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'10') == Outdated reference: A later version (-26) exists of draft-ietf-mmusic-sdp-new-24 Summary: 9 errors (**), 0 flaws (~~), 3 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIPPING Working Group G. Camarillo 3 Internet-Draft Ericsson 4 Expires: December 3, 2005 June 1, 2005 6 Framework for Transcoding with the Session Initiation Protocol (SIP) 7 draft-ietf-sipping-transc-framework-02.txt 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on December 3, 2005. 34 Copyright Notice 36 Copyright (C) The Internet Society (2005). 38 Abstract 40 This document defines a framework for transcoding with SIP. This 41 framework includes how to discover the need of transcoding services 42 in a session and how to invoke those transcoding services. Two 43 models for transcoding services invocation are discussed: the 44 conference bridge model and the third party call control model. Both 45 models meet the requirements for SIP regarding transcoding services 46 invocation to support deaf, hard of hearing, and speech-impaired 47 individuals. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 2. Discovery of the Need for Transcoding Services . . . . . . . . 3 53 3. Transcoding Services Invocation . . . . . . . . . . . . . . . 4 54 3.1 Third Party Call Control Transcoding Model . . . . . . . . 5 55 3.2 Conference Bridge Transcoding Model . . . . . . . . . . . 6 56 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 57 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 58 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8 59 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 60 7.1 Normative References . . . . . . . . . . . . . . . . . . . 8 61 7.2 Informational References . . . . . . . . . . . . . . . . . 9 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9 63 Intellectual Property and Copyright Statements . . . . . . . . 10 65 1. Introduction 67 Two user agents involved in a SIP [3] dialog may find it impossible 68 to establish a media session due to a variety of incompatibilities. 69 Assuming that both user agents understand the same session 70 description format (e.g., SDP [11]), incompatibilities can be found 71 at the user agent level and at the user level. At the user agent 72 level, both terminals may not support any common codec or may not 73 support common media types (e.g., a text-only terminal and an audio- 74 only terminal). At the user level, a deaf person will not understand 75 anything said over an audio stream. 77 In order to make communications possible in the presence of 78 incompatibilities, user agents need to introduce intermediaries that 79 provide transcoding services to a session. From the SIP point of 80 view, the introduction of a transcoder is done in the same way to 81 resolve both user level and user agent level incompatibilities. So, 82 the invocation mechanisms described in this document are generally 83 applicable to any type of incompatibility related to how the 84 information that needs to be communicated is encoded. 86 Furthermore, although this framework focuses on transcoding, the 87 mechanisms described are applicable to media manipulation in 88 general. It would be possible to use them, for example, to invoke 89 a server that simply increased the volume of an audio stream. 91 This document does not describe media server discovery. That is an 92 orthogonal problem that one can address using user agent provisioning 93 or other methods. 95 The remainder of this document is organized as follows. Section 2 96 deals with the discovery of the need of transcoding services for a 97 particular session. Section 3 introduces the third party call 98 control and conference bridge transcoding invocation models, which 99 are further described in Section 3.1 and Section 3.2 respectively. 100 Both models meet the requirements regarding transcoding services 101 invocation in RFC3351 [6] to support deaf, hard of hearing and 102 speech-impaired individuals. 104 2. Discovery of the Need for Transcoding Services 106 According to the one-party consent model defined in RFC 3238 [2], 107 services that involve media manipulation invocation are best invoked 108 by one of the end-points involved in the communication, as opposed to 109 being invoked by an intermediary in the network. Following this 110 principle, one of the end-points should be the one detecting that 111 transcoding is needed for a particular session. 113 In order to decide whether or not transcoding is needed, a user agent 114 needs to know the capabilities of the remote user agent. A user 115 agent acting as an offerer typically obtains this knowledge by 116 downloading a presence document that includes media capabilities 117 (e.g., Bob is available on a terminal that only supports audio) or by 118 getting an SDP description of media capabilities as defined in RFC 119 3264 [4]. 