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Andreasen 7 Cisco Systems 8 January 4, 2013 10 Session Description Protocol (SDP) Media Capabilities Negotiation 11 draft-ietf-mmusic-sdp-media-capabilities-17 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 This document updates the IANA Considerations of RFC 5939. 24 Status of this Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on July 8, 2013. 41 Copyright Notice 43 Copyright (c) 2013 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 This document may contain material from IETF Documents or IETF 57 Contributions published or made publicly available before November 58 10, 2008. The person(s) controlling the copyright in some of this 59 material may not have granted the IETF Trust the right to allow 60 modifications of such material outside the IETF Standards Process. 61 Without obtaining an adequate license from the person(s) controlling 62 the copyright in such materials, this document may not be modified 63 outside the IETF Standards Process, and derivative works of it may 64 not be created outside the IETF Standards Process, except to format 65 it for publication as an RFC or to translate it into languages other 66 than English. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 71 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 8 73 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8 74 3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9 75 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15 76 3.3.1. The Media Format Capability Attributes . . . . . . . . 15 77 3.3.2. The Media Format Parameter Capability Attribute . . . 18 78 3.3.3. The Media-Specific Capability Attribute . . . . . . . 20 79 3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22 80 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24 81 3.3.6. Enhanced Potential Configuration Attribute . . . . . . 26 82 3.3.7. Substitution of Media Payload Type Numbers in 83 Capability Attribute Parameters . . . . . . . . . . . 30 84 3.3.8. The Session Capability Attribute . . . . . . . . . . . 31 85 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35 86 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 36 87 3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 39 88 3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 43 89 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 44 90 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 91 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45 92 4.2. Alternative Combinations of Codecs (Session 93 Configurations) . . . . . . . . . . . . . . . . . . . . . 48 94 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48 95 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 96 5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51 97 5.2. New SDP Capability Negotiation Option Tag . . . . . . . . 52 98 5.3. SDP Capability Negotiation Configuration Parameters 99 Registry . . . . . . . . . . . . . . . . . . . . . . . . . 52 100 5.4. SDP Capability Negotiation Configuration Parameter 101 Registrations . . . . . . . . . . . . . . . . . . . . . . 53 102 6. Security Considerations . . . . . . . . . . . . . . . . . . . 55 103 7. Changes from previous versions . . . . . . . . . . . . . . . . 56 104 7.1. Changes from version 16 . . . . . . . . . . . . . . . . . 56 105 7.2. Changes from version 15 . . . . . . . . . . . . . . . . . 56 106 7.3. Changes from version 14 . . . . . . . . . . . . . . . . . 56 107 7.4. Changes from version 13 . . . . . . . . . . . . . . . . . 56 108 7.5. Changes from version 12 . . . . . . . . . . . . . . . . . 56 109 7.6. Changes from version 11 . . . . . . . . . . . . . . . . . 57 110 7.7. Changes from version 10 . . . . . . . . . . . . . . . . . 57 111 7.8. Changes from version 09 . . . . . . . . . . . . . . . . . 58 112 7.9. Changes from version 08 . . . . . . . . . . . . . . . . . 58 113 7.10. Changes from version 04 . . . . . . . . . . . . . . . . . 58 114 7.11. Changes from version 03 . . . . . . . . . . . . . . . . . 58 115 7.12. Changes from version 02 . . . . . . . . . . . . . . . . . 59 116 7.13. Changes from version 01 . . . . . . . . . . . . . . . . . 60 117 7.14. Changes from version 00 . . . . . . . . . . . . . . . . . 60 118 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 61 119 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62 120 9.1. Normative References . . . . . . . . . . . . . . . . . . . 62 121 9.2. Informative References . . . . . . . . . . . . . . . . . . 62 122 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 64 124 1. Introduction 126 Session Description Protocol (SDP) capability negotiation [RFC5939] 127 provides a general framework for indicating and negotiating 128 capabilities in SDP [RFC4566]. The base framework defines only 129 capabilities for negotiating transport protocols and attributes. 131 RFC 5939 [RFC5939] lists some of the issues with the current SDP 132 capability negotiation process. An additional real life case is to 133 be able to offer one media stream (e.g. audio) but list the 134 capability to support another media stream (e.g. video) without 135 actually offering it concurrently. 137 In this document, we extend the framework by defining media 138 capabilities that can be used to indicate and negotiate media types 139 and their associated format parameters. This document also adds the 140 ability to declare support for media streams, the use of which can be 141 offered and negotiated later, and the ability to specify session 142 configurations as combinations of media stream configurations. The 143 definitions of new attributes for media capability negotiation are 144 chosen to make the translation from these attributes to 145 "conventional" SDP [RFC4566] media attributes as straightforward as 146 possible in order to simplify implementation. This goal is intended 147 to reduce processing in two ways: each proposed configuration in an 148 offer may be easily translated into a conventional SDP media stream 149 record for processing by the receiver; and the construction of an 150 answer based on a selected proposed configuration is straightforward. 152 This document updates RFC 5939 [RFC5939] by updating the IANA 153 Considerations. All other extensions defined in this document are 154 considered extensions above and beyond RFC 5939 [RFC5939]. 156 2. Terminology 158 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 159 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 160 document are to be interpreted as described in RFC 2119 [RFC2119] and 161 indicate requirement levels for compliant implementations. 163 "Actual Configuration": An actual configuration specifies which 164 combinations of SDP session parameters and media stream components 165 can be used in the current offer/answer exchange and with what 166 parameters. Use of an actual configuration does not require any 167 further negotiation in the offer/answer exchange. See RFC 5939 168 [RFC5939] for further details. 170 "Base Attributes": Conventional SDP attributes appearing in the base 171 configuration of a media block. 173 "Base Configuration": The media configuration represented by a media 174 block exclusive of all the capability negotiation attributes defined 175 in this document, the base capability negotiation document [RFC5939], 176 or any other capability negotiation document. In an offer SDP, the 177 base configuration corresponds to the actual configuration as defined 178 in RFC 5939 [RFC5939]. 