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Andreasen 7 Cisco Systems 8 February 28, 2011 10 SDP Media Mapabilities Negotiation 11 draft-ietf-mmusic-sdp-media-capabilities-11 13 Abstract 15 Session Description Protocol (SDP) capability negotiation provides a 16 general framework for indicating and negotiating capabilities in SDP. 17 The base framework defines only capabilities for negotiating 18 transport protocols and attributes. In this document, we extend the 19 framework by defining media capabilities that can be used to 20 negotiate media types and their associated parameters. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on September 1, 2011. 39 Copyright Notice 41 Copyright (c) 2011 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 70 3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 6 71 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 72 3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 7 73 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 13 74 3.3.1. The Media Format Capability Attribute . . . . . . . . 13 75 3.3.2. The Media Format Parameter Capability Attribute . . . 15 76 3.3.3. The Media-Specific Capability Attribute . . . . . . . 17 77 3.3.4. New Configuration Parameters . . . . . . . . . . . . . 19 78 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 20 79 3.3.6. Enhanced Potential Configuration Attribute . . . . . . 23 80 3.3.7. Substitution of Media Payload Type Numbers in 81 Capability Attribute Parameters . . . . . . . . . . . 26 82 3.3.8. The Session Capability Attribute . . . . . . . . . . . 27 83 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 31 84 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 31 85 3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 32 86 3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 32 87 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 32 88 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 89 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 33 90 4.2. Alternative Combinations of Codecs (Session 91 Configurations) . . . . . . . . . . . . . . . . . . . . . 36 92 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 36 93 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 94 5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 39 95 5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 40 96 5.3. New SDP Capability Negotiation Parameters . . . . . . . . 40 97 6. Security Considerations . . . . . . . . . . . . . . . . . . . 41 98 7. Changes from previous versions . . . . . . . . . . . . . . . . 42 99 7.1. Changes from version 10 . . . . . . . . . . . . . . . . . 42 100 7.2. Changes from version 09 . . . . . . . . . . . . . . . . . 42 101 7.3. Changes from version 08 . . . . . . . . . . . . . . . . . 42 102 7.4. Changes from version 04 . . . . . . . . . . . . . . . . . 43 103 7.5. Changes from version 03 . . . . . . . . . . . . . . . . . 43 104 7.6. Changes from version 02 . . . . . . . . . . . . . . . . . 44 105 7.7. Changes from version 01 . . . . . . . . . . . . . . . . . 44 106 7.8. Changes from version 00 . . . . . . . . . . . . . . . . . 44 107 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45 108 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46 109 9.1. Normative References . . . . . . . . . . . . . . . . . . . 46 110 9.2. Informative References . . . . . . . . . . . . . . . . . . 46 111 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47 113 1. Introduction 115 Session Description Protocol (SDP) capability negotiation [RFC5939] 116 provides a general framework for indicating and negotiating 117 capabilities in SDP[RFC4566]. The base framework defines only 118 capabilities for negotiating transport protocols and attributes. 120 The [RFC5939] document lists some of the issues with the current SDP 121 capability negotiation process. An additional real life case is to 122 be able to offer one media stream (e.g. audio) but list the 123 capability to support another media stream (e.g. video) without 124 actually offering it concurrently. 126 In this document, we extend the framework by defining media 127 capabilities that can be used to indicate and negotiate media types 128 and their associated format parameters. This document also adds the 129 ability to declare support for media streams, the use of which can be 130 offered and negotiated later, and the ability to specify session 131 configurations as combinations of media stream configurations. The 132 definitions of new attributes for media capability negotiation are 133 chosen to make the translation from these attributes to 134 "conventional" SDP [RFC4566] media attributes as straightforward as 135 possible in order to simplify implementation. This goal is intended 136 to reduce processing in two ways: each proposed configuration in an 137 offer may be easily translated into a conventional SDP media stream 138 record for processing by the receiver; and the construction of an 139 answer based on a selected proposed configuration is straightforward. 141 2. Terminology 143 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 144 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 145 document are to be interpreted as described in RFC2119 [RFC2119] and 146 indicate requirement levels for compliant RTP implementations. 148 "Base Attributes": Conventional SDP attributes appearing in the base 149 configuration of a media block. 151 "Base Configuration": The media configuration represented by a media 152 block exclusive of all the capability negotiation attributes defined 153 in this document, the base capability document[RFC5939], or any 154 future capability negotiation document. In an offer SDP, the base 155 configuration corresponds to the actual configuration as defined in 156 [RFC5939]. 158 "Conventional Attribute": Any SDP attribute other than those defined 159 by the series of capability negotiation specifications. 161 "Conventional SDP": An SDP record devoid of capability negotiation 162 attributes. 164 "Media Capability": A media encoding, typically a media subtype such 165 as PCMU, H263-1998, or T38. 167 3. SDP Media Capabilities 169 The SDP capability negotiation [RFC5939] discusses the use of any SDP 170 [RFC4566] attribute (a=) under the attribute capability "acap". The 171 limitations of using acap for fmtp and rtpmap in a potential 172 configuration are described in [RFC5939]; for example they can be 173 used only at the media level since they are media level attributes. 174 The [RFC5939] does not provide a way to exchange media-level 175 capabilities prior to the actual offer of one or more media streams. 176 This section provides an overview of extensions providing an SDP 177 Media Capability negotiation solution offering more robust 178 capabilities negotiation. This is followed by definitions of new SDP 179 attributes for the solution and its associated updated offer/answer 180 procedures [RFC3264] 182 3.1. Requirements 184 The capability negotiation extensions described herein are described 185 as follows. 187 REQ-01: Support the specification of alternative (combinations of) 188 media formats (codecs) in a single media block. 190 REQ-02: Support the specification of alternative media format 191 parameters for each media format. 193 REQ-03: Retain backward compatibility with conventional SDP. 194 Ensure that each and every offered configuration can be easily 195 translated into a corresponding SDP media block expressed with 196 conventional SDP lines. 198 REQ-04: Ensure the scheme operates within the offer/answer model in 199 such a way that media formats and parameters can be agreed upon 200 with a single exchange. 202 REQ-05: Provide the ability to express offers in such a way that 203 the offerer can receive media as soon as the offer is sent. (Note 204 that the offerer may not be able to render received media prior to 205 exchange of keying material.) 207 REQ-06: Provide the ability to offer latent media configurations 208 for future negotiation. 210 REQ-07: Provide reasonable efficiency in the expression of 211 alternative media formats and/or format parameters, especially in 212 those cases in which many combinations of options are offered. 214 REQ-08: Retain the extensibility of the base capability negotiation 215 mechanism. 217 REQ-09: Provide the ability to specify acceptable combinations of 218 media streams and media formats. For example, offer a PCMU audio 219 stream with an H264 video stream, or a G729 audio stream with an 220 H263 video stream. This ability would give the offerer a means to 221 limit processing requirements for simultaneous streams. This 222 would also permit an offer to include the choice of an audio/T38 223 stream or an image/T38 stream, but not both. 225 Other possible extensions have been discussed, but have not been 226 treated in this document. They may be considered in the future. 227 Three such extensions are: 229 FUT-01: Provide the ability to mix, or change, media types within a 230 single media block. Conventional SDP does not support this 231 capability explicitly; the usual technique is to define a media 232 subtype that represents the actual format within the nominal media 233 type. For example, T.38 FAX as an alternative to audio/PCMU 234 within an audio stream is identified as audio/T38; a separate FAX 235 stream would use image/T38. 237 FUT-02: Provide the ability to support multiple transport protocols 238 within an active media stream without reconfiguration. This is 239 not explicitly supported by conventional SDP. 241 FUT-03: Provide capability negotiation attributes for all media- 242 level SDP line types in the same manner as already done for the 243 attribute type, with the exception of the media line type itself. 244 The media line type is handled in a special way to permit compact 245 expression of media coding/format options. The lines types are 246 bandwidth ("b="), information ("i="), connection data ("c="), and, 247 possibly, the deprecated encryption key ("k="). 249 3.2. Solution Overview 251 The solution consists of new capability attributes corresponding to 252 conventional SDP line types, new parameters for the pcfg, acfg, and 253 the new lcfg attributes extending the base attributes from [RFC5939], 254 and a use of the pcfg attribute to return capability information in 255 the SDP answer. 257 Three new attributes are defined in a manner that can be related to 258 the capabilities specified in a media line, and its corresponding 259 rtpmap and fmtp attributes. 