware of its E.164 number, it MUST 908 reject the circuit-switched media by setting the port number to zero 909 in the Answer. 911 The Answerer MUST select those correlation mechanisms from the Offer 912 it supports, and include an "a=cs-correlation" attribute line in the 913 Answer containing those mechanisms it supports. The Answerer MUST 914 NOT add any mechanisms which were not included in the offer. 916 If the Answerer becomes the active party, it MUST add parameter 917 values to the "callerid", "uuie" or "dtmf" sub-fields. 919 If the Answerer becomes the passive party, it MUST NOT add values to 920 the "callerid", "uuie" and/or "dtmf" sub-fields. 922 After generating and sending the Answer, if the Answerer became the 923 active party, it 925 o MUST extract the E.164 address from the "c=" line of the Offer and 926 MUST establish a circuit-switched bearer to that address. 928 o if the SDP Answer contained a value for the "callerid" sub-field, 929 must set the Calling Party Number Information Element to that 930 number, 932 o if the SDP Answer contained a value for the "uuie" sub-field, MUST 933 send the User-User Information element according to the rules 934 defined for the circuit-switched technology used, and set the 935 value of the Information Element to that received in the SDP 936 Offer, 938 o if the SDP Answer contained a value for the "dtmf" sub-field, MUST 939 send those DTMF digits according to the circuit-switched 940 technology used. 942 If, on the other hand, the Answerer became the passive party, it 944 o MUST be prepared to receive a circuit-switched bearer, 946 o if the Offer contained a value for the "callerid" sub-field, MUST 947 compare that value to the Calling Party Number Information Element 948 of the circuit-switched bearer, 950 o if the Offer contained a value for the "dtmf" sub-field, MUST be 951 prepared to receive and collect DTMF digits once the circuit- 952 switched bearer is set up. The Answerer MUST compare the received 953 DTMF digits to the value of the "dtmf" sub-field. If the received 954 DTMF digits match the value of the "dtmf" sub-field in the "cs- 955 correlation" attribute, the call SHOULD be treated as correlated 956 to the ongoing session. 958 o if the Offer contained a value for the "uuie" sub-field, MUST be 959 prepared to receive a User-User Information element once the 960 circuit-switched bearer is set up. The Answerer MUST compare the 961 received UUI to the value of the "uuie" sub-field. If the value 962 of the received UUI matches the value of the "uuie" sub-field, the 963 call SHOULD be treated as correlated to the ongoing session. 965 5.5.3. Offerer processing the Answer 967 When receiving the Answer, if the SDP does not contain "a=cs- 968 correlation" attribute line, the Offerer should take that as an 969 indication that the other party does not support or is not willing to 970 use the procedures defined in the document for this session, and MUST 971 revert to normal processing of SDP. 973 When receiving the Answer, the Offerer MUST first determine whether 974 it becomes the active or passive party, as described in Section 975 Section 5.3.1. 977 If the Offerer becomes the active party, it 979 o MUST extract the E.164 address from the "c=" line and MUST 980 establish a circuit-switched bearer to that address. 982 o if the SDP Answer contained a value for the "uuie" sub-field, MUST 983 send the User-User Information element according to the rules 984 defined for the circuit-switched technology used, and set the 985 value of the Information Element to that received in the SDP 986 Answer, 988 o if the SDP Answer contained a value for the "dtmf" sub-field, MUST 989 send those DTMF digits according to the circuit-switched 990 technology used. 992 If the Offerer becomes the passive party, it 994 o MUST be prepared to receive a circuit-switched bearer, 996 o if the Answer contained a value for the "dtmf" sub-field, MUST be 997 prepared to receive and collect DTMF digits once the circuit- 998 switched bearer is set up. The Offerer SHOULD compare the 999 received DTMF digits to the value of the "dtmf" sub-field. If the 1000 received DTMF digits match the value of the "dtmf" sub-field in 1001 the "cs-correlation" attribute, the call SHOULD be treated as 1002 correlated to the ongoing session. 