180 "Conventional Attribute": Any SDP attribute other than those defined 181 by the series of capability negotiation specifications. 183 "Conventional SDP": An SDP record devoid of capability negotiation 184 attributes. 186 "Media Format Capability": A media format, typically a media subtype 187 such as PCMU, H263-1998, or T38, expressed in the form of a 188 capability. 190 "Media Format Parameter Capability": A media format parameter 191 ("a=fmtp" in conventional SDP) expressed in the form of a capability. 192 The media format parameter capability is associated with a media 193 format capability. 195 "Media Capability": The combined set of capabilities associated with 196 expressing a media format and its relevant parameters (e.g. media 197 format parameters and media specific parameters). 199 "Potential Configuration": A potential configuration indicates which 200 combinations of capabilities can be used for the session and its 201 associated media stream components. Potential configurations are not 202 ready for use, however they are offered for potential use in the 203 current offer/answer exchange. They provide an alternative that may 204 be used instead of the actual configuration, subject to negotiation 205 in the current offer/answer exchange. See RFC 5939 [RFC5939] for 206 further details. 208 "Latent Configuration": A latent configuration indicates which 209 combinations of capabilities could be used in a future negotiation 210 for the session and its associated media stream components. Latent 211 configurations are neither ready for use, nor are they offered for 212 actual or potential use in the current offer/answer exchange. Latent 213 configurations merely inform the other side of possible 214 configurations supported by the entity. Those latent configurations 215 may be used to guide subsequent offer/answer exchanges, but they are 216 not offered for use as part of the current offer/answer exchange. 218 3. SDP Media Capabilities 220 The SDP capability negotiation [RFC5939] discusses the use of any SDP 221 [RFC4566] attribute (a=) under the attribute capability "acap". The 222 limitations of using acap for fmtp and rtpmap in a potential 223 configuration are described in RFC 5939 [RFC5939]; for example they 224 can be used only at the media level since they are media level 225 attributes. RFC 5939 [RFC5939] does not provide a way to exchange 226 media-level capabilities prior to the actual offer of the associated 227 media stream. This section provides an overview of extensions 228 providing an SDP Media Capability negotiation solution offering more 229 robust capabilities negotiation. This is followed by definitions of 230 new SDP attributes for the solution and its associated updated offer/ 231 answer procedures [RFC3264] 233 3.1. Requirements 235 The capability negotiation extensions requirements considered herein 236 are as follows. 238 REQ-01: Support the specification of alternative (combinations of) 239 media formats (codecs) in a single media block. 241 REQ-02: Support the specification of alternative media format 242 parameters for each media format. 244 REQ-03: Retain backward compatibility with conventional SDP. 245 Ensure that each and every offered configuration can be easily 246 translated into a corresponding SDP media block expressed with 247 conventional SDP lines. 249 REQ-04: Ensure the scheme operates within the offer/answer model in 250 such a way that media formats and parameters can be agreed upon 251 with a single exchange. 253 REQ-05: Provide the ability to express offers in such a way that 254 the offerer can receive media as soon as the offer is sent. (Note 255 that the offerer may not be able to render received media prior to 256 exchange of keying material.) 258 REQ-06: Provide the ability to offer latent media configurations 259 for future negotiation. 261 REQ-07: Provide reasonable efficiency in the expression of 262 alternative media formats and/or format parameters, especially in 263 those cases in which many combinations of options are offered. 265 REQ-08: Retain the extensibility of the base capability negotiation 266 mechanism. 268 REQ-09: Provide the ability to specify acceptable combinations of 269 media streams and media formats. For example, offer a PCMU audio 270 stream with an H264 video stream, or a G729 audio stream with an 271 H263 video stream. This ability would give the offerer a means to 272 limit processing requirements for simultaneous streams. This 273 would also permit an offer to include the choice of an audio/T38 274 stream or an image/T38 stream, but not both. 276 Other possible extensions have been discussed, but have not been 277 treated in this document. They may be considered in the future. 278 Three such extensions are: 280 FUT-01: Provide the ability to mix, or change, media types within a 281 single media block. Conventional SDP does not support this 282 capability explicitly; the usual technique is to define a media 283 subtype that represents the actual format within the nominal media 284 type. For example, T.38 FAX as an alternative to audio/PCMU 285 within an audio stream is identified as audio/T38; a separate FAX 286 stream would use image/T38. 288 FUT-02: Provide the ability to support multiple transport protocols 289 within an active media stream without reconfiguration. This is 290 not explicitly supported by conventional SDP. 292 FUT-03: Provide capability negotiation attributes for all media- 293 level SDP line types in the same manner as already done for the 294 attribute type, with the exception of the media line type itself. 295 The media line type is handled in a special way to permit compact 296 expression of media coding/format options. The line types are 297 bandwidth ("b="), information ("i="), connection data ("c="), and, 298 possibly, the deprecated encryption key ("k="). 300 3.2. Solution Overview 302 The solution consists of new capability attributes corresponding to 303 conventional SDP line types, new parameters for the pcfg, acfg, and 304 the new lcfg attributes extending the base attributes from RFC 5939 305 [RFC5939], and a use of the pcfg attribute to return capability 306 information in the SDP answer. 308 Several new attributes are defined in a manner that can be related to 309 the capabilities specified in a media line, and its corresponding 310 rtpmap and fmtp attributes. 312 o A new attribute ("a=rmcap") defines RTP-based media format 313 capabilities in the form of a media subtype (e.g. "PCMU"), and 314 its encoding parameters (e.g. "/8000/2"). Each resulting media 315 format type/subtype capability has an associated handle called a 316 media capability number. The encoding parameters are as specified 317 for the rtpmap attribute defined in SDP [RFC4566], without the 318 payload type number part. 320 o A new attribute ("a=omcap") defines other (non RTP-based) media 321 format capabilities in the form of a media subtype only (e.g. 322 "T38"). Each resulting media format type/subtype capability has 323 an associated handle called a media capability number. 