261 o A new media attribute ("a=mcap") defines media capabilities in the 262 form of a media subtype (e.g. "PCMU"), and its encoding 263 parameters (e.g. "/8000/2"). Each resulting media format type/ 264 subtype capability has an associated handle called a media 265 capability number. The encoding parameters are as specified for 266 the rtpmap attribute defined in [RFC4566], without the payload 267 type number part. 269 o A new attribute ("a=mfcap") specifies media format parameters 270 associated with one or more media capabilities. The mfcap 271 attribute is used primarily to associate the formatting 272 capabilities normally carried in the fmtp attribute. 274 o A new attribute ("a=mscap") that specifies media parameters 275 associated with one or more media capabilities. The mscap 276 attribute is used to associate capabilities with attributes other 277 than fmtp or rtpmap, for example, the rtcp-fb attribute defined in 278 [RFC4585]. 280 o A new attribute ("a=lcfg") specifies latent media stream 281 configurations when no corresponding media line ("m=") is offered. 282 An example is the offer of latent configurations for video even 283 though no video is currently offered. If the peer indicates 284 support for one or more of offered latent configurations, the 285 corresponding media stream(s) may be added via a new offer/answer 286 exchange. 288 o A new attribute ("a=sescap") is used to specify an acceptable 289 combination of simultaneous media streams and their configurations 290 as a list of potential and/or latent configurations. 292 New parameters are defined for the potential configuration (pcfg), 293 latent configuration (lcfg), and accepted configuration (acfg) 294 attributes to associate the new attributes with particular 295 configurations. 297 o A new parameter type ("m=") is added to the potential 298 configuration ("a=pcfg:") attribute and the actual configuration 299 ("a=acfg:") attribute defined in [RFC5939], and to the new latent 300 configuration ("a=lcfg:") attribute. This permits specification 301 of media capabilities (including their associated parameters) and 302 combinations thereof for the configuration. For example, the 303 "a=pcfg:" line might specify PCMU and telephone events [RFC4733] 304 or G.729B and telephone events as acceptable configurations. The 305 "a=acfg:" line in the answer would specify the configuration 306 chosen. 308 o A new parameter type ("pt=") is added to the potential 309 configuration, actual configuration, and latent configuration 310 attributes. This parameter associates RTP payload type numbers 311 with the referenced media capabilities, and is appropriate only 312 when the transport protocol uses RTP. 314 o A new parameter type ("mt=") is used to specify the media type for 315 latent configurations. 317 Special processing rules are defined for capability attribute 318 arguments in order to reduce the need to replicate essentially- 319 identical attribute lines for the base configuration and potential 320 configurations. 322 o A substitution rule is defined for any capability attribute to 323 permit the replacement of the (escaped) media capability number 324 with the media format identifier (e.g., the payload type number in 325 audio/video profiles). 327 o Replacement rules are defined for the conventional SDP equivalents 328 of the mfcap and mscap capability attributes. This reduces the 329 necessity to use the deletion qualifier in the pcfg a= parameter 330 in order to ignore rtpmap, fmtp, and certain other attributes in 331 the base configuration. 333 o An argument concatenation rule is defined for mfcap attributes 334 which refer to the same media capability number. This makes it 335 convenient to combine format options concisely by associating 336 multiple mfcap lines with multiple media capabilities. 338 This document extends the base protocol extensions to the offer/ 339 answer model that allow for capabilities and potential configurations 340 to be included in an offer. Media capabilities constitute 341 capabilities that can be used in potential and latent configurations. 342 Whereas potential configurations constitute alternative offers that 343 may be accepted by the answerer instead of the actual 344 configuration(s) included in the "m=" line(s) and associated 345 parameters, latent configurations merely inform the other side of 346 possible configurations supported by the entity. Those latent 347 configurations may be used to guide subsequent offer/answer 348 exchanges, but they are not part of the current offer/answer 349 exchange. 351 The mechanism is illustrated by the offer/answer exchange below, 352 where Alice sends an offer to Bob: 354 Alice Bob 355 | (1) Offer (SRTP and RTP) | 356 |--------------------------------->| 357 | | 358 | (2) Answer (RTP) | 359 |<---------------------------------| 360 | | 362 Alice's offer includes RTP and SRTP as alternatives. RTP is the 363 default, but SRTP is the preferred one (long lines are folded to fit 364 the margins): 366 v=0 367 o=- 25678 753849 IN IP4 192.0.2.1 368 s= 369 c=IN IP4 192.0.2.1 370 t=0 0 371 a=creq:med-v0 372 m=audio 3456 RTP/AVP 0 18 373 a=tcap:1 RTP/SAVP RTP/AVP 374 a=rtpmap:0 PCMU/8000/1 375 a=rtpmap:18 G729/8000/1 376 a=fmtp:18 annexb=yes 377 a=mcap:1,4 g729/8000/1 378 a=mcap:2 PCMU/8000/1 379 a=mcap:5 telephone-event/8000 380 a=mfcap:1 annexb=no 381 a=mfcap:4 annexb=yes 382 a=mfcap:5 0-11 383 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \ 384 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 385 a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102 386 a=pcfg:2 m=2 t=1 a=1 pt=2:103 387 a=pcfg:3 m=4 t=2 pt=4:18 389 The required base and extensions are provided by the "a=creq" 390 attribute defined in [RFC5939], with the option tag "med-v0", which 391 indicates that the extension framework defined here, must be 392 supported. The Base level support is implied since it is required 393 for the extensions. 395 The "m=" line indicates that Alice is offering to use plain RTP with 396 PCMU or G.729B. The media line implicitly defines the default 397 transport protocol (RTP/AVP in this case) and the default actual 398 configuration. 400 The "a=tcap:1" line, specified in the base protocol, defines 401 transport protocol capabilities, in this case Secure RTP (SAVP 402 profile) as the first option and RTP (AVP profile) as the second 403 option. 405 The "a=mcap:1,4" line defines two G.729 media format capabilities, 406 numbered 1 and 4, and their encoding rate. The capabilities are of 407 media type "audio" and subtype G729. Note that the media subtype is 408 explicitly specified here, rather than RTP payload type numbers. 409 This permits the assignment of payload type numbers in the media 410 stream configuration specification. In this example, two G.729 411 subtype capabilities are defined. This permits the declaration of 412 two sets of formatting parameters for G.729. 414 The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2. 416 The "a=mcap:5" line defines an audio telephone-event capability, 417 numbered 5. 419 The "a=mfcap:1" line specifies the fmtp formatting parameters for 420 capability 1 (offerer will not accept G.729 Annex B packets). 422 The "a=mfcap:4" line specifies the fmtp formatting parameters for 423 capability 4 (offerer will accept G.729 Annex B packets). 425 The "a=mfcap:5" line specifies the fmtp formatting parameters for 426 capability 5 (the DTMF touchtones 0-9,*,#). 428 The "a=acap:1" line specified in the base protocol provides the 429 "crypto" attribute which provides the keying material for SRTP using 430 SDP security descriptions. 432 The "a=pcfg:" attributes provide the potential configurations 433 included in the offer by reference to the media capabilities, 434 transport capabilities, attribute capabilities and specified payload 435 type number mappings. Three explicit alternatives are provided; the 436 lowest-numbered one is the preferred one. The "a=pcfg:1 ..." line 437 specifies media capabilities 4 and 5, i.e., G.729B and DTMF, or media 438 capability 1 and 5, i.e., G.729 and DTMF. Furthermore, it specifies 439 transport protocol capability 1 (i.e. the RTP/SAVP profile - secure 440 RTP), and the attribute capability 1, i.e. the crypto attribute 441 provided. Lastly, it specifies a payload type number mapping for 442 media capabilities 1, 4, and 5, thereby permitting the offerer to 443 distinguish between encrypted media and unencrypted media received 444 prior to receipt of the answer. 446 Use of unique payload type numbers is not required; codecs such as 447 AMR-WB [RFC4867] have the potential for so many combinations of 448 options that it may be impractical to define unique payload type 449 numbers for all supported combinations. If unique payload type 450 numbers cannot be specified, then the offerer will be obliged to wait 451 for the SDP answer before rendering received media. For SRTP using 452 SDES inline keying [RFC4568], the offerer will still need to receive 453 the answer before being able to decrypt the stream. 455 The second alternative ("a=pcfg:2 ...") specifies media capability 2, 456 i.e. PCMU, under the RTP/SAVP profile, with the same SRTP key 457 material. 459 The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; it's 460 only purpose in this example is to show a preference for G.729B over 461 PCMU. 463 The media line, with any qualifying attributes such as fmtp or 464 rtpmap, is itself considered a valid configuration; it is assumed to 465 be the lowest preference. 467 Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU, 468 and telephone events over RTP, but not SRTP, hence he accepts the 469 potential configuration 3 for RTP provided by Alice. Bob generates 470 the following answer: 472 v=0 473 o=- 24351 621814 IN IP4 192.0.2.2 474 s= 475 c=IN IP4 192.0.2.2 476 t=0 0 477 a=csup:med-v0 478 m=audio 4567 RTP/AVP 18 479 a=rtpmap:18 G729/8000 480 a=fmtp:18 annexb=yes 481 a=acfg:3 m=4 t=2 pt=4:18 483 Bob includes the "a=csup" and "a=acfg" attributes in the answer to 484 inform Alice that he can support the med-v0 level of capability 485 negotiations. Note that in this particular example, the answerer 486 supported the capability extensions defined here, however had he not, 487 he would simply have processed the offer based on the offered PCMU 488 and G.729 codecs under the RTP/AVP profile only. Consequently, the 489 answer would have omitted the "a=csup" attribute line and chosen one 490 or both of the PCMU and G.729 codecs instead. The answer carries the 491 accepted configuration in the m line along with corresponding rtpmap 492 and/or fmtp parameters, as appropriate. 