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'9') (Obsoleted by RFC 4733, RFC 4734) Summary: 10 errors (**), 0 flaws (~~), 8 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force Gonzalo Camarillo 3 Internet draft Jan Holler 4 Goran AP Eriksson 5 Ericsson 7 Henning Schulzrinne 8 Columbia University 10 August 2001 11 Expires February 2002 12 14 Grouping of media lines in SDP 16 Status of this Memo 18 This document is an Internet-Draft and is in full conformance with 19 all provisions of Section 10 of RFC2026. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. Internet-Drafts are draft documents valid for a maximum of 25 six months and may be updated, replaced, or obsoleted by other 26 documents at any time. It is inappropriate to use Internet- Drafts 27 as reference material or to cite them other than as "work in 28 progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 Abstract 37 This document defines two SDP attributes: "group" and "mid". They 38 allow to group together several "m" lines for two different 39 purposes: for lip synchronization and for receiving media from a 40 single flow (several media streams), encoded in different formats 41 during a particular session, in different ports and host interfaces. 43 Camarillo/Holler/Eriksson/Schulzrinne 1 44 Grouping of media lines in SDP 46 TABLE OF CONTENTS 48 1 Introduction...............................................2 49 2 Terminology................................................3 50 3 Media stream identification attribute......................3 51 4 Group attribute............................................3 52 5 Use of "group" and "mid"...................................3 53 6 Lip Synchronization (LS)...................................4 54 6.1 Example of LS..............................................4 55 7 Flow Identification (FID)..................................5 56 7.1 SIP and cellular access....................................5 57 7.2 DTMF tones.................................................5 58 7.3 Media flow definition......................................6 59 7.4 FID semantics..............................................6 60 7.4.1 Examples of FID............................................6 61 8 Scenarios that FID does not cover..........................9 62 8.1 Parallel encoding using different codecs...................9 63 8.2 Layered encoding..........................................10 64 8.3 Same IP address and port number...........................10 65 9 Usage of the "group" attribute in SIP.....................11 66 9.1 Mid value in responses....................................11 67 9.1.1 Example...................................................11 68 9.2 Group value in responses..................................12 69 9.2.1 Example...................................................13 70 9.3 Capability negotiation....................................14 71 9.3.1 Example...................................................14 72 9.4 Backward compatibility....................................14 73 9.4.1 Client does not support "group"...........................15 74 9.4.2 Server does not support "group"...........................15 75 10 IANA considerations.......................................15 76 11 Acknowledgements..........................................15 77 12 References................................................15 78 13 Authors� Addresses........................................16 80 1 Introduction 82 An SDP session description typically contains a number (one or more) 83 of media lines - they are commonly known as "m" lines. When a 84 session description contains more than one "m" line, SDP does not 85 provide any means to express a particular relationship between two 86 or more of them. When an application receives an SDP session 87 description with more than one "m" line it is up to the application 88 what to do with them. SDP does not carry any information about 89 grouping media streams. 91 While in some environments this information can be carried out of 92 band, it would be desirable to have extensions to SDP that allowed 93 to express how different media streams within a session description 94 relate to each other. This document defines such extensions. 