325 o A new attribute ("a=mfcap") specifies media format parameters 326 associated with one or more media format capabilities. The mfcap 327 attribute is used primarily to associate the media format 328 parameters normally carried in the fmtp attribute. Note that 329 media format parameters can be used with RTP and non-RTP based 330 media formats. 332 o A new attribute ("a=mscap") that specifies media parameters 333 associated with one or more media format capabilities. The mscap 334 attribute is used to associate capabilities with attributes other 335 than fmtp or rtpmap, for example, the rtcp-fb attribute defined in 336 RFC 4585 [RFC4585]. 338 o A new attribute ("a=lcfg") specifies latent media stream 339 configurations when no corresponding media line ("m=") is offered. 340 An example is the offer of latent configurations for video even 341 though no video is currently offered. If the peer indicates 342 support for one or more offered latent configurations, the 343 corresponding media stream(s) may be added via a new offer/answer 344 exchange. 346 o A new attribute ("a=sescap") is used to specify an acceptable 347 combination of simultaneous media streams and their configurations 348 as a list of potential and/or latent configurations. 350 New parameters are defined for the potential configuration (pcfg), 351 latent configuration (lcfg), and accepted configuration (acfg) 352 attributes to associate the new attributes with particular 353 configurations. 355 o A new parameter type ("m=") is added to the potential 356 configuration ("a=pcfg:") attribute and the actual configuration 357 ("a=acfg:") attribute defined in RFC 5939 [RFC5939], and to the 358 new latent configuration ("a=lcfg:") attribute. This permits 359 specification of media capabilities (including their associated 360 parameters) and combinations thereof for the configuration. For 361 example, the "a=pcfg:" line might specify PCMU and telephone 362 events [RFC4733] or G.729B and telephone events as acceptable 363 configurations. The "a=acfg:" line in the answer would specify 364 the configuration chosen. 366 o A new parameter type ("pt=") is added to the potential 367 configuration, actual configuration, and latent configuration 368 attributes. This parameter associates RTP payload type numbers 369 with the referenced RTP-based media format capabilities, and is 370 appropriate only when the transport protocol uses RTP. 372 o A new parameter type ("mt=") is used to specify the media type for 373 latent configurations. 375 Special processing rules are defined for capability attribute 376 arguments in order to reduce the need to replicate essentially- 377 identical attribute lines for the base configuration and potential 378 configurations. 380 o A substitution rule is defined for any capability attribute to 381 permit the replacement of the (escaped) media capability number 382 with the media format identifier (e.g., the payload type number in 383 audio/video profiles). 385 o Replacement rules are defined for the conventional SDP equivalents 386 of the mfcap and mscap capability attributes. This reduces the 387 necessity to use the deletion qualifier in the a=pcfg parameter in 388 order to ignore rtpmap, fmtp, and certain other attributes in the 389 base configuration. 391 o An argument concatenation rule is defined for mfcap attributes 392 which refer to the same media capability number. This makes it 393 convenient to combine format options concisely by associating 394 multiple mfcap lines with multiple media format capabilities. 396 This document extends the base protocol extensions to the offer/ 397 answer model that allow for capabilities and potential configurations 398 to be included in an offer. Media capabilities constitute 399 capabilities that can be used in potential and latent configurations. 400 Whereas potential configurations constitute alternative offers that 401 may be accepted by the answerer instead of the actual 402 configuration(s) included in the "m=" line(s) and associated 403 parameters, latent configurations merely inform the other side of 404 possible configurations supported by the entity. Those latent 405 configurations may be used to guide subsequent offer/answer 406 exchanges, but they are not part of the current offer/answer 407 exchange. 409 The mechanism is illustrated by the offer/answer exchange below, 410 where Alice sends an offer to Bob: 412 Alice Bob 413 | (1) Offer (SRTP and RTP) | 414 |--------------------------------->| 415 | | 416 | (2) Answer (RTP) | 417 |<---------------------------------| 418 | | 420 Alice's offer includes RTP and SRTP as alternatives. RTP is the 421 default, but SRTP is the preferred one (long lines are folded to fit 422 the margins): 424 v=0 425 o=- 25678 753849 IN IP4 192.0.2.1 426 s= 427 c=IN IP4 192.0.2.1 428 t=0 0 429 a=creq:med-v0 430 m=audio 3456 RTP/AVP 0 18 431 a=tcap:1 RTP/SAVP RTP/AVP 432 a=rtpmap:0 PCMU/8000/1 433 a=rtpmap:18 G729/8000/1 434 a=fmtp:18 annexb=yes 435 a=rmcap:1,4 G729/8000/1 436 a=rmcap:2 PCMU/8000/1 437 a=rmcap:5 telephone-event/8000 438 a=mfcap:1 annexb=no 439 a=mfcap:4 annexb=yes 440 a=mfcap:5 0-11 441 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \ 442 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 443 a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102 444 a=pcfg:2 m=2 t=1 a=1 pt=2:103 445 a=pcfg:3 m=4 t=2 pt=4:18 447 The required base and extensions are provided by the "a=creq" 448 attribute defined in RFC 5939 [RFC5939], with the option tag 449 "med-v0", which indicates that the extension framework defined here, 450 must be supported. The base level capability negotiation support 451 ("cap-v0" [RFC5939]) is implied since it is required for the 452 extensions. 454 The "m=" line indicates that Alice is offering to use plain RTP with 455 PCMU or G.729B. The media line implicitly defines the default 456 transport protocol (RTP/AVP in this case) and the default actual 457 configuration. 459 The "a=tcap:1" line, specified in the SDP Capability Negotiation base 460 protocol [RFC5939], defines transport protocol capabilities, in this 461 case Secure RTP (SAVP profile) as the first option and RTP (AVP 462 profile) as the second option. 464 The "a=rmcap:1,4" line defines two G.729 RTP-based media format 465 capabilities, numbered 1 and 4, and their encoding rate. The 466 capabilities are of media type "audio" and subtype G729. Note that 467 the media subtype is explicitly specified here, rather than RTP 468 payload type numbers. This permits the assignment of payload type 469 numbers in the media stream configuration specification. In this 470 example, two G.729 subtype capabilities are defined. This permits 471 the declaration of two sets of formatting parameters for G.729. 473 The "a=rmcap:2" line defines a G.711 mu-law capability, numbered 2. 475 The "a=rmcap:5" line defines an audio telephone-event capability, 476 numbered 5. 478 The "a=mfcap:1" line specifies the fmtp formatting parameters for 479 capability 1 (offerer will not accept G.729 Annex B packets). 481 The "a=mfcap:4" line specifies the fmtp formatting parameters for 482 capability 4 (offerer will accept G.729 Annex B packets). 484 The "a=mfcap:5" line specifies the fmtp formatting parameters for 485 capability 5 (the DTMF touchtones 0-9,*,#). 487 The "a=acap:1" line specified in the base protocol provides the 488 "crypto" attribute which provides the keying material for SRTP using 489 SDP security descriptions. 