494 Note that per the base protocol, after the above, Alice MAY generate 495 a new offer with an actual configuration ("m=" line, etc.) 496 corresponding to the actual configuration referenced in Bob's answer 497 (not shown here). 499 3.3. New Capability Attributes 501 In this section, we present the new attributes associated with 502 indicating the media capabilities for use by the SDP Capability 503 negotiation. The approach taken is to keep things similar to the 504 existing media capabilities defined by the existing media 505 descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" 506 attributes. We use media subtypes and "media capability numbers" 507 instead of payload type numbers to link the relevant media capability 508 parameters. This permits the capabilities to be defined at the 509 session level and be used for multiple streams, if desired. Payload 510 types are then specified at the media level (see Section 3.3.4.2). 512 A media capability merely indicates possible support for the media 513 type and media format(s) in question. In order to actually use a 514 media capability in an offer/answer exchange, it must be referenced 515 in a potential configuration. 517 Media capabilities can be provided at the session-level and/or the 518 media-level. Media capabilities provided at the session level may be 519 referenced in any pcfg or lcfg attribute at the media level 520 (consistent with the media type), whereas media capabilities provided 521 at the media level may be referenced only by the pcfg or lcfg 522 attribute within that media stream only. In either case, the scope 523 of the is the entire session description. This enables 524 each media capability to be uniquely referenced across the entire 525 session description (e.g. in a potential configuration). 527 3.3.1. The Media Format Capability Attribute 529 Media subtypes can be expressed as media format capabilities by use 530 of the "a=mcap" attribute, which is formatted as follows: 532 a=mcap: / 533 [/] 535 where is a (list of) media capability number(s) 536 used to number a media format capability, the is the 537 media subtype e.g. H263-1998 or PCMU, is the encoding 538 rate, and are the media encoding parameters for the 539 media subtype;. All media format capabilities in the list are 540 assigned to the same media type/subtype. Each occurrence of the mcap 541 attribute MUST use unique values in its ; the 542 media capability numbers must be unique across the entire SDP 543 session. In short, the mcap attribute defines media capabilities and 544 associates them with a media capability number in the same manner as 545 the rtpmap attribute defines them and associates them with a payload 546 type number. Additionally, the mcap attribute allows multiple 547 capability numbers to be defined for the media format in question. 548 This permits the media format to be associated with different media 549 parameters in different configurations. 551 In ABNF, we have: 553 media-capability-line = "a=mcap:" media-cap-num-list 554 1*WSP encoding-name "/" clock-rate 555 ["/" encoding-parms] 556 media-cap-num-list = media-cap-num-element 557 *["," media-cap-num-element] 558 media-cap-num-element = media-cap-num 559 / media-cap-num-range 560 media-cap-num-range = media-cap-num "-" media-cap-num 561 media-cap-num = 1*10(DIGIT) 562 encoding-name = token ; defined in RFC4566 563 clock-rate = 1*10(DIGIT) 564 encoding-parms = token 566 The encoding-name, clock-rate and encoding-params are as defined to 567 appear in an rtpmap attribute for each media type/subtype. Thus, it 568 is easy to convert an mcap attribute line into one or more rtpmap 569 attribute lines, once a payload type number is assigned to a media- 570 cap-num (see Section 3.3.5). 572 The "mcap" attribute can be provided at the session-level and/or the 573 media-level. There can be more than one mcap attribute at the 574 session or media level. Each media-cap-num must be unique within the 575 entire SDP record; it is used to identify that media capability in 576 potential, latent and actual configurations, and in other attribute 577 lines as explained below. When used in a potential, latent or actual 578 configuration it is, in effect, a media level attribute regardless if 579 it is specified at the session or media level. In other words, the 580 media capability applies to the specific media description associated 581 with the configuration which invokes it. 583 For example: 585 v=0 586 a=mcap:1 L16/8000/1 587 a=mcap:2 L16/16000/2 588 a=mcap:3,4 H263-1998/90000 589 m=audio 54320 RTP/AVP 0 590 a=pcfg:1 m=1|2, pt=1:99,2:98 591 m=video 66544 RTP/AVP 100 592 a=rtpmap:100 H264/90000 593 a=pcfg:10 m=3 pt=3:101 595 3.3.2. The Media Format Parameter Capability Attribute 597 This attribute is used to associate media format specific format 598 parameters with one or more media capabilities. The form of the 599 attribute is: 601 a=mfcap: 603 where permits the list of parameters to be associated 604 with one or more media capabilities and the format parameters are 605 specific to the type of media format. The mfcap lines map to a 606 single traditional SDP fmtp attribute line (one for each entry in 607 ) of the form 609 a=fmtp: 611 where is the media format description defined in RFC 4566 612 [RFC4566], as appropriate for the particular media stream. The mfcap 613 attribute MUST be used to encode attributes for media capabilities, 614 which would conventionally appear in an fmtp attribute. The existing 615 acap attribute MUST NOT be used to encode fmtp attributes. 617 The mfcap attribute adheres to [RFC4566] attribute production rules 618 with 620 media-format-capability = "a=mfcap:" media-cap-num-list 1*WSP 621 fmt-specific-param-list 622 fmt-specific-param-list = text ; defined in RFC4566 624 3.3.2.1. Media Format Parameter Concatenation Rule 626 The appearance of media subtypes with a large number of formatting 627 options (e.g., AMR-WB [RFC4867]) coupled with the restriction that 628 only a single fmtp attribute can appear per media format, suggests 629 that it is useful to create a combining rule for mfcap parameters 630 which are associated with the same media capability number. 631 Therefore, different mfcap lines MAY include the same media-cap-num 632 in their media-cap-num-list. When a particular media capability is 633 selected for processing, the parameters from each mfcap line which 634 references the particular capability number in its media-cap-num-list 635 are concatenated together via ";", in the order the mfcap attributes 636 appear in the SDP record, to form the equivalent of a single fmtp 637 attribute line. This permits one to define a separate mfcap line for 638 a single parameter and value that is to be applied to each media 639 capability designated in the media-cap-num-list. This provides a 640 compact method to specify multiple combinations of format parameters 641 when using codecs with multiple format options. Note that order- 642 dependent parameters MAY be placed in a single mfcap line to avoid 643 possible problems with line rearrangement by a middlebox. 645 Format parameters are not parsed by SDP; their content is specific to 646 the media type/subtype. When format parameters for a specific media 647 capability are combined from multiple a=mfcap lines which reference 648 that media capability, the format-specific parameters are 649 concatenated together and separated by "; " for construction of the 650 corresponding format attribute (a=fmtp). The resulting format 651 attribute will look something like the following: 653 a=fmtp: ; 654 ; 655 ... 657 where depends on the transport protocol in the manner defined 658 in RFC4566. SDP cannot assess the legality of the resulting 659 parameter list in the "a=fmtp" line; the user must take care to 660 ensure that legal parameter lists are generated. 662 The "mfcap" attribute can be provided at the session-level and the 663 media-level. There can be more than one mfcap attribute at the 664 session or media level. The unique media-cap-num is used to 665 associate the parameters with a media capability. 667 As a simple example, a G.729 capability is, by default, considered to 668 support comfort noise as defined by Annex B. Capabilities for G.729 669 with and without comfort noise support may thus be defined by: 671 a=mcap:1,2 audio G729/8000 672 a=mfcap:2 annexb:no 674 Media format capability 1 supports G.729 with Annex B, whereas media 675 format capability 2 supports G.729 without Annex B. 677 Example for H.263 video: 679 a=mcap:1 video H263-1998/90000 680 a=mcap:2 video H263-2000/90000 681 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 682 a=mfcap:2 profile=2;level=2.2 684 Finally, for six format combinations of the Adaptive MultiRate codec: 686 a=mcap:1-3 AMR/8000/1 687 a=mcap:4-6 AMR-WB/16000/1 688 a=mfcap:1,2,3,4 mode-change-capability=1 689 a=mfcap:5,6 mode-change-capability=2 690 a=mfcap:1,2,3,5 max-red=220 691 a=mfcap:3,4,5,6 octet-align=1 692 a=mfcap:1,3,5 mode-set=0,2,4,7 693 a=mfcap:2,4,6 mode-set=0,3,5,6 695 So that AMR codec #1, when specified in a pcfg attribute within an 696 audio stream block (and assigned payload type number 98) as in 698 a=pcfg:1 m=1 pt=1:98 700 is essentially equivalent to the following 702 m=audio 49170 RTP/AVP 98 703 a=rtpmap:98 AMR/8000/1 704 a=fmtp:98 mode-change-capability=1; \ 705 max-red=220; mode-set=0,2,4,7 707 and AMR codec #4 with payload type number 99,depicted by the 708 potential configuration: 710 a=pcfg:4 m=4, pt=4:99 712 is equivalent to the following: 714 m=audio 49170 RTP/AVP 99 715 a=rtpmap:99 AMR-WB/16000/1 716 a=fmtp:99 mode-change-capability=1; octet-align=1; \ 717 mode-set=0,3,5,6 719 and so on for the other four combinations. SDP could thus convert 720 the media capabilities specifications into one or more alternative 721 media stream specifications, one of which can be chosen for the 722 answer. 724 3.3.3. The Media-Specific Capability Attribute 726 Media-specific attributes, beyond the rtpmap and fmtp attributes, may 727 be associated with media capability numbers via a new media-specific 728 attribute, mscap, of the following form: 730 a=mscap: 732 Where is a (list of) media capability number(s), is the attribute name, and is the value field for 734 the named attribute. In ABNF, we have: 736 media-specific-capability = "a=mscap:" 737 media-caps ; defined in 3.3.2 738 1*WSP att-field ; from RFC4566 739 1*WSP att-value ; from RFC4566 741 Given an association between a media capability and a payload type 742 number as specified by the pt= parameters in an lcfg or pcfg 743 attribute line, a mscap line may be translated easily into a 744 conventional SDP attribute line of the form 746 a=":" ; defined in [RFC4566] 748 A resulting attribute that is not a legal SDP attribute as specified 749 by RFC4566 MUST be ignored by the receiver. 