96 Camarillo/Holler/Eriksson/Schulzrinne 2 97 Grouping of media lines in SDP 99 2 Terminology 101 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 102 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 103 and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] 104 and indicate requirement levels for compliant implementations. 106 3. Media stream identification attribute 108 A new "media stream identification" media attribute is defined. It 109 is used for identifying media streams within a session description. 110 Its formatting in SDP [2] is described by the following BNF: 112 mid-attribute = "a=mid:" identification-tag 113 identification-tag = token 115 The identification tag MUST be unique within an SDP session 116 description. 118 4. Group attribute 120 A new "group" session level attribute is defined. It is used for 121 grouping together different media streams. Its formatting in SDP is 122 described by the following BNF: 124 group-attribute = "a=group:" semantics 125 *(space identification-tag) 126 semantics = "LS" | "FID" 128 This document defines two standard semantics: LS (Lip 129 Synchronization) and FID (Flow Identification). If in the future it 130 was needed to standardize further semantics they would need to be 131 defined in a standards track document. However, defining new 132 semantics apart from LS and FID is discouraged. Instead, it is 133 RECOMMENDED to use other session description mechanisms such as 134 SDPng [3]. 136 5. Use of "group" and "mid" 138 All the "m" lines of a session description that uses "group" MUST be 139 identified with an "mid" attribute regardless of whether they appear 140 or not in the group line(s). If a session description contains at 141 least one "m" line that has no "mid" identification the application 142 MUST NOT perform any grouping of media lines. 144 "a=group" lines are used to group together several "m" lines that 145 are identified by their "mid" attribute. "a=group" lines that 146 contain identification-tags that do not correspond to any "m" line 147 within the session description MUST be simply ignored. The 148 application acts as if the "a=group" line did not exist. The 149 behavior of an application receiving an SDP with grouped "m" lines 150 is defined by the semantics field in the "a=group" line. 152 Camarillo/Holler/Eriksson/Schulzrinne 3 153 Grouping of media lines in SDP 155 There MAY be several "a=group" lines in a session description. 157 An application that wants to be compliant to this specification MUST 158 support both "group" and "mid". An application that supported just 159 one of them would not be compliant. 161 6. Lip Synchronization (LS) 163 An application that receives a session description that contains "m" 164 lines that are grouped together using LS semantics MUST synchronize 165 the play out of the corresponding media streams. Note that LS 166 semantics not only apply to a video stream that has to be 167 synchronized with an audio stream. The play out of two streams of 168 the same type can perfectly be synchronized as well. 170 For RTP streams synchronization is typically performed using RTCP, 171 which provides enough information to map time stamps from the 172 different streams into a wall clock. However, the concept of media 173 stream synchronization MAY also apply to media streams that do not 174 make use of RTP. If this is the case, the application MUST recover 175 the original timing relationship between the streams using whatever 176 available mechanism. 178 6.1 Example of LS 180 The following example shows a session description of a conference 181 that is being multicast. The first media stream (mid:1) contains the 182 voice of the speaker, who speaks in English. The second media stream 183 (mid:2) contains the video component and the third (mid:3) media 184 stream carries the translation to Spanish of what he is saying. The 185 first and the second media streams MUST be synchronized. 187 v=0 188 o=Laura 289083124 289083124 IN IP4 one.example.com 189 t=0 0 190 c=IN IP4 224.2.17.12/127 191 a=group:LS 1 2 192 m=audio 30000 RTP/AVP 0 193 a=mid:1 194 m=video 30002 RTP/AVP 31 195 a=mid:2 196 m=audio 30004 RTP/AVP 0 197 i=This media stream contains the Spanish translation 198 a=mid:3 200 Note that although the third media stream is not present in the 201 group line it still MUST contain an mid attribute (mid:3), as stated 202 before. 