491 The "a=pcfg:" attributes provide the potential configurations 492 included in the offer by reference to the media capabilities, 493 transport capabilities, attribute capabilities and specified payload 494 type number mappings. Three explicit alternatives are provided; the 495 lowest-numbered one is the preferred one. The "a=pcfg:1 ..." line 496 specifies media capabilities 4 and 5, i.e., G.729B and DTMF (incl. 497 their associated media format parameters), or media capability 1 and 498 5, i.e., G.729 and DTMF (incl. their associated media format 499 parameters). Furthermore, it specifies transport protocol capability 500 1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute 501 capability 1, i.e. the crypto attribute provided. Lastly, it 502 specifies a payload type number mapping for (RTP-based) media 503 capabilities 1, 4, and 5, thereby permitting the offerer to 504 distinguish between encrypted media and unencrypted media received 505 prior to receipt of the answer. 507 Use of unique payload type numbers in alternative configurations is 508 not required; codecs such as AMR-WB [RFC4867] have the potential for 509 so many combinations of options that it may be impractical to define 510 unique payload type numbers for all supported combinations. If 511 unique payload type numbers cannot be specified, then the offerer 512 will be obliged to wait for the SDP answer before rendering received 513 media. For SRTP using SDES inline keying [RFC4568], the offerer will 514 still need to receive the answer before being able to decrypt the 515 stream. 517 The second alternative ("a=pcfg:2 ...") specifies media capability 2, 518 i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key 519 material. 521 The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; its 522 only purpose in this example is to show a preference for G.729B over 523 PCMU. 525 Per RFC 5939 [RFC5939], the media line, with any qualifying 526 attributes such as fmtp or rtpmap, is itself considered a valid 527 configuration (the current actual configuration); it has the lowest 528 preference (per RFC 5939 [RFC5939]). 530 Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU, 531 and telephone events over RTP, but not SRTP, hence he accepts the 532 potential configuration 3 for RTP provided by Alice. Bob generates 533 the following answer: 535 v=0 536 o=- 24351 621814 IN IP4 192.0.2.2 537 s= 538 c=IN IP4 192.0.2.2 539 t=0 0 540 a=csup:med-v0 541 m=audio 4567 RTP/AVP 18 542 a=rtpmap:18 G729/8000 543 a=fmtp:18 annexb=yes 544 a=acfg:3 m=4 t=2 pt=4:18 546 Bob includes the "a=csup" and "a=acfg" attributes in the answer to 547 inform Alice that he can support the med-v0 level of capability 548 negotiations. Note that in this particular example, the answerer 549 supported the capability extensions defined here, however had he not, 550 he would simply have processed the offer based on the offered PCMU 551 and G.729 codecs under the RTP/AVP profile only. Consequently, the 552 answer would have omitted the "a=csup" attribute line and chosen one 553 or both of the PCMU and G.729 codecs instead. The answer carries the 554 accepted configuration in the "m=" line along with corresponding 555 rtpmap and/or fmtp parameters, as appropriate. 557 Note that per the base protocol, after the above, Alice MAY generate 558 a new offer with an actual configuration ("m=" line, etc.) 559 corresponding to the actual configuration referenced in Bob's answer 560 (not shown here). 562 3.3. New Capability Attributes 564 In this section, we present the new attributes associated with 565 indicating the media capabilities for use by the SDP Capability 566 negotiation. The approach taken is to keep things similar to the 567 existing media capabilities defined by the existing media 568 descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" 569 attributes. We use media subtypes and "media capability numbers" to 570 link the relevant media capability parameters. This permits the 571 capabilities to be defined at the session level and be used for 572 multiple streams, if desired. For RTP-based media formats, payload 573 types are then specified at the media level (see Section 3.3.4.2). 575 A media capability merely indicates possible support for the media 576 type and media format(s) and parameters in question. In order to 577 actually use a media capability in an offer/answer exchange, it MUST 578 be referenced in a potential configuration. 580 Media capabilities, i.e. the attributes associated with expressing 581 media capability formats, parameters, etc., can be provided at the 582 session-level and/or the media-level. Media capabilities provided at 583 the session level may be referenced in any pcfg or lcfg attribute at 584 the media level (consistent with the media type), whereas media 585 capabilities provided at the media level may be referenced only by 586 the pcfg or lcfg attribute within that media stream. In either case, 587 the scope of the is the entire session description. 588 This enables each media capability to be uniquely referenced across 589 the entire session description (e.g. in a potential configuration). 591 3.3.1. The Media Format Capability Attributes 593 Media subtypes can be expressed as media format capabilities by use 594 of the "a=rmcap" and "a=omcap" attributes. The "a=rmcap" attribute 595 MUST be used for RTP-based media whereas the "a=omcap" attribute MUST 596 be used for non-RTP-based (other) media formats. The two attributes 597 are defined as follows: 599 a=rmcap: / 600 [/] 602 a=omcap: 604 where is a (list of) media capability number(s) 605 used to number a media format capability, the or 606 is the media subtype, e.g., H263-1998, PCMU, or T38, 607 is the encoding rate, and are the media 608 encoding parameters for the media subtype. All media format 609 capabilities in the list are assigned to the same media type/subtype. 610 Each occurrence of the rmcap and omcap attribute MUST use unique 611 values in their ; the media capability numbers 612 are shared between the two attributes and the numbers MUST be unique 613 across the entire SDP session. In short, the rmcap and omcap 614 attributes define media format capabilities and associate them with a 615 media capability number in the same manner as the rtpmap attribute 616 defines them and associates them with a payload type number. 617 Additionally, the attributes allow multiple capability numbers to be 618 defined for the media format in question by specifying a range of 619 media capability numbers. This permits the media format to be 620 associated with different media parameters in different 621 configurations. When a range of capability numbers is specified, the 622 first (leftmost) capability number MUST be strictly smaller than the 623 second (rightmost), i.e. the range increases and covers at least two 624 numbers. 626 In ABNF [RFC5234], we have: 628 media-capability-line = rtp-mcap / non-rtp-mcap 630 rtp-mcap = "a=rmcap:" media-cap-num-list 631 1*WSP encoding-name "/" clock-rate 632 ["/" encoding-parms] 633 non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name 634 media-cap-num-list = media-cap-num-element 635 *("," media-cap-num-element) 636 media-cap-num-element = media-cap-num 637 / media-cap-num-range 638 media-cap-num-range = media-cap-num "-" media-cap-num 639 media-cap-num = NonZeroDigit *9(DIGIT) 640 encoding-name = token ;defined in RFC4566 641 clock-rate = NonZeroDigit *9(DIGIT) 642 encoding-parms = token 643 format-name = token ;defined in RFC4566 644 NonZeroDigit = %x31-39 ; 1-9 646 The encoding-name, clock-rate and encoding-params are as defined to 647 appear in an rtpmap attribute for each media type/subtype. Thus, it 648 is easy to convert an rmcap attribute line into one or more rtpmap 649 attribute lines, once a payload type number is assigned to a media- 650 cap-num (see Section 3.3.5). 