751 A single mscap line may refer to multiple media capabilities; this is 752 equivalent to multiple mscap lines, each with the same attribute 753 values, one line per media capability. 755 Multiple mscap lines may refer to the same media capability, but, 756 unlike the mfcap attribute, no concatenation operation is defined. 757 Hence, multiple mscap lines applied to the same media capability is 758 equivalent to multiple lines of the specified attribute in a 759 conventional media record. 761 Here is an example with the rtcp-fb attribute, modified from an 762 example in [RFC5104] (with the session-level and audio media 763 omitted). If the offer contains a media block like the following, 765 m=video 51372 RTP/AVP 98 766 a=rtpmap:98 H263-1998/90000 767 a=tcap:1 RTP/AVPF 768 a=mcap:1 H263-1998/90000 769 a=mscap:1 rtcp-fb ccm tstr 770 a=mscap:1 rtcp-fb ccm fir 771 a=mscap:* rtcp-fb ccm tmmbr smaxpr=120 772 a=pcfg:1 t=1 m=1 pt=1:98 774 and if the proposed configuration is chosen, then the equivalent 775 media block would look like 777 m=video 51372 RTP/AVPF 98 778 a=rtpmap:98 H263-1998/90000 779 a=rtcp-fb:98 ccm tstr 780 a=rtcp-fb:98 ccm fir 781 a=rtcp-fb:* ccm tmmbr smaxpr=120 783 3.3.4. New Configuration Parameters 785 Along with the new attributes for media capabilities, new extension 786 parameters are defined for use in the potential configuration, the 787 actual configuration, and/or the new latent configuration defined in 788 Section 3.3.5. 790 3.3.4.1. The Media Configuration Parameter (m=) 792 The media configuration parameter is used to specify the media 793 encoding(s) and related parameters for a potential, actual, or latent 794 configuration. Adhering to the ABNF for extension-config-list in 795 [RFC5939] with 797 ext-cap-name = "m" 798 ext-cap-list = media-cap-num-list 799 [*(BAR media-cap-num-list)] 801 we have 803 media-config-list = ["+"]"m=" media-cap-num-list 804 [*(BAR media-cap-num-list)] 805 ; BAR is defined in RFC5939 806 ; media-cap-num-list is defined above 808 Alternative media configurations are separated by a vertical bar 809 ("|"). The alternatives are ordered by preference, most-preferred 810 first. When media capabilities are not included in a potential 811 configuration at the media level, the media type and media format 812 from the associated "m=" line will be used. The use of the plus sign 813 ("+") is described in RFC5939. 815 3.3.4.2. The Payload Type Number Mapping Parameter (pt=) 817 The payload type number mapping parameter is used to specify the 818 payload type number to be associated with each media type in a 819 potential, actual, or latent configuration. We define the payload 820 type number mapping parameter, payload-number-config-list, in 821 accordance with the extension-config-list format defined in 822 [RFC5939]. In ABNF: 824 payload-number-config-list = ["+"]"pt=" media-map-list 825 media-map-list = media-map *["," media-map] 826 media-map = media-cap-num ":" payload-type-number 827 ; media-cap-num is defined in 3.3.1 828 payload-type-number = 1*3(DIGIT) ; RTP payload type number 829 / "*" ; dummy 831 The example in Section 3.3.7 shows how the parameters from the mcap 832 line are mapped to payload type numbers from the pcfg "pt" parameter. 833 The use of the plus sign ("+") is desribed in RFC5939. 835 The "*" value for payload-type-number is used in cases such as BFCP 836 [RFC4583] in which the fmt list in the m-line is ignored. 838 A latent configuration represents a future capability, hence the pt= 839 parameter is not directly meaningful in the lcfg attribute because no 840 actual media session is being offered or accepted; it is permitted in 841 order to tie any payload type number parameters within attributes to 842 the proper media format. A primary example is the case of format 843 parameters for the RED payload, which are payload type numbers. 844 Specific payload type numbers used in a latent configuration may be 845 interpreted as suggestions to be used in any future offer based on 846 the latent configuration, but they are not binding; the offerer 847 and/or answerer may use any payload type numbers each deems 848 appropriate. The use of explicit payload type numbers for latent 849 configurations can be avoided by use of the parameter substitution 850 rule of Section 3.3.7 . Future extensions are also permitted. 852 3.3.4.3. The Media Type Parameter 854 When a latent configuration is specified (always at the session 855 level), indicating the ability to support an additional media stream, 856 it is necessary to specify the media type (audio, video, etc.) as 857 well as the format and transport type. The media type parameter is 858 defined in ABNF as 860 media-type = ["+"] "mt=" media; media defined in RFC4566 862 At present, the media-type parameter is used only in the latent 863 configuration attribute, and the use of the "+" prefix to specify 864 that the entire attribute line is to be ignored if the mt= parameter 865 is not understood, is unnecessary. However, if the media-type 866 parameter is later added to an existing capability attribute such as 867 pcfg, then the "+" would be useful. The media format(s) and 868 transport type(s) are specified using the media configuration 869 parameter ("+m=") defined above, and the transport parameter ("t=") 870 defined in [RFC5939], respectively. 872 3.3.5. The Latent Configuration Attribute 874 One of the goals of this work is to permit the exchange of 875 supportable media configurations in addition to those offered or 876 accepted for immediate use. Such configurations are referred to as 877 "latent configurations". For example, a party may offer to establish 878 a session with an audio stream, and, at the same time, announce its 879 ability to support a video stream as part of the same session. The 880 offerer can supply its video capabilities by offering one or more 881 latent video configurations along with the media stream for audio; 882 the responding party may indicate its ability and willingness to 883 support such a video session by returning a corresponding latent 884 configuration. 886 Latent configurations returned in SDP answers must match offered 887 latent configurations (or parameter subsets thereof). Therefore, it 888 is appropriate for the offering party to announce most, if not all, 889 of its capabilities in the initial offer. This choice has been made 890 in order to keep the size of the answer more compact by not requiring 891 acap, mcap, tcap, etc. lines in the answer. 893 Latent configurations may be announced by use of the latent 894 configuration attribute, which is defined in a manner very similar to 895 the potential configuration attribute. The latent configuration 896 attribute combines the properties of a media line and a potential 897 configuration. The media type (mt=) and the transport protocol(s) 898 (t=) MUST be specified since the latent configuration is independent 899 of any media line present. In most cases, the media configuration 900 (m=) parameter MUST be present as well (see Section 4 for examples). 901 The lcfg attribute is a media level attribute and, like a media line, 902 it ends the session level of the session description if it appears 903 before any media line. 905 Each media line in an SDP description represents an offered 906 simultaneous media stream, whereas each latent configuration 907 represents an additional stream which may be negotiated in a future 908 offer/answer exchange. Session capability attributes may be used to 909 determine whether a latent configuration may be used to form an offer 910 for an additional simultaneous stream or to reconfigure an existing 911 stream in a subsequent offer/answer exchange. 913 The latent configuration attribute is of the form: 915 a=lcfg: 917 which adheres to the [RFC4566] "attribute" production with att-field 918 and att-value defined as: 920 att-field = "lcfg" 921 att-value = config-number 1*WSP lcfg-cfg-list 922 config-number = 1*10(DIGIT) ; defined in RFC5234 923 lcfg-cfg-list = media-type 1*WSP pot-cfg-list 924 ; as defined in RFC5939 925 ; and extended herein 927 The media-type (mt=) parameter identifies the media type (audio, 928 video, etc.) to be associated with the latent media stream, and MUST 929 be present. The pot-cfg-list MUST contain a transport-protocol- 930 config-list (t=) parameter and a media-config-list (m=) parameter. 931 The pot-cfg-list MUST NOT contain more than one instance of each type 932 of parameter list. As specified in [RFC5939], the use of the "+" 933 prefix with a parameter indicates that the entire configuration MUST 934 be ignored if the parameter is not understood; otherwise, the 935 parameter itself may be ignored. 937 Media stream payload numbers are not assigned by a latent 938 configuration. Assignment will take place if and when the 939 corresponding stream is actually offered via an m-line in a later 940 exchange. The payload-number-config-list is included as a parameter 941 to the lcfg attribute in case it is necessary to tie payload numbers 942 in attribute capabilities to specific media capabilities. 944 If an lcfg attribute invokes an acap attribute that appears at the 945 session level, then that attribute will be expected to appear at the 946 session level of a subsequent offer when and if a corresponding media 947 stream is offered. Otherwise, acap attributes which appear at the 948 media level represent media-level attributes. Note, however, that 949 mcap, mfcap, mscap, tcap attributes may appear at the session level 950 because they always result in media-level attributes or m-line 951 parameters. 953 The configuration numbers for latent configurations do not imply a 954 preference; the offerer will imply a preference when actually 955 offering potential configurations derived from latent configurations 956 negotiated earlier. Note however that the offerer of latent 957 configurations MAY specify preferences for combinations of potential 958 and latent configurations by use of the sescap attribute defined in 959 Section 3.3.8. For example, if an SDP offer contains, say, an audio 960 stream with pcfg:1, and two latent video configurations, lcfg:2, and 961 lcfg:3, then a session with one audio stream and one video stream 962 could be specified by including "a=sescap:1 1,2|3". One audio stream 963 and two video streams could be specified by including "a=sescap:2 964 1,2,3" in the offer. In order to permit combinations of latent and 965 potential configurations in session capabilities, latent 966 configuration numbers MUST be different from those used for potential 967 configurations. This restriction is especially important if the 968 offerer does not require cmed-v0 capability and the recipient of the 969 offer doesn't aupport it. If the lcfg attribute is not recognized, 970 the capability attributes intended to be associated with it may be 971 confused with those associated with a potential configuration of some 972 other media stream. 