1004 o if the Answer contained a value for the "uuie" sub-field, MUST be 1005 prepared to receive a User-User Information element once the 1006 circuit-switched bearer is set up. The Offerer SHOULD compare the 1007 received UUI to the value of the "uuie" sub-field. If the value 1008 of the received UUI matches the value of the "uuie" sub-field, the 1009 call SHOULD be treated as correlated to the ongoing session. 1011 5.5.4. Modifying the session 1013 If, at a later time, one of the parties wishes to modify the session, 1014 e.g., by adding new media stream, or by changing properties used on 1015 an existing stream, it may do so via the mechanisms defined for An 1016 Offer/Answer Model with SDP [RFC3264]. 1018 If either party removes the circuit-switched media from the session 1019 (by setting the port number to zero), it MUST terminate the circuit- 1020 switched bearer using whatever mechanism is appropriate for the 1021 technology in question. 1023 5.6. Formal Syntax 1025 The following is the formal Augmented Backus-Naur Form (ABNF) 1026 [RFC5234] syntax that supports the extensions defined in this 1027 specification. The syntax is built above the SDP [RFC4566] grammar. 1028 Implementations according to this specification MUST be compliant 1029 with this syntax. 1031 Figure 2 shows the formal syntax of the extensions defined in this 1032 memo. 1034 ; extension to the connection field originally specified 1035 ; in RFC 4566 1037 connection-field = [%x63 "=" nettype SP addrtype SP 1038 connection-address CRLF] 1039 ;nettype and addrtype are defined in RFC 4566 1041 connection-address /= e164-address / "-" 1042 e164-address = ["+"] 1*15DIGIT 1043 ; DIGIT is specified in RFC 5234 1045 ;subrules for correlation attribute 1046 attribute /= cs-correlation-attr 1047 ; attribute defined in RFC 4566 1048 cs-correlation-attr= "cs-correlation:" corr-mechanisms 1049 corr-mechanisms = corr-mech *(SP corr-mech) 1050 corr-mech = caller-id-mech / uuie-mech / dtmf-mech / ext-mech 1051 caller-id-mech = "callerid" [":" caller-id-value] 1052 caller-id-value = ["+"] 1*DIGIT 1053 uuie-mech = "uuie" [":" uuie-value] 1054 uuie-value = 1*32(ALPHA/DIGIT) 1055 dtmf-mech = "dtmf" [":" dtmf-value] 1056 dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) 1057 ;0-9, A-D, '#' and '*' 1058 ext-mech = ext-mech-name[":" ext-mech-value] 1059 ext-mech-name = token 1060 ext-mech-value = token 1061 ; token is specified in RFC4566 1063 Figure 2: Syntax of the SDP extensions 1065 6. Example 1067 Alice Bob 1068 | | 1069 | (1) SDP Offer (PSTN audio) | 1070 |--------------------------------->| 1071 | | 1072 | (2) SDP Answer (PSTN audio) | 1073 |<---------------------------------| 1074 | | 1075 | PSTN call setup | 1076 |<---------------------------------| 1077 | | 1078 |<==== media over PSTN bearer ====>| 1079 | | 1080 Figure 3: Basic flow 1082 Figure 3 shows a basic example that describes a single audio media 1083 stream over a circuit-switched bearer. Alice generates a SDP Offer 1084 which is show in Figure 4. The Offer describes a PSTN circuit- 1085 switched bearer in the "m=" and "c=" line where it also indicates its 1086 E.164 number in the international format. Additionally, Alice 1087 expresses that she can initiate the circuit-switched bearer or be the 1088 recipient of it in the "a=setup" attribute line. The SDP Offer also 1089 includes a correlation identifiers that this endpoint will be 1090 inserting the Calling Party Number and/or User-User Information 1091 Element of the PSTN call setup if eventually this endpoint initiates 1092 the PSTN call. 1094 v=0 1095 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5 1096 s= 1097 t=0 0 1098 m=audio 9 PSTN - 1099 c=PSTN E164 +15551234 1100 a=setup:actpass 1101 a=connection:new 1102 a=cs-correlation:callerid:+15551234 uuie:2890W284hAT452612908awudfjang908 1104 Figure 4: SDP offer (1) 1106 Bob generates a SDP Answer (Figure 5), describing a PSTN audio media 1107 on port 9 without information on the media sub-type on the "m=" line. 1108 The "c=" line contains Bob's E.164 number in international format. 