652 The format-name is a media format description for non-RTP based media 653 as defined for the part of the media description ("m=" line) in 654 SDP [RFC4566]. In simple terms, it is the name of the media format, 655 e.g. "t38". This form can also be used in cases such as BFCP 656 [RFC4585] where the fmt list in the m-line is effectively ignored 657 (BFCP uses "*"). 659 The "rmcap" and "omcap" attributes can be provided at the session- 660 level and/or the media-level. There can be more than one rmcap and 661 more than one omcap attribute at both the session and media level 662 (i.e., more than one of each at the session-level and more than one 663 of each in each media description). Media capability numbers cannot 664 include leading zeroes, and each media-cap-num MUST be unique within 665 the entire SDP record; it is used to identify that media capability 666 in potential, latent and actual configurations, and in other 667 attribute lines as explained below. Note that the media-cap-num 668 values are shared between the rmcap and omcap attributes, and hence 669 the uniqueness requirement applies to the union of them. When the 670 media capabilities are used in a potential, latent or actual 671 configuration, the media formats referred by those configurations 672 apply at the media level, irrespective of whether the media 673 capabilities themselves were specified at the session or media level. 674 In other words, the media capability applies to the specific media 675 description associated with the configuration which invokes it. 677 For example: 679 v=0 680 o=- 24351 621814 IN IP4 192.0.2.2 681 s= 682 c=IN IP4 192.0.2.2 683 t=0 0 684 a=rmcap:1 L16/8000/1 685 a=rmcap:2 L16/16000/2 686 a=rmcap:3 H263-1998/90000 687 a=omcap:4 example 688 m=audio 54320 RTP/AVP 0 689 a=pcfg:1 m=1|2, pt=1:99,2:98 690 m=video 66544 RTP/AVP 100 691 a=rtpmap:100 H264/90000 692 a=pcfg:10 m=3 pt=3:101 693 a=tcap:1 TCP 694 a=pcfg:11 m=4 t=1 696 3.3.2. The Media Format Parameter Capability Attribute 698 This attribute is used to associate media format specific parameters 699 with one or more media format capabilities. The form of the 700 attribute is: 702 a=mfcap: 704 where permits the list of parameters to be associated 705 with one or more media format capabilities and the format parameters 706 are specific to the type of media format. The mfcap lines map to a 707 single traditional SDP fmtp attribute line (one for each entry in 708 ) of the form 710 a=fmtp: 712 where is the media format parameter defined in RFC 4566 713 [RFC4566], as appropriate for the particular media stream. The mfcap 714 attribute MUST be used to encode attributes for media capabilities, 715 which would conventionally appear in an fmtp attribute. The existing 716 acap attribute MUST NOT be used to encode fmtp attributes. 718 The mfcap attribute adheres to SDP [RFC4566] attribute production 719 rules with 721 media-format-parameter-capability = 722 "a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list 723 fmt-specific-param-list = text ; defined in RFC4566 725 Note that media format parameters can be used with RTP-based and non- 726 RTP based media formats. 728 3.3.2.1. Media Format Parameter Concatenation Rule 730 The appearance of media subtypes with a large number of formatting 731 options (e.g., AMR-WB [RFC4867]) coupled with the restriction that 732 only a single fmtp attribute can appear per media format, suggests 733 that it is useful to create a combining rule for mfcap parameters 734 which are associated with the same media capability number. 735 Therefore, different mfcap lines MAY include the same media-cap-num 736 in their media-cap-num-list. When a particular media capability is 737 selected for processing, the parameters from each mfcap line which 738 references the particular capability number in its media-cap-num-list 739 are concatenated together via ";", in the order the mfcap attributes 740 appear in the SDP record, to form the equivalent of a single fmtp 741 attribute line. This permits one to define a separate mfcap line for 742 a single parameter and value that is to be applied to each media 743 capability designated in the media-cap-num-list. This provides a 744 compact method to specify multiple combinations of format parameters 745 when using codecs with multiple format options. Note that order- 746 dependent parameters SHOULD be placed in a single mfcap line to avoid 747 possible problems with line rearrangement by a middlebox. 749 Format parameters are not parsed by SDP; their content is specific to 750 the media type/subtype. When format parameters for a specific media 751 capability are combined from multiple a=mfcap lines which reference 752 that media capability, the format-specific parameters are 753 concatenated together and separated by ";" for construction of the 754 corresponding format attribute (a=fmtp). The resulting format 755 attribute will look something like the following (without line 756 breaks): 758 a=fmtp: ; 759 ; 760 ... 762 where depends on the transport protocol in the manner defined 763 in RFC4566. SDP cannot assess the legality of the resulting 764 parameter list in the "a=fmtp" line; the user must take care to 765 ensure that legal parameter lists are generated. 767 The "mfcap" attribute can be provided at the session-level and the 768 media-level. There can be more than one mfcap attribute at the 769 session or media level. The unique media-cap-num is used to 770 associate the parameters with a media capability. 772 As a simple example, a G.729 capability is, by default, considered to 773 support comfort noise as defined by Annex B. Capabilities for G.729 774 with and without comfort noise support may thus be defined by: 776 a=rmcap:1,2 G729/8000 777 a=mfcap:2 annexb:no 779 Media capability 1 supports G.729 with Annex B, whereas media 780 capability 2 supports G.729 without Annex B. 782 Example for H.263 video: 784 a=rmcap:1 H263-1998/90000 785 a=rmcap:2 H263-2000/90000 786 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 787 a=mfcap:2 profile=2;level=2.2 789 Finally, for six format combinations of the Adaptive MultiRate codec: 791 a=rmcap:1-3 AMR/8000/1 792 a=rmcap:4-6 AMR-WB/16000/1 793 a=mfcap:1,2,3,4 mode-change-capability=1 794 a=mfcap:5,6 mode-change-capability=2 795 a=mfcap:1,2,3,5 max-red=220 796 a=mfcap:3,4,5,6 octet-align=1 797 a=mfcap:1,3,5 mode-set=0,2,4,7 798 a=mfcap:2,4,6 mode-set=0,3,5,6 800 So that AMR codec #1, when specified in a pcfg attribute within an 801 audio stream block (and assigned payload type number 98) as in 803 a=pcfg:1 m=1 pt=1:98 805 is essentially equivalent to the following 807 m=audio 49170 RTP/AVP 98 808 a=rtpmap:98 AMR/8000/1 809 a=fmtp:98 mode-change-capability=1; \ 810 max-red=220; mode-set=0,2,4,7 812 and AMR codec #4 with payload type number 99,depicted by the 813 potential configuration: 815 a=pcfg:4 m=4, pt=4:99 817 is equivalent to the following: 819 m=audio 49170 RTP/AVP 99 820 a=rtpmap:99 AMR-WB/16000/1 821 a=fmtp:99 mode-change-capability=1; octet-align=1; \ 822 mode-set=0,3,5,6 824 and so on for the other four combinations. SDP could thus convert 825 the media capabilities specifications into one or more alternative 826 media stream specifications, one of which can be chosen for the 827 answer. 