974 If a cryptographic attribute, such as the SDES "a=crypto:" attribute 976 [RFC4568], is referenced by a latent configuration through an acap 977 attribute, any key material REQUIRED in the conventional attribute, 978 such as the SDES key/salt string, MUST be included in order to 979 satisfy formatting rules for the attribute. The actual value(s) of 980 the key material SHOULD be meaningless, and the receiver of the lcfg 981 attribute MUST ignore the values. 983 3.3.6. Enhanced Potential Configuration Attribute 985 The present work requires new extensions (parameters) for the pcfg: 986 attribute defined in the base protocol [RFC5939] The parameters and 987 their definitions are "borrowed" from the definitions provided for 988 the latent configuration attribute in Section 3.3.5. The expanded 989 ABNF definition of the pcfg attribute is 991 a=pcfg: [] 993 where 995 config-number = 1*DIGIT ;defined in [RFC5234] 996 pot-cfg-list = pot-config *(1*WSP pot-config) 997 pot-config = / attribute-config-list / def in [RFC5939] 998 transport-protocol-config-list / ;defined in [RFC5939] 999 extension-config-list / ;[RFC5939] 1000 media-config-list / ; Section 3.3.4.1 1001 payload-number-config-list ; Section 3.3.4.2 1003 Except for the extension-config-list, the pot-cfg-list MUST NOT 1004 contain more than one instance of each parameter list. 1006 3.3.6.1. Returning Capabilities in the Answer 1008 Potential and/or latent configuration attributes may be returned 1009 within an answer SDP to indicate the ability of the answerer to to 1010 support alternative configurations of the corresponding stream(s). 1011 For example, an offer may include multiple potential configurations 1012 for a media stream and/or latent configurations for additional 1013 streams; the corresponding answer will indicate (via an acfg 1014 attribute) the configuration accepted and used to construct the base 1015 configuration for each active media stream in the reply, but the 1016 reply MAY also contain potential and/or latent configuration 1017 attributes, with parameters, to indicate which other offered 1018 configurations would be acceptable. This information is useful if it 1019 becomes desirable to reconfigure a media stream, e.g., to reduce 1020 resource consumption. 1022 When potential and/or latent configurations are returned in an 1023 answer, all numbering MUST refer to the configuration and capability 1024 attribute numbering of the offer. The offered capability attributes 1025 need not be returned in the answer. The answer MAY include 1026 additional capability attributes and/or configuratons (with distinct 1027 numbering). The parameter values of any returned pcfg or lcfg 1028 attributes MUST be a subset of those included in the offered 1029 configurations or those added by the answerer; values may be omitted 1030 only if they were indicated as alternative sets, or optional, in the 1031 original offer. The parameter set indicated in the returned acfg 1032 attribute need not be repeated in a returned pcfg attribute. The 1033 answerer may return more than one pcfg attribute with the same 1034 configuration number if it is necessary to describe selected 1035 combinations of optional or alternative parameters. 1037 Similarly, one or more session capability attributes (a=sescap) may 1038 be returned to indicate which of the offered session capabilities is/ 1039 are supportable by the answerer (see Section 3.3.8.) 1041 Note that, although the answerer MAY return capabilities beyond those 1042 included by the offerer, these capabilities MUST NOT be used to form 1043 any base level media description in the answer. For this reason, it 1044 seems advisable for the offerer to include most, if not all, 1045 potential and latent configurations it can support in the initial 1046 offer. Either party MAY later announce additional capabilities by 1047 renegotiating the session in a second offer/answer exchange. 1049 3.3.6.2. Payload Type Number Mapping 1051 When media capabilities defined in mcap attributes are used in 1052 potential configuration lines, and the transport protocol uses RTP, 1053 it is necessary to assign payload type numbers to them. In some 1054 cases, it is desirable to assign different payload type numbers to 1055 the same media capability when used in different potential 1056 configurations. One example is when configurations for AVP and SAVP 1057 are offered: the offerer would like the answerer to use different 1058 payload type numbers for encrypted and unencrypted media so that it 1059 (the offerer) can decide whether or not to render early media which 1060 arrives before the answer is received. This association of distinct 1061 payload type number(s) with different transport protocols requires a 1062 separate pcfg line for each protocol. Clearly, this technique cannot 1063 be used if the number of potential configurations exceeds the number 1064 of possible payload type numbers. 1066 3.3.6.3. Processing of Media-Format-Related Conventional Attributes for 1067 Potential Configurations 1069 In cases in which media capabilities negotiation is employed, SDP 1070 records are likely to contain conventional attributes such as rtpmap, 1071 fmtp, and other media-format-related lines, as well as capability 1072 attributes such as mcap, mfcap, and mscap which map into those 1073 conventional attributes when invoked by a potential configuration. 1074 In such cases, it MAY be appropriate to employ the a delete- 1075 attributes option in the attribute configuration list parameter in 1076 order to avoid the generation of conflicting fmtp attributes for a 1077 particular configuration. Any media-specific attributes in the media 1078 block which refer to payload type numbers not used by the potential 1079 configuration MUST be ignored. 1081 For example: 1083 v=0 1084 o=- 25678 753849 IN IP4 192.0.2.1 1085 s= 1086 c=IN IP4 192.0.2.1 1087 t=0 0 1088 a=creq:med-v0 1089 m=audio 3456 RTP/AVP 0 18 100 1090 a=rtpmap:100 telephone-events 1091 a=fmtp:100 0-11 1092 a=mcap:1 PCMU/8000 1093 a=mcap:2 g729/8000 1094 a=mcap:3 telephone-events/8000 1095 a=mfcap:3 0-15 1096 a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100 1097 a=pcfg:2 1099 In this example, PCMU is media capability 1, G729 is media capability 1100 2, and telephone-event is media capability 3. The a=pcfg:1 line 1101 specifies that the preferred configuration is G.729 with extended 1102 dtmf events, second is G.711 mu-law with extended dtmf events, and 1103 the base media-level attributes are to be deleted. Intermixing of 1104 G.729, G.711, and "commercial" dtmf events is least preferred (the 1105 base configuration provided by the "m=" line, which is, by default, 1106 the least preferred configuration). The rtpmap and fmtp attributes 1107 of the base configuration are replaced by the mcap and mfcap 1108 attributes when invoked by the proposed configuration. 1110 If the preferred configuration is selected, the SDP answer will look 1111 like 1113 v=0 1114 o=- 25678 753849 IN IP4 192.0.2.1 1115 s= 1116 c=IN IP4 192.0.2.1 1117 t=0 0 1118 a=csup:med-v0 1119 m=audio 6543 RTP/AVP 18 100 1120 a=rtpmap:100 telephone-events/8000 1121 a=fmtp:100 0-15 1122 a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100 1123 a=pcfg:2 1124 a=acfg:1 m=2,3 pt=1:0,2:18,3:100 1126 3.3.7. Substitution of Media Payload Type Numbers in Capability 1127 Attribute Parameters 1129 In some cases, for example, when an RFC 2198 redundancy audio subtype 1130 (RED) capability is defined in an mfcap attribute, the parameters to 1131 an attribute may contain payload type numbers. Two options are 1132 available for specifying such payload type numbers. They may be 1133 expressed explicitly, in which case they are bound to actual payload 1134 types by means of the payload type number parameter (pt=) in the 1135 appropriate potential or latent configuration. For example, the 1136 following SDP fragment defines a potential configuration with 1137 redundant G.711 mu-law: 1139 m=audio 45678 RTP/AVP 0 1140 a=rtpmap:0 PCMU/8000 1141 a=mcap:1 PCMU/8000 1142 a=mcap:2 RED/8000 1143 a=mfcap:2 0/0 1144 a=pcfg:1 m=2,1 pt=2:98,1:0 1146 The potential configuration is then equivalent to 1148 m=audio 45678 RTP/AVP 98 0 1149 a=rtpmap:0 PCMU/8000 1150 a=rtpmap:98 RED/8000 1151 a=fmtp:98 0/0 1153 A more general mechanism is provided via the parameter substitution 1154 rule. When an mfcap, mscap, or acap attribute is processed, its 1155 arguments will be scanned for a payload type number escape sequences 1156 of the following form (in ABNF): 1158 ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1 1160 If the sequence is found, the sequence is replaced by the payload 1161 type number assigned to the media capability number, as specified by 1162 the pt= parameter in the selected potential configuration. The 1163 sequence "%%" (null digit string) is replaced by a single percent 1164 sign and processing continues with the next character, if any. 1166 For example, the above offer sequence could have been written as 1167 m=audio 45678 RTP/AVP 0 1168 a=rtpmap:0 PCMU/8000 1169 a=mcap:1 PCMU/8000 1170 a=mcap:2 RED/8000 1171 a=mfcap:2 %m=1%/%m=1% 1172 a=pcfg:1 m=2,1 pt=2:98,1:0 1174 and the equivalent SDP is the same as above. 1176 3.3.8. The Session Capability Attribute 1178 The session capability attribute provides a means for the offerer 1179 and/or the answerer to specify combinations of specific media stream 1180 configurations which it is willing and able to support. Each session 1181 capability in an offer or answer MAY be expressed as a list of 1182 required potential configurations, and MAY include a list of optional 1183 potential and/or latent configurations. 1185 The choices of session capabilities may be based on processing load, 1186 total bandwidth, or any other criteria of importance to the 1187 communicating parties. If the answerer supports media capabilities 1188 negotiation, and session configurations are offered, it MUST accept 1189 one of the offered configurations, or it MUST refuse the session. 