204 Camarillo/Holler/Eriksson/Schulzrinne 4 205 Grouping of media lines in SDP 207 7. Flow Identification (FID) 209 An "m" line in an SDP session description defines a media stream. 210 However, SDP does not define what a media stream is. To find the 211 definition of a media stream we have to go to the RTSP 212 specification. The RTSP RFC [4] defines a media stream as "a single 213 media instance, e.g., an audio stream or a video stream as well as a 214 single whiteboard or shared application group. When using RTP, a 215 stream consists of all RTP and RTCP packets created by a source 216 within an RTP session". 218 This definition assumes that a single audio (or video) stream maps 219 into an RTP session. To find the definition of an RTP session we go 220 to the RTP specification. The RTP RFC [5] defines an RTP session as 221 follows: "For each participant, the session is defined by a 222 particular pair of destination transport addresses (one network 223 address plus a port pair for RTP and RTCP)". 225 While the previous definitions cover the most common cases, there 226 are situations where a single media instance, (e.g., an audio stream 227 or a video stream) is sent using more than one RTP session. Two 228 examples (among many others) of this kind of situation are cellular 229 systems using SIP [6] and systems receiving DTMF tones on a 230 different host than the voice. 232 7.1 SIP and cellular access 234 Systems using a cellular access and SIP as a signalling protocol 235 need to receive media over the air. During a session the media can 236 be encoded using different codecs. The encoded media has to traverse 237 the radio interface. The radio interface is generally characterized 238 by being bit error prone and associated with relatively high packet 239 transfer delays. In addition, radio interface resources in a 240 cellular environment are scarce and thus expensive, which calls for 241 special measures in providing a highly efficient transport [7]. In 242 order to get an appropriate speech quality in combination with an 243 efficient transport, precise knowledge of codec properties are 244 required so that a proper radio bearer for the RTP session can be 245 configured before transferring the media. These radio bearers are 246 dedicated bearers per media type, i.e. codec. 248 Cellular systems typically configure different radio bearers on 249 different port numbers. Therefore, incoming media has to have 250 different destination port numbers for the different possible codecs 251 in order to be routed properly to the correct radio bearer. Thus, 252 this is an example in which several RTP sessions are used to carry a 253 single media instance (the encoded speech from the sender). 255 7.2 DTMF tones 257 Some voice sessions include DTMF tones. Sometimes the voice handling 258 is performed by a different host than the DTMF handling. [8] 259 contains several examples of how application servers in the network 261 Camarillo/Holler/Eriksson/Schulzrinne 5 262 Grouping of media lines in SDP 264 gather DTMF tones for the user while the user receives the encoded 265 speech on his user agent. In this situations it is necessary to 266 establish two RTP sessions: one for the voice and the other for the 267 DTMF tones. Both RTP sessions are logically part of the same media 268 instance. 270 7.3 Media flow definition 272 The previous examples show that the definition of a media stream in 273 [4] do not cover some scenarios. It cannot be assumed that a single 274 media instance maps into a single RTP session. Therefore, we 275 introduce the definition of a media flow: 277 Media flow consists of a single media instance, e.g., an audio 278 stream or a video stream as well as a single whiteboard or shared 279 application group. When using RTP, a media flow comprises one or 280 more RTP sessions. 