829 3.3.3. The Media-Specific Capability Attribute 831 Attributes and parameters associated with a media format are 832 typically specified using the "rtpmap" and "fmtp" attributes in SDP, 833 and the similar "rmcap" and "mfcap" attributes in SDP Media 834 Capabilities. Some SDP extensions define other attributes that need 835 to be associated with media formats, for example the "rtcp-fb" 836 attribute defined in RFC 4585 [RFC4585]. Such media-specific 837 attributes, beyond the rtpmap and fmtp attributes, may be associated 838 with media capability numbers via a new media-specific attribute, 839 mscap, of the following form: 841 a=mscap: 843 where is a (list of) media capability number(s), 844 is the attribute name, and is the value field 845 for the named attribute. Note that the media capability numbers 846 refer to media format capabilities specified elsewhere in the SDP 847 ("rmcap" and/or "omcap"). If a range of capability numbers is 848 specified, the first (leftmost) capability number MUST be strictly 849 smaller than the second (rightmost). The media capability numbers 850 may include a wildcard ("*"), which will be used instead of any 851 payload type mappings in the resulting SDP (see, e.g. RFC 4585 852 [RFC4585] and the example below). In ABNF, we have: 854 media-specific-capability = "a=mscap:" 855 media-caps-star 856 1*WSP att-field ; from RFC4566 857 1*WSP att-value ; from RFC4566 858 media-caps-star = media-cap-star-element 859 *("," media-cap-star-element) 860 media-cap-star-element = (media-cap-num [wildcard]) 861 / (media-cap-num-range [wildcard]) 862 wildcard = "*" 864 Given an association between a media capability and a payload type 865 number as specified by the pt= parameters in a pcfg attribute line, a 866 mscap line may be translated easily into a conventional SDP attribute 867 line of the form 869 a=":" ; defined in SDP 870 [RFC4566] 872 A resulting attribute that is not a legal SDP attribute as specified 873 by RFC4566 MUST be ignored by the receiver. 875 If a media capability number (or range) contains a wildcard character 876 at the end, any payload type mapping specified for that media 877 specific capability (or range of capabilities) will use the wildcard 878 character in the resulting SDP instead of the payload type specified 879 in the payload type mapping ("pt" parameter) in the configuration 880 attribute. 882 A single mscap line may refer to multiple media capabilities by use 883 of a capability number range; this is equivalent to multiple mscap 884 lines, each with the same attribute values (but different media 885 capability numbers), one line per media capability. 887 Multiple mscap lines may refer to the same media capability, but, 888 unlike the mfcap attribute, no concatenation operation is defined. 889 Hence, multiple mscap lines applied to the same media capability is 890 equivalent to multiple lines of the specified attribute in a 891 conventional media record. 893 Here is an example with the rtcp-fb attribute, modified from an 894 example in RFC 5104 [RFC5104] (with the session-level and audio media 895 omitted). If the offer contains a media block like the following 896 (note the wildcard character), 898 m=video 51372 RTP/AVP 98 899 a=rtpmap:98 H263-1998/90000 900 a=tcap:1 RTP/AVPF 901 a=rmcap:1 H263-1998/90000 902 a=mscap:1 rtcp-fb ccm tstr 903 a=mscap:1 rtcp-fb ccm fir 904 a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120 905 a=pcfg:1 t=1 m=1 pt=1:98 907 and if the proposed configuration is chosen, then the equivalent 908 media block would look like 910 m=video 51372 RTP/AVPF 98 911 a=rtpmap:98 H263-1998/90000 912 a=rtcp-fb:98 ccm tstr 913 a=rtcp-fb:98 ccm fir 914 a=rtcp-fb:* ccm tmmbr smaxpr=120 916 3.3.4. New Configuration Parameters 918 Along with the new attributes for media capabilities, new extension 919 parameters are defined for use in the potential configuration, the 920 actual configuration, and/or the new latent configuration defined in 921 Section 3.3.5. 923 3.3.4.1. The Media Configuration Parameter (m=) 925 The media configuration parameter is used to specify the media 926 format(s) and related parameters for a potential, actual, or latent 927 configuration. Adhering to the ABNF for extension-config-list in RFC 928 5939 [RFC5939] with 930 ext-cap-name = "m" 931 ext-cap-list = media-cap-num-list 932 [*(BAR media-cap-num-list)] 934 we have 935 media-config-list = ["+"] "m=" media-cap-num-list 936 *(BAR media-cap-num-list) 937 ;BAR is defined in RFC5939 938 ;media-cap-num-list is defined above 940 Alternative media configurations are separated by a vertical bar 941 ("|"). The alternatives are ordered by preference, most-preferred 942 first. When media capabilities are not included in a potential 943 configuration at the media level, the media type and media format 944 from the associated "m=" line will be used. The use of the plus sign 945 ("+") is described in RFC5939. 947 3.3.4.2. The Payload Type Number Mapping Parameter (pt=) 949 The payload type number mapping parameter is used to specify the 950 payload type number to be associated with each RTP-based media format 951 in a potential, actual, or latent configuration. We define the 952 payload type number mapping parameter, payload-number-config-list, in 953 accordance with the extension-config-list format defined in RFC 5939 954 [RFC5939]. In ABNF: 956 payload-number-config-list = ["+"] "pt=" media-map-list 957 media-map-list = media-map *("," media-map) 958 media-map = media-cap-num ":" payload-type-number 959 ; media-cap-num is defined in 3.3.1 960 payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload 961 ; type number 963 The example in Section 3.3.7 shows how the parameters from the rmcap 964 line are mapped to payload type numbers from the pcfg "pt" parameter. 965 The use of the plus sign ("+") is described in RFC 5939 [RFC5939]. 967 A latent configuration represents a future capability, hence the pt= 968 parameter is not directly meaningful in the lcfg attribute because no 969 actual media session is being offered or accepted; it is permitted in 970 order to tie any payload type number parameters within attributes to 971 the proper media format. A primary example is the case of format 972 parameters for the Redundant Audio Data (RED) payload, which are 973 payload type numbers. Specific payload type numbers used in a latent 974 configuration MAY be interpreted as suggestions to be used in any 975 future offer based on the latent configuration, but they are not 976 binding; the offerer and/or answerer may use any payload type numbers 977 each deems appropriate. The use of explicit payload type numbers for 978 latent configurations can be avoided by use of the parameter 979 substitution rule of Section 3.3.7. Future extensions are also 980 permitted. Note that leading zeroes are not permitted. 