1190 Therefore, if the offer includes any session capabilities, it SHOULD 1191 include all the session capabilities the offerer is willing to 1192 support. 1194 The session capability attribute is described by: 1196 "a=sescap:" 1198 which corresponds to the standard attribute definition with 1200 att-field = "sescap" 1201 att-value = session-num 1*WSP list-of-configs 1202 [1*WSP optional-configs] 1203 session-num = 1*DIGIT ; defined in RFC5234 1204 list-of-configs = alt-config *["," alt-config] 1205 optional-configs = "[" list-of-configs "]" 1206 alt-config = config-number *["|" config-number] 1207 ; config-number defined in RFC5939 1209 The session-num identifies the session; a lower-number session is 1210 preferred over a higher-numbered session. Each alt-config list 1211 specifies alternative media configurations within the session; 1212 preference is based on config-num as specified in [RFC5939]. Note 1213 that the session preference order, when present, takes precedence 1214 over the individual media stream configuration preference order. 1216 Use of session capability attributes requires that configuration 1217 numbers assigned to potential and latent configurations MUST be 1218 unique across the entire session; [RFC5939] requires only that pcfg 1219 configuration numbers be unique within a media description. 1221 As an example, consider an endpoint that is capable of supporting an 1222 audio stream with either one H.264 video stream or two H.263 video 1223 streams with a floor control stream. The SDP offer might look like 1224 the following: 1226 v=0 1227 o=- 25678 753849 IN IP4 192.0.2.1 1228 s= 1229 c=IN IP4 192.0.2.1 1230 t=0 0 1231 a=creq:med-v0 1232 a=sescap:2 1,2,3,5 1233 a=sescap:1 1,4 1234 m=audio 54322 RTP/AVP 0 1235 a=rtpmap:0 PCMU/8000 1236 a=pcfg:1 1237 m=video 22344 RTP/AVP 102 1238 a=rtpmap:102 H263-1998/90000 1239 a=fmtp:102 CIF=4;QCIF=2;F=1;K=1 1240 i= main video stream 1241 a=label:11 1242 a=pcfg:2 1243 a=mcap:1 H264/90000 1244 a=mfcap:1 profile-level-id=42A01E; packetization-mode=2 1245 a=acap:1 label:13 1246 a=pcfg:4 m=1 a=1 pt=1:104 1247 m=video 33444 RTP/AVP 103 1248 a=rtpmap:103 H263-1998/90000 1249 a=fmtp:103 CIF=4;QCIF=2;F=1;K=1 1250 i= secondary video (slides) 1251 a=label:12 1252 a=pcfg:3 1253 m=application 33002 TCP/BFCP * 1254 a=setup:passive 1255 a=connection:new 1256 a=floorid:1 m-stream:11 12 1257 a=floor-control:s-only 1258 a=confid:4321 1259 a=userid:1234 1260 a=pcfg:5 1262 If the answerer understands MediaCapNeg, but cannot support the 1263 Binary Floor Control Protocol, then it would respond with: 1265 v=0 1266 o=- 25678 753849 IN IP4 192.0.2.1 1267 s= 1268 c=IN IP4 192.0.2.22 1269 t=0 0 1270 a=cusp:med-v0 1271 a=sescap:1 1,4 1272 m=audio 23456 RTP/AVP 0 1273 a=rtpmap:0 PCMU/8000 1274 a=acfg:1 1275 m=video 41234 RTP/AVP 104 1276 a=rtpmap:100 H264/90000 1277 a=fmtp:104 profile-level-id=42A01E; packetization-mode=2 1278 a=acfg:4 m=1 a=1 pt=1:104 1279 a=pcfg:2 1280 m=video 0 RTP/AVP 103 1281 a=acfg:3 1282 m=application 0 TCP/BFCP * 1283 a=acfg:5 1285 An endpoint that doesn't support Media capabilities negotiation, but 1286 does support H.263 video, would respond with one or two H.263 video 1287 streams. In the latter case, the answerer may issue a second offer 1288 to reconfigure the session to one audio and one video channel using 1289 H.264 or H.263. 1291 Session capabilities can include latent capabilities as well. Here's 1292 a similar example in which the offerer wishes to initially establish 1293 an audio stream, and prefers to later establish two video streams 1294 with chair control. If the answerer doesn't understand Media CapNeg, 1295 or cannot support the dual video streams or flow control, then it may 1296 support a single H.264 video stream. Note that establishment of the 1297 most favored configuration will require two offer/answer exchanges. 1299 h 1301 v=0 1302 o=- 25678 753849 IN IP4 192.0.2.1 1303 s= 1304 c=IN IP4 192.0.2.1 1305 t=0 0 1306 a=creq:med-v0 1307 a=sescap:1 1,3,4,5 1308 a=sescap:2 1,2 1309 a=sescap:3 1 1310 a=mcap:1 H263-1998/90000 1311 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 1312 a=tcap:1 RTP/AVP TCP/BFCP 1313 m=audio 54322 RTP/AVP 0 1314 a=rtpmap:0 PCMU/8000 1315 a=label:11 1316 a=pcfg:1 1317 m=video 22344 RTP/AVP 102 1318 a=rtpmap:102 H264/90000 1319 a=fmtp:102 profile-level-id=42A01E; packetization-mode=2 1320 a=label:11 1321 a=content:main 1322 a=pcfg:2 1323 a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3 1324 a=acap:31 label:12 1325 a=acap:32 content:main 1326 a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4 1327 a=acap:41 label:13 1328 a=acap:42 content:slides 1329 a=lcfg:5 mt=application m=51 t=51 1330 a=tcap:51 TCP/BFCP 1331 a=mcap:51 * 1332 a=acap:51 setup:passive 1333 a=acap:52 connection:new 1334 a=acap:53 floorid:1 m-stream:12 13 1335 a=acap:54 floor-control:s-only 1336 a=acap:55 confid:4321 1337 a=acap:56 userid:1234 1339 In this example, the default offer, as seen by endpoints which do not 1340 understand capabilities negotiation, proposes a PCMU audio stream and 1341 an H.264 video stream. Note that the offered lcfg lines for the 1342 video streams don't carry pt= parameters because they're not needed 1343 (payload type numbers will be assigned in the offer/answer exchange 1344 that establishes the streams). Note also that the three mcap, mfcap, 1345 and tcap attributes used by lcfg:3 and lcfg:4 are included at the 1346 session level so they may be referenced by both latent 1347 configurations. As per Section 3.3, the media attributes generated 1348 from the mcap, mfcap, and tcap attributes are always media-level 1349 attributes. If the answerer supports Media CapNeg, and supports the 1350 most desired configuration, it would return the following SDP: 1352 v=0 1353 o=- 25678 753849 IN IP4 192.0.2.1 1354 s= 1355 c=IN IP4 192.0.2.22 1356 t=0 0 1357 a=csup:med-v0 1358 a=sescap:1 1,3,4,5 1359 a=sescap:2 1,2 1360 a=sescap:3 1 1361 m=audio 23456 RTP/AVP 0 1362 a=rtpmap:0 PCMU/8000 1363 a=acfg:1 1364 m=video 0 RTP/AVP 102 1365 a=pcfg:2 1366 a=lcfg:3 mt=video t=1 m=1 a=31,32 1367 a=lcfg:4 mt=video t=1 m=1 a=41,42 1368 a=lcfg:5 mt=application t=2 1370 This exchange supports immediate establishment of an audio stream for 1371 preliminary conversation. This exchange would presumably be followed 1372 at the appropriate time with a "reconfiguration" offer/answer 1373 exchange to add the video and chair control streams. 1375 3.4. Offer/Answer Model Extensions 1377 In this section, we define extensions to the offer/answer model 1378 defined in RFC3264 [RFC3264] and [RFC5939] to allow for media 1379 capabilities, bandwidth capabilities, and latent configurations to be 1380 used with the SDP Capability Negotiation framework. 1382 The [RFC5939] provides a relatively compact means to offer the 1383 equivalent of an ordered list of alternative media stream 1384 configurations (as would be described by separate m= lines and 1385 associated attributes). The attributes acap, mscap, mfcap and mcap 1386 are designed to map somewhat straightforwardly into equivalent m= 1387 lines and conventional attributes when invoked by a pcfg, lcfg, or 1388 acfg attribute with appropriate parameters. The a=pcfg: lines, along 1389 with the m= line itself, represent offered media configurations. The 1390 a=lcfg: lines represent alternative capabilities for future use. 1392 3.4.1. Generating the Initial Offer 1394 When an endpoint generates an initial offer and wants to use the 1395 functionality described in the current document, it should identify 1396 and define the codecs it can support via mcap, mfcap and mscap 1397 attributes. The SDP media line(s) should be made up with the 1398 configuration to be used if the other party does not understand 1399 capability negotiations (by default, this is the least preferred 1400 configuration). Typically, the media line configuration will contain 1401 the minimum acceptable capabilities. The offer MUST include the 1402 level of capability negotiation extensions needed to support this 1403 functionality in a "creq" attribute. 1405 Preferred configurations for each media stream are identified 1406 following the media line. The present offer may also include latent 1407 configuration (lcfg) attributes, at the session level, describing 1408 media streams and/or configurations the offerer is not now offering, 1409 but which it is willing to support in a future offer/answer exchange. 1410 A simple example might be the inclusion of a latent video 1411 configuration in an offer for an audio stream. 1413 3.4.2. Generating the Answer 1415 When the answering party receives the offer and if it supports the 1416 required capability negotiation extensions, it should select the 1417 most-preferred configuration it can support for each media stream, 1418 and build its answer accordingly. The configuration selected for 1419 each accepted media stream is placed into the answer as a media line 1420 with associated parameters and attributes. If a proposed 1421 configuration is chosen, the answer must include the supported 1422 extension attribute and each media stream for which a proposed 1423 configuration was chosen must contain an actual configuration (acfg) 1424 attribute to indicate just which pcfg attribute was used to build the 1425 answer. The answer should also include any potential or latent 1426 configurations the answerer can support, especially any 1427 configurations compatible with other potential or latent 1428 configurations received in the offer. The answerer should make note 1429 of those configurations it might wish to offer in the future. 1431 3.4.3. Offerer Processing of the Answer 1433 When the offerer receives the answer, it should make note of any 1434 capabilities and/or latent configurations for future use. The media 1435 line(s) must be processed in the normal way to identify the media 1436 stream(s) accepted by the answer, if any. The acfg attribute, if 1437 present, may be used to verify the proposed configuration used to 1438 form the answer, and to infer the lack of acceptability of higher- 1439 preference configurations that were not chosen. Note that the base 1440 specification [RFC5939] requires the answerer to choose the highest 1441 preference configuration it can support, subject to local policies. 