282 7.4 FID semantics 284 Several "m" lines grouped together using FID semantics form a media 285 flow. A media agent handling a media flow that comprises several "m" 286 lines MUST send a copy of the media to every "m" line part of the 287 flow as long as the codecs and the direction attribute present in a 288 particular "m" line allow it. 290 It is assumed that the application uses only one codec at a time to 291 encode the media produced. This codec MAY change dynamically during 292 the session, but at any certain moment only one codec is in use. 294 The application encodes the media using the current codec and checks 295 one by one all the "m" lines that are part of the flow. If a 296 particular "m" line contains the codec being used and the direction 297 attribute is "sendonly" or "sendrecv" a copy of the encoded media is 298 sent to the address/port specified in that particular media stream. 299 If either the "m" line does not contain the codec being used or the 300 direction attribute is neither "sendonly" nor "sendrecv" nothing is 301 sent over this media stream. 303 The application typically ends up sending media to different 304 destinations (IP address/port number) depending on the codec used at 305 any moment. 307 7.4.1 Examples of FID 309 The session description below would be the SDP sent by a SIP user 310 agent using a cellular access. The user agent supports GSM on port 311 30000 and AMR on port 30002. When the remote party sends GSM it will 312 send RTP packets to port number 30000. When AMR is the codec chosen, 313 packets will be sent to port 30002. Note that the remote party can 314 switch between both codecs dynamically in the middle of the session. 315 However, in this example, only one media stream at a time carries 317 Camarillo/Holler/Eriksson/Schulzrinne 6 318 Grouping of media lines in SDP 320 voice. The other remains "muted" while its corresponding codec is 321 not in use. 323 v=0 324 o=Laura 289083124 289083124 IN IP4 two.example.com 325 t=0 0 326 c=IN IP4 131.160.1.112 327 a=group:FID 1 2 328 m=audio 30000 RTP/AVP 3 329 a=rtpmap:3 GSM/8000 330 a=mid:1 331 m=audio 30002 RTP/AVP 97 332 a=rtpmap:97 AMR/8000 333 a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- 334 neighbor; maxframes=1 335 a=mid:2 337 In the previous example a system receives media on the same IP 338 address on different port numbers. The following example shows how a 339 system can receive different codecs on different IP addresses. 341 v=0 342 o=Laura 289083124 289083124 IN IP4 three.example.com 343 t=0 0 344 c=IN IP4 131.160.1.112 345 a=group:FID 1 2 346 m=audio 20000 RTP/AVP 0 347 c=IN IP4 131.160.1.111 348 a=rtpmap:0 PCMU/8000 349 a=mid:1 350 m=audio 30002 RTP/AVP 97 351 a=rtpmap:97 AMR/8000 352 a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- 353 neighbor; maxframes=1 354 a=mid:2 356 The cellular terminal of this example only supports the AMR codec. 357 However, many current IP phones only support PCM (payload 0). In 358 order to be able to interoperate with them, the cellular terminal 359 uses a transcoder whose IP address is 131.160.1.111. The cellular 360 terminal includes in its SDP support for PCM at that IP address. 361 Remote systems will send AMR directly to the terminal but PCM will 362 be sent to the transcoder. The transcoder will be configured (using 363 whatever method) to convert the incoming PCM audio to AMR and send 364 it to the terminal. 366 The next example shows that the "group" attribute used with FID 367 semantics allows to express uni-directional codecs for a bi- 368 directional media flow. That is, a codec that is only used in one 369 direction within a sendrecv media stream. 371 Camarillo/Holler/Eriksson/Schulzrinne 7 372 Grouping of media lines in SDP 374 v=0 375 o=Laura 289083124 289083124 IN IP4 four.example.com 376 t=0 0 377 c=IN IP4 131.160.1.112 378 a=group:FID 1 2 379 m=audio 30000 RTP/AVP 0 380 a=mid:1 381 m=audio 30002 RTP/AVP 8 382 a=recvonly 383 a=mid:2 385 A user agent that receives the SDP above knows that at a certain 386 moment it can send either PCM u-law to port number 30000 or PCM A- 387 law to port number 30002. However, the media agent also knows that 388 the other end will only send PCM u-law (payload 0). 390 The following example shows a session description with different "m" 391 lines grouped together using FID semantics that contain the same 392 codec. 