1109 In the "a=setup" line Bob indicates that he is willing to become the 1110 active endpoint when establishing the PSTN call, and he also includes 1111 the "a=cs-correlation" attribute line containing the values he is 1112 going to include in the Calling Party Number and User-User IE of the 1113 PSTN call establishment. 1115 v=0 1116 o=- 2890973824 2890987289 IN IP4 192.0.2.7 1117 s= 1118 t=0 0 1119 m=audio 9 PSTN - 1120 c=PSTN E164 +15554321 1121 a=setup:active 1122 a=connection:new 1123 a=cs-correlation:callerid:+15554321 uuie:2127W49uThi455916adjfhtow9619361 1125 Figure 5: SDP Answer with circuit-switched media 1127 When Alice receives the Answer, she examines that Bob is willing to 1128 become the active endpoint when setting up the PSTN call. Alice 1129 temporarily stores Bob's E.164 number and the User-User IE value of 1130 the "cs-correlation" attribute, and waits for a circuit-switched 1131 bearer establishment. 1133 Bob initiates a circuit-switched bearer using whatever circuit- 1134 switched technology is available for him. The called party number is 1135 set to Alice's number, and calling party number is set to Bob's own 1136 number. Bob also sets the User-User Information Element value to the 1137 on contained in the SDP Answer. 1139 When Alice receives the circuit-switched bearer establishment, she 1140 examines the UUIE and the calling party number, and by comparing 1141 those received during O/A exchange determines that the call is 1142 related to the SDP session. 1144 It may also be that neither the UUIE nor the calling party number is 1145 received by the called party, or the format of the calling party 1146 number is changed by the PSTN. Implementations may still accept such 1147 call establishment attempts as being related to the session that was 1148 established in the IP network. As it cannot be guaranteed that the 1149 values used for correlation are always passed intact through the 1150 network, they should be treated as additional hints that the circuit- 1151 switched bearer is actually related to the session. 1153 7. Security Considerations 1155 This document provides an extension on top of RFC 4566 [RFC4566], and 1156 RFC 3264 [RFC3264]. As such, the security considerations of those 1157 documents apply. 1159 This memo provides mechanisms to agree on a correlation identifier or 1160 identifiers that are used to evaluate whether an incoming circuit- 1161 switched bearer is related to an ongoing session in the IP domain. 1162 If an attacker replicates the correlation identifer and establishes a 1163 call within the time window the receiving endpoint is expecting a 1164 call, the attacker may be able to hijack the circuit-switched bearer. 1165 These types of attacks are not specific to the mechanisms presented 1166 in this memo. For example, caller ID spoofing is a well known attack 1167 in the PSTN. Users are advised to use the same caution before 1168 revealing sensitive information as they would on any other phone 1169 call. Furthermore, users are advised that mechanisms that may be in 1170 use in the IP domain for securing the media, like Secure RTP (SRTP) 1171 [RFC3711], are not available in the CS domain. 1173 8. IANA Considerations 1175 This document instructs IANA to register a number of SDP tokens 1176 according to the following data. 1178 8.1. Registration of new cs-correlation SDP attribute 1180 Contact: Miguel Garcia 1182 Attribute name: cs-correlation 1184 Long-form attribute name: PSTN Correlation Identifier 1186 Type of attribute: media level only 1188 This attribute is subject to the charset attribute 1190 Description: This attribute provides the Correlation Identifier 1191 used in PSTN signaling 1193 Specification: RFC XXXX 1195 The IANA is requested the create a subregistry for 'cs-correlation' 1196 attribute under the Session Description Protocol (SDP) Parameters 1197 registry. The initial values for the subregistry are presented in 1198 the following, and IANA is requested to add them into its database: 1200 Value of 'cs-correlation' attribute Reference Description 1201 ----------------------------------- --------- ----------- callerid 1202 RFC XXXX Caller ID uuie RFC XXXX User-User Information Element dtmf 1203 RFC XXXX Dual-tone Multifrequency 1205 Note for the RFC Editor: 'RFC XXXX' above should be replaced by a 1206 reference to the RFC number of this draft. 