1443 3.4.4. Modifying the Session 1445 If, at a later time, one of the parties wishes to modify the 1446 operating parameters of a session, e.g., by adding a new media 1447 stream, or by changing the properties used on an existing stream, it 1448 may do so via the mechanisms defined for offer/answer [RFC3264]. If 1449 the initiating party has remembered the codecs, potential 1450 configurations, and latent configurations announced by the other 1451 party in the earlier negotiation, it may use this knowledge to 1452 maximize the likelihood of a successful modification of the session. 1453 Alternatively, the initiator may perform a new capabilities exchange 1454 as part of the reconfiguration. In such a case, the new capabilities 1455 will replace the previously-negotiated capabilities. This may be 1456 useful if conditions change on the endpoint. 1458 4. Examples 1460 In this section, we provide examples showing how to use the Media 1461 Capabilities with the SDP Capability Negotiation. 1463 4.1. Alternative Codecs 1465 This example provide a choice of one of six variations of the 1466 adaptive multirate codec. In this example, the default configuration 1467 as specified by the media line is the same as the most preferred 1468 configuration. Each configuration uses a different payload type 1469 number so the offerer can interpret early media. 1471 v=0 1472 o=- 25678 753849 IN IP4 192.0.2.1 1473 s= 1474 c=IN IP4 192.0.2.1 1475 t=0 0 1476 a=creq:med-v0 1477 m=audio 54322 RTP/AVP 96 1478 rtpmap:96 AMR-WB/16000/1 1479 a=fmtp:96 mode-change-capability=1; max-red=220; \ 1480 mode-set=0,2,4,7 1481 a=mcap:1,3,5 audio AMR-WB/16000/1 1482 a=mcap:2,4,6 audio AMR/8000/1 1483 a=mfcap:1,2,3,4 mode-change-capability=1 1484 a=mfcap:5,6 mode-change-capability=2 1485 a=mfcap:1,2,3,5 max-red=220 1486 a=mfcap:3,4,5,6 octet-align=1 1487 a=mfcap:1,3,5 mode-set=0,2,4,7 1488 a=mfcap:2,4,6 mode-set=0,3,5,6 1489 a=pcfg:1 m=1 pt=1:96 1490 a=pcfg:2 m=2 pt=2:97 1491 a=pcfg:3 m=3 pt=3:98 1492 a=pcfg:4 m=4 pt=4:99 1493 a=pcfg:5 m=5 pt=5:100 1494 a=pcfg:6 m=6 pt=6:101 1496 In the above example, media capability 1 could have been excluded 1497 from the first mcap declaration and from the corresponding mfcap 1498 attributes, and the pcfg:1 attribute line could have been simply 1499 "pcfg:1". 1501 The next example offers a video stream with three options of H.264 1502 and 4 transports. It also includes an audio stream with different 1503 audio qualities: four variations of AMR, or AC3. The offer looks 1504 something like: 1506 v=0 1507 o=- 25678 753849 IN IP4 192.0.2.1 1508 s=An SDP Media NEG example 1509 c=IN IP4 192.0.2.1 1510 t=0 0 1511 a=creq:med-v0 1512 a=ice-pwd:speEc3QGZiNWpVLFJhQX 1513 m=video 49170 RTP/AVP 100 1514 c=IN IP4 192.0.2.56 1515 a=maxprate:1000 1516 a=rtcp:51540 1517 a=sendonly 1518 a=candidate 12345 1 UDP 9 192.0.2.56 49170 host 1519 a=candidate 23456 2 UDP 9 192.0.2.56 51540 host 1520 a=candidate 34567 1 UDP 7 10.0.0.1 41345 srflx raddr \ 1521 192.0.2.56 rport 49170 1522 a=candidate 45678 2 UDP 7 10.0.0.1 52567 srflx raddr \ 1523 192.0.2.56 rport 51540 1524 a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \ 1525 192.0.2.56 rport 49170 1526 a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \ 1527 192.0.2.56 rport 51540 1528 b=AS:10000 1529 b=TIAS:10000000 1530 b=RR:4000 1531 b=RS:3000 1532 a=rtpmap:100 H264/90000 1533 a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \ 1534 sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \ 1535 sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \ 1536 sprop-init-buf-time=102478; deint-buf-cap=128000 1537 a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF 1538 a=mcap:1-3,7-9 H264/90000 1539 a=mcap:4-6 rtx/90000 1540 a=mfcap:1-9 profile-level-id=42A01E 1541 a=mfcap:1-9 aMljiA== 1542 a=mfcap:1,4,7 packetization-mode=0 1543 a=mfcap:2,5,8 packetization-mode=1 1544 a=mfcap:3,6,9 packetization-mode=2 1545 a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI 1546 a=mfcap:1,7 sprop-interleaving-depth=45; \ 1547 sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \ 1548 deint-buf-cap=128000 1549 a=mfcap:4 apt=100 1550 a=mfcap:5 apt=99 1551 a=mfcap:6 apt=98 1552 a=mfcap:4-6 rtx-time=3000 1553 a=mscap:1-6 rtcp-fb nack 1554 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \ 1555 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 1556 a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97 1557 a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96 1558 a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95 1559 a=pcfg:4 t=2 m=7 a=1 pt=7:100 1560 a=pcfg:5 t=2 m=8 a=1 pt=8:99 1561 a=pcfg:6 t=2 m=9 a=1 pt=9:98 1562 a=pcfg:7 t=3 m=1,3 pt=1:100,4:97 1563 a=pcfg:8 t=3 m=2,4 pt=2:99,4:96 1564 a=pcfg:9 t=3 m=3,6 pt=3:98,6:95 1565 m=audio 49176 RTP/AVP 101 100 99 98 1566 c=IN IP4 192.0.2.56 1567 a=ptime:60 1568 a=maxptime:200 1569 a=rtcp:51534 1570 a=sendonly 1571 a=candidate 12345 1 UDP 9 192.0.2.56 49176 host 1572 a=candidate 23456 2 UDP 9 192.0.2.56 51534 host 1573 a=candidate 34567 1 UDP 7 10.0.0.1 41348 srflx \ 1574 raddr 192.0.2.56 rport 49176 1575 a=candidate 45678 2 UDP 7 10.0.0.1 52569 srflx \ 1576 raddr 192.0.2.56 rport 51534 1577 a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \ 1578 raddr 192.0.2.56 rport 49176 1579 a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \ 1580 raddr 192.0.2.56 rport 51534 1581 b=AS:512 1582 b=TIAS:512000 1583 b=RR:4000 1584 b=RS:3000 1585 a=maxprate:120 1586 a=rtpmap:98 AMR-WB/16000 1587 a=fmtp:98 octet-align=1; mode-change-capability=2 1588 a=rtpmap:99 AMR-WB/16000 1589 a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2 1590 a=rtpmap:100 AMR-WB/16000/2 1591 a=fmtp:100 octet-align=1; interleaving=30 1592 a=rtpmap:101 AMR-WB+/72000/2 1593 a=fmtp:101 interleaving=50; int-delay=160000; 1594 a=mcap:14 ac3/48000/6 1595 a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \ 1596 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 1597 a=tcap:4 RTP/SAVP 1598 a=pcfg:10 t=4 a=23 1599 a=pcfg:11 t=4 m=14 a=23 pt=14:102 1601 This offer illustrates the advantage in compactness that arises if 1602 one can avoid deleting the base configuration attributes and 1603 recreating them in acap attributes for the potential configurations. 1605 4.2. Alternative Combinations of Codecs (Session Configurations) 1607 If an endpoint has limited signal processing capacity, it might be 1608 capable of supporting, say, a G.711 mu-law audio stream in 1609 combination with an H.264 video stream, or a G.729B audio stream in 1610 combination with an H.263-1998 video stream. It might then issue an 1611 offer like the following: 1613 v=0 1614 o=- 25678 753849 IN IP4 192.0.2.1 1615 s= 1616 c=IN IP4 192.0.2.1 1617 t=0 0 1618 a=creq:med-v0 1619 a=sescap:1 2,4 1620 a=sescap:2 1,3 1621 m=audio 54322 RTP/AVP 18 1622 a=rtpmap:18 G729/8000 1623 a=fmtp:18 annexb=yes 1624 a=mcap:1 PCMU/8000 1625 a=pcfg:1 m=1 pt=1:0 1626 a=pcfg:2 1627 m=video 54344 RTP/AVP 100 1628 a=rtpmap:100 H263-1998/90000 1629 a=mcap:2 H264/90000 1630 a=mfcap:2 profile-level-id=42A01E; packetization-mode=2 1631 a=pcfg:3 m=2 pt=2:101 1632 a=pcfg:4 1634 Note that the preferred session configuration (and the default as 1635 well) is G.729B with H.263. This overrides the individual media 1636 stream preferences which are PCMU and H.264 by the potential 1637 configuration numbering rule. 1639 4.3. Latent Media Streams 1641 Consider a case in which the offerer can support either G.711 mu-law, 1642 or G.729B, along with DTMF telephony events for the 12 common 1643 touchtone signals, but is willing to support simple G.711 mu-law 1644 audio as a last resort. In addition, the offerer wishes to announce 1645 its ability to support video in the future, but does not wish to 1646 offer a video stream at present. The offer might look like the 1647 following: 1649 v=0 1650 o=- 25678 753849 IN IP4 192.0.2.1 1651 s= 1652 c=IN IP4 192.0.2.1 1653 t=0 0 1654 a=creq:med-v0 1655 m=audio 23456 RTP/AVP 0 1656 a=rtpmap:0 PCMU/8000 1657 a=mcap:1 PCMU/8000 1658 a=mcap:2 g729/8000 1659 a=mcap:3 telephone-event/8000 1660 a=mfcap:3 0-11 1661 a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 a=lcfg:10 mt=video t=1 1662 m=10|11 1663 a=mcap:10 H263-1998/90000 1664 a=mcap:11 H264/90000 1665 a=tcap:1 RTP/AVP 1667 The lcfg attribute line announces support for H.263 and H.264 video 1668 (H.263 preferred) for future reference. The m-line and the rtpmap 1669 attribute offer an audio stream and provide the lowest precedence 1670 configuration (PCMU without any DTMF encoding). The mcap lines 1671 define the media capabilities (PCMU, G729, and telephone-event) to be 1672 offered in potential configurations. The mfcap attribute provides 1673 the format parameters for telephone-events, specifying the 12 1674 commercial DTMF 'digits'. The pcfg attribute line defines the most- 1675 preferred media configuration as PCMU plus DTMF events and the next- 1676 most-preferred configuration as G.729B plus DTMF events. 1678 If the answerer is able to support all the potential configurations, 1679 and also support H.263 video (but not H.264), it would reply with an 1680 answer like: 1682 v=0 1683 o=- 24351 621814 IN IP4 192.0.2.2 1684 s= 1685 c=IN IP4 192.0.2.2 1686 t=0 0 1687 a=csup:med-v0 1688 m=audio 54322 RTP/AVP 0 100 1689 a=rtpmap:0 PCMU/8000 1690 a=rtpmap:100 telephone-event/8000 1691 a=fmtp:100 0-11 1692 a=acfg:1 m=1,3 pt=1:0,3:100 1693 a=pcfg:1 m=2,3 pt=2:18,3:100 a=lcfg:1 mt=video t=1 m=10 1695 The lcfg attribute line announces the capability to support H.263 1696 video at a later time. The media line and subsequent rtpmap and fmtp 1697 attribute lines present the selected configuration for the media 1698 stream. The acfg attribute line identifies the potential 1699 configuration from which it was taken, and the pcfg attribute line 1700 announces the potential capability to support G.729 with DTMF events 1701 as well. If, at some later time, congestion becomes a problem in the 1702 network, either party may, with expectation of success, offer a 1703 reconfiguration of the media stream to use G.729 in order to reduce 1704 packet sizes. 