394 v=0 395 o=Laura 289083124 289083124 IN IP4 five.example.com 396 t=0 0 397 c=IN IP4 131.160.1.112 398 a=groupe:FID 1 2 3 399 m=audio 30000 RTP/AVP 0 400 a=mid:1 401 m=audio 30002 RTP/AVP 8 402 a=mid:2 403 m=audio 20000 RTP/AVP 0 8 404 c=IN IP4 131.160.1.111 405 a=recvonly 406 a=mid:3 408 At a particular point of time, if the media agent is sending PCM u- 409 law (payload 0) it sends RTP packets to 131.160.1.112 on port 30000 410 and to 131.160.1.111 on port 20000 (first and third "m" lines). If 411 it is sending PCM A-law (payload 8) it sends RTP packets to 412 131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000 413 (second and third "m" lines). 415 The system that generated the SDP above supports PCM u-law on port 416 30000 and PCM A-law on port 30002. Besides, it uses an application 417 server whose IP address is 131.160.1.111 that records all the 418 conversation. That is why the application server always receives a 419 copy of the audio stream regardless of the codec being used at any 420 given moment (it actually performs an RTP dump, so it can 421 effectively receive any codec). 423 Remember that if several "m" lines grouped together using FID 424 semantics contain the same codec the media agent MUST send media 425 over several RTP sessions at the same time. 427 Camarillo/Holler/Eriksson/Schulzrinne 8 428 Grouping of media lines in SDP 430 The last example of this section deals with DTMF tones. DTMF tones 431 can be transmitted using a regular voice codec or can be transmitted 432 as telephony events. The RTP payload for DTMF tones treated as 433 telephone events is described in RFC 2833 [9]. Below there is an 434 example of an SDP session description using FID semantics and this 435 payload type. 437 v=0 438 o=Laura 289083124 289083124 IN IP4 six.example.com 439 t=0 0 440 c=IN IP4 131.160.1.112 441 a=group:FID 1 2 442 m=audio 30000 RTP/AVP 0 443 a=mid:1 444 m=audio 20000 RTP/AVP 97 445 c=IN IP4 131.160.1.111 446 a=rtpmap:97 telephone-events 447 a=mid:2 449 The remote party would send PCM encoded voice (payload 0) to 450 131.160.1.112 and DTMF tones encoded as telephony events to 451 131.160.1.111. Note that only voice or DTMF is sent at a particular 452 point of time. When DTMF tones are sent the first media stream does 453 not carry any data and when voice is sent there is no data in the 454 second media stream. FID semantics provide different destinations 455 for alternative codecs. 457 8 Scenarios that FID does not cover 459 It is worthwhile mentioning some scenarios where the "group" 460 attribute using existing semantics (particularly FID) might seem to 461 be applicable but it is not. This section has been included because 462 we have observed some confusion within the community regarding the 463 three scenarios described below. This section helps clarify them. 465 8.1 Parallel encoding using different codecs 467 FID semantics are useful when the application only uses one codec at 468 a time. When a particular application encodes the same media using 469 different codecs FID MUST NOT be used. Some systems that handle DTMF 470 tones are a typical example of parallel encoding using different 471 codecs. 473 Some systems implement the RTP payload defined in RFC 2833, but when 474 they send DTMF tones they do not mute the voice channel. Therefore, 475 effectively they are sending two copies of the same DTMF tone: 476 encoded as voice and encoded as a telephony event. When the receiver 477 gets both copies it typically uses the telephony event rather than 478 the tone encoded as voice. FID semantics MUST NOT be used in this 479 context to group both media streams since such a system is not using 480 alternative codecs but rather different parallel encodings for the 481 same information. 483 Camarillo/Holler/Eriksson/Schulzrinne 9 484 Grouping of media lines in SDP 486 8.2 Layered encoding 488 Layered encoding schemes encode media in different layers. Quality 489 at the receiver varies depending on the number of layers received. 490 SDP provides a means to group together contiguous multicast 491 addresses that transport different layers. The "c" line below: 493 c=IN IP4 224.2.1.1/127/3 495 is equivalent to the following three "c" lines: 497 c=IN IP4 224.2.1.1/127 498 c=IN IP4 224.2.1.2/127 499 c=IN IP4 224.2.1.3/127 501 FID MUST NOT be used to group "m" lines that contain the different 502 layers of layered encoding scheme. Besides, we do not define new 503 group semantics to provide a more flexible way of grouping different 504 layers because the already existing SDP mechanism covers the most 505 useful scenarios. 