982 3.3.4.3. The Media Type Parameter 984 When a latent configuration is specified (always at the media level), 985 indicating the ability to support an additional media stream, it is 986 necessary to specify the media type (audio, video, etc.) as well as 987 the format and transport type. The media type parameter is defined 988 in ABNF as 990 media-type = ["+"] "mt=" media; media defined in RFC4566 992 At present, the media-type parameter is used only in the latent 993 configuration attribute, and the use of the "+" prefix to specify 994 that the entire attribute line is to be ignored if the mt= parameter 995 is not understood, is unnecessary. However, if the media-type 996 parameter is later added to an existing capability attribute such as 997 pcfg, then the "+" would be useful. The media format(s) and 998 transport type(s) are specified using the media configuration 999 parameter ("+m=") defined above, and the transport parameter ("t=") 1000 defined in RFC 5939 [RFC5939], respectively. 1002 3.3.5. The Latent Configuration Attribute 1004 One of the goals of this work is to permit the exchange of 1005 supportable media configurations in addition to those offered or 1006 accepted for immediate use. Such configurations are referred to as 1007 "latent configurations". For example, a party may offer to establish 1008 a session with an audio stream, and, at the same time, announce its 1009 ability to support a video stream as part of the same session. The 1010 offerer can supply its video capabilities by offering one or more 1011 latent video configurations along with the media stream for audio; 1012 the responding party may indicate its ability and willingness to 1013 support such a video session by returning a corresponding latent 1014 configuration. 1016 Latent configurations returned in SDP answers MUST match offered 1017 latent configurations (or parameter subsets thereof). Therefore, it 1018 is appropriate for the offering party to announce most, if not all, 1019 of its capabilities in the initial offer. This choice has been made 1020 in order to keep the size of the answer more compact by not requiring 1021 acap, rmcap, tcap, etc. lines in the answer. 1023 Latent configurations may be announced by use of the latent 1024 configuration attribute, which is defined in a manner very similar to 1025 the potential configuration attribute. The latent configuration 1026 attribute combines the properties of a media line and a potential 1027 configuration. A latent configuration MUST include a media type 1028 (mt=) and a transport protocol configuration parameter since the 1029 latent configuration is independent of any media line present. In 1030 most cases, the media configuration (m=) parameter needs to be 1031 present as well (see Section 4 for examples). The lcfg attribute is 1032 a media level attribute. 1034 The lcfg attribute is defined as a media level attribute since it 1035 specifies a possible future media stream. However the lcfg 1036 attribute is not necessarily related to the media description 1037 within which it is provided. Session capabilities ("sescap") may 1038 be used to indicate this. 1040 Each media line in an SDP description represents an offered 1041 simultaneous media stream, whereas each latent configuration 1042 represents an additional stream which may be negotiated in a future 1043 offer/answer exchange. Session capability attributes may be used to 1044 determine whether a latent configuration may be used to form an offer 1045 for an additional simultaneous stream or to reconfigure an existing 1046 stream in a subsequent offer/answer exchange. 1048 The latent configuration attribute is of the form: 1050 a=lcfg: 1052 which adheres to the SDP [RFC4566] "attribute" production with att- 1053 field and att-value defined as: 1055 att-field = "lcfg" 1056 att-value = config-number 1*WSP lcfg-cfg-list 1057 config-number = NonZeroDigit *9(DIGIT) ; DIGIT defined in RFC5234 1058 lcfg-cfg-list = media-type 1*WSP pot-cfg-list 1059 ; as defined in RFC5939 1060 ; and extended herein 1062 The media-type (mt=) parameter identifies the media type (audio, 1063 video, etc.) to be associated with the latent media stream, and MUST 1064 be present. The pot-cfg-list MUST contain a transport-protocol- 1065 config-list (t=) parameter and a media-config-list (m=) parameter. 1066 The pot-cfg-list MUST NOT contain more than one instance of each type 1067 of parameter list. As specified in RFC 5939 [RFC5939], the use of 1068 the "+" prefix with a parameter indicates that the entire 1069 configuration MUST be ignored if the parameter is not understood; 1070 otherwise, the parameter itself may be ignored. 1072 Media stream payload numbers are not assigned by a latent 1073 configuration. Assignment will take place if and when the 1074 corresponding stream is actually offered via an m-line in a later 1075 exchange. The payload-number-config-list is included as a parameter 1076 to the lcfg attribute in case it is necessary to tie payload numbers 1077 in attribute capabilities to specific media capabilities. 1079 If an lcfg attribute invokes an acap attribute that appears at the 1080 session level, then that attribute will be expected to appear at the 1081 session level of a subsequent offer when and if a corresponding media 1082 stream is offered. Otherwise, acap attributes which appear at the 1083 media level represent media-level attributes. Note, however, that 1084 rmcap, omcap, mfcap, mscap, and tcap attributes may appear at the 1085 session level because they always result in media-level attributes or 1086 m-line parameters. 1088 The configuration numbers for latent configurations do not imply a 1089 preference; the offerer will imply a preference when actually 1090 offering potential configurations derived from latent configurations 1091 negotiated earlier. Note however that the offerer of latent 1092 configurations MAY specify preferences for combinations of potential 1093 and latent configurations by use of the sescap attribute defined in 1094 Section 3.3.8. For example, if an SDP offer contains, say, an audio 1095 stream with pcfg:1, and two latent video configurations, lcfg:2, and 1096 lcfg:3, then a session with one audio stream and one video stream 1097 could be specified by including "a=sescap:1 1,2|3". One audio stream 1098 and two video streams could be specified by including "a=sescap:2 1099 1,2,3" in the offer. In order to permit combinations of latent and 1100 potential configurations in session capabilities, latent 1101 configuration numbers MUST be different from those used for potential 1102 configurations. This restriction is especially important if the 1103 offerer does not require cmed-v0 capability and the recipient of the 1104 offer doesn't support it. If the lcfg attribute is not recognized, 1105 the capability attributes intended to be associated with it may be 1106 confused with those associated with a potential configuration of some 1107 other media stream. Note also that leading zeroes are not permitted 1108 in configuration numbers. 1110 If a cryptographic attribute, such as the SDES "a=crypto:" attribute 1111 [RFC4568], is referenced by a latent configuration through an acap 1112 attribute, any keying material required in the conventional 1113 attribute, such as the SDES key/salt string, MUST be included in 1114 order to satisfy formatting rules for the attribute. Since the 1115 keying material will be visible but not actually used at this stage 1116 (since it's a latent configuration), the value(s) of the keying 1117 material MUST NOT be a real value used for real exchange of media, 1118 and the receiver of the lcfg attribute MUST ignore the values. 