1208 As per the terminology in [RFC2434], the registration policy for new 1209 values of 'cs-correlation' parameter is 'Specification Required'. 1211 8.2. Registration of a new "nettype" value 1213 This memo provides instructions to IANA to register a new "nettype" 1214 in the Session Description Protocol Parameters registry [1]. The 1215 registration data, according to RFC 4566 [RFC4566] follows. 1217 Type SDP Name Reference 1218 ---- ------------------ --------- 1219 nettype PSTN [RFCxxxx] 1221 8.3. Registration of new "addrtype" values 1223 This memo provides instructions to IANA to register a new "addrtype" 1224 in the Session Description Protocol Parameters registry [1]. The 1225 registration data, according to RFC 4566 [RFC4566] follows. 1227 Type SDP Name Reference 1228 ---- ------------------ --------- 1229 addrtype E164 [RFCxxxx] 1230 - [RFCxxxx] 1232 8.4. Registration of a new "proto" value 1234 This memo provides instructions to IANA to register a new "proto" in 1235 the Session Description Protocol Parameters registry [1]. The 1236 registration data, according to RFC 4566 [RFC4566] follows. 1238 Type SDP Name Reference 1239 -------------- --------------------------- --------- 1240 proto PSTN [RFCxxxx] 1242 9. Acknowledgments 1244 The authors want to thank Flemming Andreasen, Thomas Belling, John 1245 Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan Rosenberg, 1246 Ingemar Johansson, Christer Holmberg, and Alf Heidermark for 1247 providing their insight and comments on this document. 1249 10. References 1251 10.1. Normative References 1253 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1254 Requirement Levels", BCP 14, RFC 2119, March 1997. 1256 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1257 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 1258 October 1998. 1260 [RFC3108] Kumar, R. and M. Mostafa, "Conventions for the use of the 1261 Session Description Protocol (SDP) for ATM Bearer 1262 Connections", RFC 3108, May 2001. 1264 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1265 with Session Description Protocol (SDP)", RFC 3264, 1266 June 2002. 1268 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1269 the Session Description Protocol (SDP)", RFC 4145, 1270 September 2005. 1272 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1273 Description Protocol", RFC 4566, July 2006. 1275 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1276 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1278 [RFC5939] Andreasen, F., "Session Description Protocol (SDP) 1279 Capability Negotiation", RFC 5939, September 2010. 1281 10.2. Informative References 1283 [ITU.E164.1991] 1284 International Telecommunications Union, "The International 1285 Public Telecommunication Numbering Plan", ITU- 1286 T Recommendation E.164, 1991. 1288 [ITU.Q931.1998] 1289 "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN 1290 User - Network Interface Layer 3 Specification for Basic 1291 Call Control", ISO Standard 9594-1, May 1998. 1293 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1294 A., Peterson, J., Sparks, R., Handley, M., and E. 1295 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1296 June 2002. 1298 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1299 Jacobson, "RTP: A Transport Protocol for Real-Time 1300 Applications", STD 64, RFC 3550, July 2003. 1302 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 1303 Video Conferences with Minimal Control", STD 65, RFC 3551, 1304 July 2003. 1306 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1307 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1308 RFC 3711, March 2004. 1310 [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message 1311 Session Relay Protocol (MSRP)", RFC 4975, September 2007. 1313 [TS.24.008] 1314 3GPP, "Mobile radio interface Layer 3 specification; Core 1315 network protocols; Stage 3", 3GPP TS 24.008 3.20.0, 1316 December 2005. 1318 URIs 1320 [1] 1322 Authors' Addresses 1324 Miguel A. Garcia-Martin 1325 Ericsson 1326 Calle Via de los Poblados 13 1327 Madrid, ES 28033 1328 Spain 1330 Email: miguel.a.garcia@ericsson.com 1332 Simo Veikkolainen 1333 Nokia 1334 P.O. Box 407 1335 NOKIA GROUP, FI 00045 1336 Finland 1338 Phone: +358 50 486 4463 1339 Email: simo.veikkolainen@nokia.com