507 8.3 Same IP address and port number 509 If several codecs have to be sent to the same IP address and port, 510 the traditional SDP syntax of listing several codecs in the same "m" 511 line MUST be used. FID MUST NOT be used to group "m" lines with the 512 same IP address/port. Therefore, an SDP like the one below MUST NOT 513 be generated. 515 v=0 516 o=Laura 289083124 289083124 IN IP4 six.example.com 517 t=0 0 518 c=IN IP4 131.160.1.112 519 a=group:FID 1 2 520 m=audio 30000 RTP/AVP 0 521 a=mid:1 522 m=audio 30000 RTP/AVP 8 523 a=mid:2 525 The correct SDP for the session above would be the following one: 527 v=0 528 o=Laura 289083124 289083124 IN IP4 six.example.com 529 t=0 0 530 c=IN IP4 131.160.1.112 531 m=audio 30000 RTP/AVP 0 8 533 9. Usage of the "group" attribute in SIP 535 SDP descriptions are used by several different protocols, SIP among 536 them. We include a section about SIP because the "group" attribute 537 will most likely be used mainly by SIP systems. 539 Camarillo/Holler/Eriksson/Schulzrinne 10 540 Grouping of media lines in SDP 542 SIP [6] is an application layer protocol for establishing, 543 terminating and modifying multimedia sessions. SIP carries session 544 descriptions in the bodies of the SIP messages but is independent 545 from the protocol used for describing sessions. SDP [2] is one of 546 the protocols that can be used for this purpose. 548 At session establishment SIP provides a three-way handshake (INVITE- 549 200 OK-ACK) between end systems. However, just two of these three 550 messages carry SDP. SDPs MAY be present in INVITE and 200 OK or in 551 200 OK and ACK. The following sections assume that INVITE and 200 OK 552 are the ones carrying SDP for the shake of clarity, but everything 553 is also applicable to the other possible scenario (200 OK and ACK). 555 9.1 Mid value in responses 557 The "mid" attribute is an identifier for a particular media stream. 558 Therefore, the "mid" value in the response MUST be the same as the 559 "mid" value in the request. Besides, subsequent requests such as re- 560 INVITEs SHOULD use the same "mid" value for the already existing 561 media streams. 563 Appendix B of [6] describes the usage of SDP in relation to SIP. It 564 states: "The caller and callee align their media description so that 565 the nth media stream ("m=" line) in the caller�s session description 566 corresponds to the nth media stream in the callee�s description." 568 The presence of the "group" attribute in an SDP session description 569 does not modify this behavior. 571 Since the "mid" attribute provides a means to label "m" lines it 572 would be possible to perform media alignment using "mid" labels 573 rather than matching nth "m" lines. However this would not bring any 574 gain and would add complexity to implementations. Therefore SIP 575 systems MUST perform media alignment matching nth lines regardless 576 of the presence of the "group" or "mid" attributes. 578 If a media stream that contained a particular "mid" identifier in 579 the request contains a different identifier in the response the 580 application ignores all the "mid" and "group" lines that might 581 appear in the session description. The following example illustrates 582 this scenario: 584 9.1.1 Example 586 Two SIP entities exchange SDPs during session establishment. The 587 INVITE contained the SDP below: 589 v=0 590 o=Laura 289083124 289083124 IN IP4 seven.example.com 591 t=0 0 592 c=IN IP4 131.160.1.112 593 a=groupe:FID 1 2 595 Camarillo/Holler/Eriksson/Schulzrinne 11 596 Grouping of media lines in SDP 598 m=audio 30000 RTP/AVP 0 8 599 a=mid:1 600 m=audio 30002 RTP/AVP 0 8 601 a=mid:2 603 The 200 OK response contains the following SDP: 605 v=0 606 o=Bob 289083122 289083122 IN IP4 eigth.example.com 607 t=0 0 608 c=IN IP4 131.160.1.113 609 a=groupe:FID 1 2 610 m=audio 25000 RTP/AVP 0 8 611 a=mid:2 612 m=audio 25002 RTP/AVP 0 8 613 a=mid:1 615 Since alignment of "m" lines is performed based on matching of nth 616 lines, the first stream had "mid:1" in the INVITE and "mid:2" in the 617 200 OK. Therefore, the application MUST ignore every "mid" and 618 "group" lines contained in the SDP. 620 A well-behaved SIP user agent would have returned the SDP below in 621 the 200 OK: 623 v=0 624 o=Bob 289083122 289083122 IN IP4 nine.example.com 625 t=0 0 626 c=IN IP4 131.160.1.113 627 a=groupe:FID 1 2 628 m=audio 25002 RTP/AVP 0 8 629 a=mid:1 630 m=audio 25000 RTP/AVP 0 8 631 a=mid:2 633 9.2 Group value in responses 635 A SIP entity that receives a request that contains an "a=group" line 636 with semantics that it does not understand MUST return a response 637 without the "group" line. Note that, as it was described in the 638 previous section, the "mid" lines MUST still be present in the 639 response. 