1706 5. IANA Considerations 1708 5.1. New SDP Attributes 1710 The IANA is hereby requested to register the following new SDP 1711 attributes: 1713 Attribute name: mcap 1714 Long form name: media capability 1715 Type of attribute: session-level and media-level 1716 Subject to charset: no 1717 Purpose: associate media capability number(s) with 1718 media subtype and encoding parameters 1719 Appropriate Values: see Section 3.3.1 1721 Attribute name: mfcap 1722 Long form name: media format capability 1723 Type of attribute: session-level and media-level 1724 Subject to charset: no 1725 Purpose: associate media format attributes and 1726 parameters with media format capabilities 1727 Appropriate Values: see Section 3.3.2 1729 Attribute name: mscap 1730 Long form name: media-specific capability 1731 Type of attribute: session-level and media-level 1732 Subject to charset: no 1733 Purpose: associate media-specific attributes and 1734 parameters with media capabilities 1735 Appropriate Values: see Section 3.3.3 1737 Attribute name: lcfg 1738 Long form name: latent configuration 1739 Type of attribute: media-level 1740 Subject to charset: no 1741 Purpose: to announce supportable media streams 1742 without offering them for immediate use. 1743 Appropriate Values: see Section 3.3.5 1745 Attribute name: sescap 1746 Long form name: session capability 1747 Type of attribute: session-level 1748 Subject to charset: no 1749 Purpose: to specify and prioritize acceptable 1750 combinations of media stream configurations. 1751 Appropriate Values: see Section 3.3.8 1753 5.2. New SDP Option Tag 1755 The IANA is hereby requested to add the new option tag "med-v0", 1756 defined in this document, to the SDP Capability Option Negotiation 1757 Capability registry created for [RFC5939]. 1759 5.3. New SDP Capability Negotiation Parameters 1761 The IANA is hereby requested to expand the SDP Capability Negotiation 1762 Potential Configuration Parameter Registry established by [RFC5939] 1763 to become the SDP Capability Negotiation Configuration Parameter 1764 Registry and to include parameters for the potential, actual and 1765 latent configuration attributes. The new parameters to be registered 1766 are the "m" for "media", "pt" for "payload type number", and "mt" for 1767 "media type" parameters. Note that the "mt" parameter is defined for 1768 use only in the latent configuration attribute. 1770 6. Security Considerations 1772 The security considerations of [RFC5939] apply for this document. 1774 The addition of negotiable media encoding, bandwidth attributes, and 1775 connection data in this specification can cause problems for 1776 middleboxes which attempt to control bandwidth utilization, media 1777 flows, and/or processing resource consumption as part of network 1778 policy, but which do not understand the media capability negotiation 1779 feature. As for the initial CapNeg work, the SDP answer is 1780 formulated in such a way that it always carries the selected media 1781 encoding and bandwidth parameters for every media stream selected. 1782 Pending an understanding of capabilities negotiation, the middlebox 1783 should examine the answer SDP to obtain the best picture of the media 1784 streams being established. 1786 As always, middleboxes can best do their job if they fully understand 1787 media capabilities negotiation. 1789 7. Changes from previous versions 1791 7.1. Changes from version 10 1793 o Defined the latent configuration attribute as a media-level 1794 attribute because it specifies a possible future media stream. 1795 Added text to clarify how to specify alternative configurations of 1796 a single latent stream and/or multiple streams. 1798 o Improved the definition of the session capability attribute to 1799 permit both required configurations and optional configurations - 1800 latent configurations cannot be required because they have not yet 1801 been offered. 1803 o Removed the special-case treatment of conflicts between base-level 1804 fmtp attributes and fmtp attributes generated for a configuration 1805 via invoked mcap and mfcap attributes. 1807 o Removed reference to bandwidth capability (bcap) attribute. 1809 o Changed various "must", etc., terms to normative terms ("MUST", 1810 etc.) as appropriate, in Section 3.3.5Section 3.3.6.1 1811 Section 3.3.6.3 and Section 3.3.8 1813 o Attempted to clarify the substitution mechanism in Section 3.3.7 1814 and improve its uniqueness. 1816 o Made various editorial changes, including changing the title in 1817 the header, and removing numbering from some SDP examples. 1819 7.2. Changes from version 09 1821 o Additional corrections to latent media stream example in 1822 Section 4.3 1824 o Fixed up attribute formatting examples and corresponding ABNF. 1826 o Removed preference rule for latent configurations. 1828 o Various spelling and other editorial changes were made. 1830 o updated crossreferences. 1832 7.3. Changes from version 08 1834 The major change is in Section 4.3, Latent Media Streams, fixing the 1835 syntax of the answer. All the other changes are editorial. 1837 7.4. Changes from version 04 1839 o The definitions for bcap, ccap, icap, and kcap attributes have 1840 been removed, and are to be defined in another document. 1842 o Corrected formatting of m= and p= configuration parameters to 1843 conform to extension-config-list form defined in [RFC5939] 1845 o Reorganized definitions of new parameters to make them easier to 1846 find in document. 1848 o Added ability to renegotiate capabilities when modifying the 1849 session (Section 3.4.4). 1851 o Made various editorial changes, clarifications, and typo 1852 corrections. 1854 7.5. Changes from version 03 1856 o A new session capability attribute (sescap) has been added to 1857 permit specification of acceptable media stream combinations. 1859 o Capability attribute definitions corresponding to the i, c, b, and 1860 k SDP line types have been added for completeness. 1862 o Use of the pcfg: attribute in SDP answers has been included in 1863 order to conveniently return information in the answer about 1864 acceptable configurations in the media stream offer. 1866 o The use of the lcfg: attribute(s) in SDP answers has been 1867 restricted to indicate just which latent configuration offers 1868 would be acceptable to the answerer. 1870 o A suggestion for "naive" middleboxes has been added to the 1871 Security Considerations. 1873 o Various editorial changes have been made. 1875 o Several errors/omissions have been corrected. 1877 o The description of the mscap attribute has been modified to make 1878 it clear that it should not be used to generate undefined SDP 1879 attributes, or to "extend" existing attributes. 1881 o are made optional in the mscap attribute 1882 definition. 1884 o "AMR" changed to "AMR-WB" in cases in which the sample rate is 1885 16000. 1887 7.6. Changes from version 02 1889 This version contains several detail changes intended to simplify 1890 capability processing and mapping into conventional SDP media blocks. 1892 o The "mcap" attribute is enhanced to include the role of the "ecap" 1893 attribute; the latter is eliminated. 1895 o The "fcap" attribute has been renamed "mfcap". New replacement 1896 rules vis-a-vis fmtp attributes in the base media specification 1897 have been added. 1899 o A new "mscap" attribute is defined to handle the problem of 1900 attributes (other than rtpmap and fmtp) that are specific to a 1901 particular payload type. 1903 o New rules for processing the mcap, mfcap, and mscap attributes, 1904 and overriding standard rtpmap, fmtp, or other media-specific 1905 attributes, are put forward to reduce the need to use the deletion 1906 option in the a= parameter of the potential configuration (pcfg) 1907 attribute. 1909 o A new parameter, "mt=" is added to the latent configuration 1910 attribute (lcfg) to specify the media stream type (audio, video, 1911 etc.) when the lcfg is declared at the session level. 1913 o The examples are expanded. 1915 o Numerous typos and misspellings have been corrected. 1917 7.7. Changes from version 01 1919 The documents adds a new attribute for specifying bandwidth 1920 capability and a parametr to list in the potential configuration. 1921 Other changes are to align the document with the terminolgy and 1922 attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07. 1923 The document also clarifies some previous open issues. 1925 7.8. Changes from version 00 1927 The major changes include taking out the "mcap" and "cptmap" 1928 parameter. The mapping of payload type is now in the "pt" parameter 1929 of "pcfg". Media subtype need to explictly definesd in the "cmed" 1930 attribute if referenced in the "pcfg" 1932 8. Acknowledgements 1934 This document is heavily influenced by the discussions and work done 1935 by the SDP Capability Negotiation Design team. The following people 1936 in particular provided useful comments and suggestions to either the 1937 document itself or the overall direction of the solution defined 1938 herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and 1939 Thomas Stach. 1941 We thank Ingemar Johansson and Magnus Westerlund for examples that 1942 stimulated this work, and for critical reading of the document. 1944 9. References 1946 9.1. Normative References 1948 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1949 Requirement Levels", BCP 14, RFC 2119, March 1997. 1951 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1952 with Session Description Protocol (SDP)", RFC 3264, 1953 June 2002. 1955 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1956 Description Protocol", RFC 4566, July 2006. 1958 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1959 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1961 [RFC5939] Andreasen, F., "SDP Capability Negotiation", RFC 5939, 1962 September 2010. 1964 9.2. Informative References 1966 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 1967 Description Protocol (SDP) Security Descriptions for Media 1968 Streams", RFC 4568, July 2006. 1970 [RFC4583] Camarillo, G., "Session Description Protocol (SDP) Format 1971 for Binary Floor Control Protocol (BFCP) Streams", 1972 RFC 4583, November 2006. 1974 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1975 "Extended RTP Profile for Real-time Transport Control 1976 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1977 July 2006. 1979 [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF 1980 Digits, Telephony Tones, and Telephony Signals", RFC 4733, 1981 December 2006. 1983 [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, 1984 "RTP Payload Format and File Storage Format for the 1985 Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband 1986 (AMR-WB) Audio Codecs", RFC 4867, April 2007. 1988 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 1989 "Codec Control Messages in the RTP Audio-Visual Profile 1990 with Feedback (AVPF)", RFC 5104, February 2008. 1992 Authors' Addresses 1994 Robert R Gilman 1995 Independent 1996 3243 W. 11th Ave. Dr. 1997 Broomfield, CO 80020 1998 USA 2000 Email: bob_gilman@comcast.net 2002 Roni Even 2003 Gesher Erove Ltd 2004 14 David Hamelech 2005 Tel Aviv 64953 2006 Israel 2008 Email: ron.even.tlv@gmail.com 2010 Flemming Andreasen 2011 Cisco Systems 2012 Edison, NJ 2013 USA 2015 Email: fandreas@cisco.com