121 Presence documents are typically received in a NOTIFY [5] request as 122 a result of a subscription. SDP media capabilities descriptions are 123 typically received in a 200 (OK) response to an OPTIONS request or in 124 a 488 (Not Acceptable Here) response to an INVITE. 126 It is recommended that an offerer does not invoke transcoding 127 services before making sure that the answerer does not support the 128 capabilities needed for the session. Making wrong assumptions about 129 the answerer's capabilities can lead to situations where two 130 transcoders are introduced (one by the offerer and one by the 131 answerer) in a session that would not need any transcoding services 132 at all. 134 An example of the situation above is a call between two GSM phones 135 (without using transcoding-free operation). Both phones use a GSM 136 codec, but the speech is converted from GSM to PCM by the 137 originating MSC and from PCM back to GSM by the terminating MSC. 139 Note that transcoding services can be symmetric (e.g., speech-to-text 140 plus text-to-speech) or asymmetric (e.g., a one-way speech-to-text 141 transcoding for a hearing impaired user that can talk). 143 3. Transcoding Services Invocation 145 Once the need for transcoding for a particular session has been 146 identified as described in Section 2, one of the user agents needs to 147 invoke transcoding services. 149 As we said earlier, transcoder location is outside the scope of this 150 document. So, we assume that the user agent invoking transcoding 151 services knows the URI of a server that provides them. 153 Invoking transcoding services from a server (T) for a session between 154 two user agents (A and B) involves establishing two media sessions; 155 one between A and T and another between T and B. How to invoke T's 156 services (i.e., how to establish both A-T and T-B sessions) depends 157 on how we model the transcoding service. We have considered two 158 models for invoking a transcoding service. The first is to use third 159 party call control [7], also referred to as 3pcc. The second is to 160 use a (dial-in and dial-out) conference bridge that negotiates the 161 appropriate media parameters on each individual leg (i.e., A-T and 162 T-B). 164 Section 3.1 analyzes the applicability of the third party call 165 control model and Section 3.2 analyzes the applicability of the 166 conference bridge transcoding invocation model. 168 3.1 Third Party Call Control Transcoding Model 170 In the 3pcc transcoding model, defined in [10], the user agent 171 invoking the transcoding service has a signalling relationship with 172 the transcoder and another signalling relationship with the remote 173 user agent. There is no signalling relationship between the 174 transcoder and the remote user agent, as shown in Figure 1. 176 +-------+ 177 | | 178 | T |** 179 | | ** 180 +-------+ ** 181 ^ * ** 182 | * ** 183 | * ** 184 SIP * ** 185 | * ** 186 | * ** 187 v * ** 188 +-------+ +-------+ 189 | | | | 190 | A |<-----SIP----->| B | 191 | | | | 192 +-------+ +-------+ 194 <-SIP-> Signalling 195 ******* Media 197 Figure 1: Third party call control model 199 This model is suitable for advanced end points that are able to 200 perform third party call control. It allows end-points to invoke 201 transcoding services on a stream basis. That is, the media streams 202 that need transcoding are routed through the transcoder while the 203 streams that do not need it are sent directly between the end points. 204 This model also allows to invoke one transcoder for the sending 205 direction and a different one for the receiving direction of the same 206 stream. 208 Invoking a transcoder in the middle of an ongoing session is also 209 quite simple. This is useful when session changes occur (e.g., an 210 audio session is upgraded to an audio/video session) and the end- 211 points cannot cope with the changes (e.g., they had common audio 212 codecs but no common video codecs). 214 The privacy level that is achieved using 3pcc is high, since the 215 transcoder does no see the signalling between both end-points. In 216 this model, the transcoder only has access to the information that is 217 strictly needed to perform its function. 219 3.2 Conference Bridge Transcoding Model 221 In a centralized conference, there are a number of media streams 222 between the conference server and each participant of a conference. 223 For a given media type (e.g., audio) the conference server sends, 224 over each individual stream, the media received over the rest of the 225 streams, typically performing some mixing. If the capabilities of 226 all the end points participating in the conference are not the same, 227 the conference server may have to send audio to different 228 participants using different audio codecs. 230 Consequently, we can model a transcoding service as a two-party 231 conference server that may change not only the codec in use, but also 232 the format of the media (e.g., audio to text). 