641 A SIP entity that receives a request that contains an "a=group" line 642 which semantics that are understood MUST return a response that 643 contains an "a=group" line with the same semantics. The 644 identification-tags contained in this "a=group" lines MUST be the 645 same that were received in the request or a subset of them (zero 646 identification-tags is a valid subset). When the identification-tags 647 in the response are a subset the "group" value to be used in the 648 session MUST be the one present in the response. 650 Camarillo/Holler/Eriksson/Schulzrinne 12 651 Grouping of media lines in SDP 653 SIP entities refuse media streams by setting the port to zero in the 654 corresponding "m" line. "a=group" lines MUST no contain 655 identification-tags that correspond to "m" lines with port zero. 657 Note that grouping of m lines MUST always be requested by the issuer 658 of the request (the client), never by the issuer of the response 659 (the server). Since SIP provides a two-way SDP exchange, a server 660 that requested grouping in a response would not know whether the 661 "group" attribute was accepted by the client or not. A server that 662 wants to group media lines SHOULD issue another request after having 663 responded to the first one (a re-INVITE for instance). 665 Note that, as we mentioned previously, in this section we are 666 assuming that the SDPs are present in the INVITE and in the 200 667 OK. Applying the statement above to the scenario where SDPs are 668 present in the 200 OK and in the ACK, the entity requesting 669 grouping would be the server. 671 9.2.1 Example 673 The example below shows how the callee refuses a media stream 674 offered by the caller setting its port number to zero. The "mid" 675 value corresponding to that media stream is removed from the "group" 676 value in the response. 678 SDP in the INVITE from caller to callee: 680 v=0 681 o=Laura 289083124 289083124 IN IP4 ten.example.com 682 t=0 0 683 c=IN IP4 131.160.1.112 684 a=group:FID 1 2 3 685 m=audio 30000 RTP/AVP 0 686 a=mid:1 687 m=audio 30002 RTP/AVP 8 688 a=mid:2 689 m=audio 30004 RTP/AVP 3 690 a=mid:3 692 SDP in the INVITE from callee to caller: 694 v=0 695 o=Bob 289083125 289083125 IN IP4 eleven.example.com 696 t=0 0 697 c=IN IP4 131.160.1.113 698 a=group:FID 1 3 699 m=audio 20000 RTP/AVP 0 700 a=mid:1 701 m=audio 0 RTP/AVP 8 702 a=mid:2 703 m=audio 20002 RTP/AVP 3 704 a=mid:3 706 Camarillo/Holler/Eriksson/Schulzrinne 13 707 Grouping of media lines in SDP 709 9.3 Capability negotiation 711 A client that understands "group" and "mid" but does not want to 712 make use of them in a particular session MAY want indicate that it 713 supports them. If a client decides to do that, it SHOULD add an 714 "a=group" line with zero identification-tags for every semantics it 715 understands. 717 If a server receives a request that contains empty "a=group" lines 718 it SHOULD add its capabilities also in the form of empty "a=group" 719 lines to its response. 721 9.3.1 Example 723 A system that supports both LS and FID semantics but does not want 724 to group any media stream for this particular session generates the 725 following SDP: 727 v=0 728 o=Bob 289083125 289083125 IN IP4 twelve.example.com 729 t=0 0 730 c=IN IP4 131.160.1.113 731 a=group:LS 732 a=group:FID 733 m=audio 20000 RTP/AVP 0 8 735 The server that receives that request supports FID but not LS. It 736 responds with the SDP below: 738 v=0 739 o=Laura 289083124 289083124 IN IP4 thirteen.example.com 740 t=0 0 741 c=IN IP4 131.160.1.112 742 a=group:FID 743 m=audio 30000 RTP/AVP 0 745 9.4 Backward compatibility 747 This document does not define any SIP "Require" header. Therefore, 748 if one of the SIP user agents does not understand the "group" 749 attribute the standard SDP fall back mechanism MUST be used 750 (attributes that are not understood are simply ignored). 