234 Using this model, T behaves as a B2BUA (Back-to-Back User Agent) and 235 the whole A-T-B session is established as described in 236 [draft-ietf-sipping-transc-conf]. Figure 2 shows the signalling 237 relationships between the end-points and the transcoder. 239 +-------+ 240 | |** 241 | T | ** 242 | |\ ** 243 +-------+ \\ ** 244 ^ * \\ ** 245 | * \\ ** 246 | * SIP ** 247 SIP * \\ ** 248 | * \\ ** 249 | * \\ ** 250 v * \ ** 251 +-------+ +-------+ 252 | | | | 253 | A | | B | 254 | | | | 255 +-------+ +-------+ 257 <-SIP-> Signalling 258 ******* Media 260 Figure 2: Conference bridge model 262 In the conferencing bridge model, the end-point invoking the 263 transcoder is generally involved in less signalling exchanges than in 264 the 3pcc model. This may be an important feature for end-poing using 265 low bandwidth or high-delay access links (e.g., some wireless 266 accesses). 268 On the other hand, this model is less flexible than the 3pcc model. 269 It is not possible to use different transcoders for different streams 270 or for different directions of a stream. 272 Invoking a transcoder in the middle of an ongoing session or changing 273 from one transcoder to another requires the remote end-point to 274 support the Replaces [9] extension. At present, not many user agents 275 support it. 277 Simple end-points that cannot perform 3pcc and thus cannot use the 278 3pcc model, of course, need to use the conference bridge model. 280 4. Security Considerations 282 The specifications of the 3pcc and the conferencing transcoding 283 models discuss security issues directly related to the implementation 284 of those models. Additionally, there are some considerations that 285 apply to transcoding in general. 287 In a session, a transcoder has access to at least some of the media 288 exchanged between the end points. In order to avoid rogue 289 transcoders getting access to those media, it is recommended that end 290 points authenticate the transcoder. TLS [1] and S/MIME [8] can be 291 used for this purpose. 293 To achieve a higher degree of privacy, end points following the 3pcc 294 transcoding model can use one transcoder in one direction and a 295 different one in the other direction. This way, no single transcoder 296 has access to all the media exchanged between the end points. 298 5. IANA Considerations 300 This document does not contain any IANA actions. 302 6. Contributors 304 This document is the result of discussions amongst the conferencing 305 design team. The members of this team include Eric Burger, Henning 306 Schulzrinne and Arnoud van Wijk. 308 7. References 310 7.1 Normative References 312 [1] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", 313 RFC 2246, January 1999. 315 [2] Floyd, S. and L. Daigle, "IAB Architectural and Policy 316 Considerations for Open Pluggable Edge Services", RFC 3238, 317 January 2002. 319 [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 320 Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: 321 Session Initiation Protocol", RFC 3261, June 2002. 323 [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 324 Session Description Protocol (SDP)", RFC 3264, June 2002. 326 [5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 327 Notification", RFC 3265, June 2002. 329 [6] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van 330 Wijk, "User Requirements for the Session Initiation Protocol 331 (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired 332 Individuals", RFC 3351, August 2002. 334 [7] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, 335 "Best Current Practices for Third Party Call Control (3pcc) in 336 the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, 337 April 2004. 339 [8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions 340 (S/MIME) Version 3.1 Certificate Handling", RFC 3850, 341 July 2004. 343 [9] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation 344 Protocol (SIP) "Replaces" Header", RFC 3891, September 2004. 346 [10] Camarillo, G., "Transcoding Services Invocation in the Session 347 Initiation Protocol (SIP) Using Third Party Call Control 348 (3pcc)", draft-ietf-sipping-transc-3pcc-02 (work in progress), 349 September 2004. 351 7.2 Informational References 353 [11] Handley, M., "SDP: Session Description Protocol", 354 draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005. 356 Author's Address 358 Gonzalo Camarillo 359 Ericsson 360 Hirsalantie 11 361 Jorvas 02420 362 Finland 364 Email: Gonzalo.Camarillo@ericsson.com 366 Intellectual Property Statement 368 The IETF takes no position regarding the validity or scope of any 369 Intellectual Property Rights or other rights that might be claimed to 370 pertain to the implementation or use of the technology described in 371 this document or the extent to which any license under such rights 372 might or might not be available; nor does it represent that it has 373 made any independent effort to identify any such rights. 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