752 9.4.1 Client does not support "group" 754 This situation does not represent a problem because grouping 755 requests is always performed by clients, not by servers. If the 757 Camarillo/Holler/Eriksson/Schulzrinne 14 758 Grouping of media lines in SDP 760 client does not support "group" this attribute will just not be 761 used. 763 9.4.2 Server does not support "group" 765 The server will ignore the "group" attribute, since it does not 766 understand it (it will also ignore the "mid" attribute). For LS 767 semantics, the server might decide to perform or to not perform 768 synchronization between media streams. 770 For FID semantics, the server will consider that the session 771 comprises several media streams. 773 Different implementations would behave in different ways. 775 In the case of audio and different "m" lines for different codecs an 776 implementation might decide to act as a mixer with the different 777 incoming RTP sessions, which is the correct behavior. 779 An implementation might also decide to refuse the request (e.g. 488 780 Not acceptable here or 606 Not Acceptable) because it contains 781 several "m" lines. In this case, the server does not support the 782 type of session that the caller wanted to establish. In case the 783 client is willing to establish a simpler session anyway, he SHOULD 784 re-try the request without "group" attribute and only one "m" line 785 per flow. 787 10. IANA considerations 789 As previously stated in section 4, this document defines two 790 standard semantics related to the "group" attribute: LS (Lip 791 Synchronization) and FID (Flow Identification). If in the future it 792 was needed to standardize further semantics they would need to be 793 defined in a standards track document. 795 11. Acknowledgments 797 The authors would like to thank Jonathan Rosenberg, Adam Roach, Orit 798 Levin and Joerg Ott for their feedback on this document. 800 12. References 802 [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement 803 Levels", RFC 2119, IETF; March 1997. 805 [2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC 806 2327, IETF; April 1998. 808 [3] D. Kutscher/J. Ott/C. Bormann, "Session Description and 809 Capability Negotiation", draft-ietf-mmusic-sdpng-00.txt, IETF; April 810 2001. Work in progress. 812 Camarillo/Holler/Eriksson/Schulzrinne 15 813 Grouping of media lines in SDP 815 [4] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol 816 (RTSP)", RFC 2326, IETF; April 1998. 818 [5] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A 819 Transport Protocol for Real-Time Applications", RFC 1889, IETF; 820 January 1996. 822 [6] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP: 823 Session Initiation Protocol", RFC 2543, IETF; Mach 1999. 825 [7] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access 826 Networks", draft-westberg-realtime-cellular-04.txt, IETF; June 2001. 827 Work in progress. 829 [8] J. Rosenberg/P.Mataga/H.Schulzrinne, "An Application Server 830 Component Architecture for SIP", draft-rosenberg-sip-app-components- 831 00.txt, IETF; November 2000. Work in progress. 833 [9] H. Schulzrinne/S. Petrack, "RTP Payload for DTMF Digits, 834 Telephony Tones and Telephony Signals", RFC 2833, IETF; May 2000. 836 13. Authors� Addresses 838 Gonzalo Camarillo 839 Ericsson 840 Advanced Signalling Research Lab. 841 FIN-02420 Jorvas 842 Finland 843 Phone: +358 9 299 3371 844 Fax: +358 9 299 3052 845 Email: Gonzalo.Camarillo@ericsson.com 847 Jan Holler 848 Ericsson Research 849 S-16480 Stockholm 850 Sweden 851 Phone: +46 8 58532845 852 Fax: +46 8 4047020 853 Email: Jan.Holler@era.ericsson.se 855 Goran AP Eriksson 856 Ericsson Research 857 S-16480 Stockholm 858 Sweden 859 Phone: +46 8 58531762 860 Fax: +46 8 4047020 861 Email: Goran.AP.Eriksson@era.ericsson.se 863 Henning Schulzrinne 864 Dept. of Computer Science 865 Columbia University 866 1214 Amsterdam Avenue 868 Camarillo/Holler/Eriksson/Schulzrinne 16 869 Grouping of media lines in SDP 871 New York, NY 10027 872 USA 873 Email: schulzrinne@cs.columbia.edu 875 Camarillo/Holler/Eriksson/Schulzrinne 17