1120 3.3.6. Enhanced Potential Configuration Attribute 1122 The present work requires new extensions (parameters) for the pcfg 1123 attribute defined in the SDP Capability Negotiation base protocol 1124 [RFC5939]. The parameters and their definitions are "borrowed" from 1125 the definitions provided for the latent configuration attribute in 1126 Section 3.3.5. The expanded ABNF definition of the pcfg attribute is 1127 a=pcfg: [] 1129 where 1131 config-number = 1*DIGIT ;defined in [RFC5234] 1132 pot-cfg-list = pot-config *(1*WSP pot-config) 1133 pot-config = attribute-config-list / ;def in [RFC5939] 1134 transport-protocol-config-list / ;defined in [RFC5939] 1135 extension-config-list / ;[RFC5939] 1136 media-config-list / ; Section 3.3.4.1 1137 payload-number-config-list ; Section 3.3.4.2 1139 Except for the extension-config-list, the pot-cfg-list MUST NOT 1140 contain more than one instance of each parameter list. 1142 3.3.6.1. Returning Capabilities in the Answer 1144 Potential and/or latent configuration attributes may be returned 1145 within an answer SDP to indicate the ability of the answerer to 1146 support alternative configurations of the corresponding stream(s). 1147 For example, an offer may include multiple potential configurations 1148 for a media stream and/or latent configurations for additional 1149 streams; the corresponding answer will indicate (via an acfg 1150 attribute) the configuration accepted and used to construct the base 1151 configuration for each active media stream in the reply, but the 1152 reply MAY also contain potential and/or latent configuration 1153 attributes, with parameters, to indicate which other offered 1154 configurations would be acceptable. This information is useful if it 1155 becomes desirable to reconfigure a media stream, e.g., to reduce 1156 resource consumption. 1158 When potential and/or latent configurations are returned in an 1159 answer, all numbering MUST refer to the configuration and capability 1160 attribute numbering of the offer. The offered capability attributes 1161 need not be returned in the answer. The answer MAY include 1162 additional capability attributes and/or configurations (with distinct 1163 numbering). The parameter values of any returned pcfg or lcfg 1164 attributes MUST be a subset of those included in the offered 1165 configurations and/or those added by the answerer; values MAY be 1166 omitted only if they were indicated as alternative sets, or optional, 1167 in the original offer. The parameter set indicated in the returned 1168 acfg attribute need not be repeated in a returned pcfg attribute. 1169 The answerer MAY return more than one pcfg attribute with the same 1170 configuration number if it is necessary to describe selected 1171 combinations of optional or alternative parameters. 1173 Similarly, one or more session capability attributes (a=sescap) MAY 1174 be returned to indicate which of the offered session capabilities is/ 1175 are supportable by the answerer (see Section 3.3.8.) 1177 Note that, although the answerer MAY return capabilities beyond those 1178 included by the offerer, these capabilities MUST NOT be used to form 1179 any base level media description in the answer. For this reason, it 1180 is advisable for the offerer to include most, if not all, potential 1181 and latent configurations it can support in the initial offer, unless 1182 the size of the resulting SDP is a concern. Either party MAY later 1183 announce additional capabilities by renegotiating the session in a 1184 second offer/answer exchange. 1186 3.3.6.2. Payload Type Number Mapping 1188 When media format capabilities defined in rmcap attributes are used 1189 in potential configuration lines, the transport protocol uses RTP and 1190 it is necessary to assign payload type numbers. In some cases, it is 1191 desirable to assign different payload type numbers to the same media 1192 format capability when used in different potential configurations. 1193 One example is when configurations for AVP and SAVP are offered: the 1194 offerer would like the answerer to use different payload type numbers 1195 for encrypted and unencrypted media, so the offerer can decide 1196 whether or not to render early media which arrives before the answer 1197 is received. 1199 For example, if use of AVP was selected by the answerer, then 1200 media received by the offerer is not encrypted and hence can be 1201 played out prior to receiving the answer. Conversely, if SAVP was 1202 selected, cryptographic parameters and keying material present in 1203 the answer may be needed to decrypt received media. If the offer 1204 configuration indicated that AVP media uses one set of payload 1205 types and SAVP a different set, then the offerer will know whether 1206 media received prior to the answer is encrypted or not by simply 1207 looking at the RTP payload type number in the received packet. 1209 This association of distinct payload type number(s) with different 1210 transport protocols requires a separate pcfg line for each protocol. 1211 Clearly, this technique cannot be used if the number of potential 1212 configurations exceeds the number of possible payload type numbers. 1214 3.3.6.3. Processing of Media-Format-Related Conventional Attributes for 1215 Potential Configurations 1217 When media capabilities negotiation is employed, SDP records are 1218 likely to contain conventional attributes such as rtpmap, fmtp, and 1219 other media-format-related lines, as well as capability attributes 1220 such as rmcap, omcap, mfcap, and mscap which map into those 1221 conventional attributes when invoked by a potential configuration. 1222 In such cases, it MAY be appropriate to employ the delete-attributes 1223 option [RFC5939] in the attribute configuration list parameter in 1224 order to avoid the generation of conflicting fmtp attributes for a 1225 particular configuration. Any media-specific attributes in the media 1226 block which refer to media formats not used by the potential 1227 configuration MUST be ignored. 1229 For example: 1231 v=0 1232 o=- 25678 753849 IN IP4 192.0.2.1 1233 s= 1234 c=IN IP4 192.0.2.1 1235 t=0 0 1236 a=creq:med-v0 1237 m=audio 3456 RTP/AVP 0 18 100 1238 a=rtpmap:100 telephone-event 1239 a=fmtp:100 0-11 1240 a=rmcap:1 PCMU/8000 1241 a=rmcap:2 G729/8000 1242 a=rmcap:3 telephone-event/8000 1243 a=mfcap:3 0-15 1244 a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100 1245 a=pcfg:2 1247 In this example, PCMU is media capability 1, G729 is media capability 1248 2, and telephone-event is media capability 3. The a=pcfg:1 line 1249 specifies that the preferred configuration is G.729 with extended 1250 dtmf events, second is G.711 mu-law with extended dtmf events, and 1251 the base media-level attributes are to be deleted. Intermixing of 1252 G.729, G.711, and "commercial" dtmf events is least preferred (the 1253 base configuration provided by the "m=" line, which is, by default, 1254 the least preferred configuration). The rtpmap and fmtp attributes 1255 of the base configuration are replaced by the rmcap and mfcap 1256 attributes when invoked by the proposed configuration. 1258 If the preferred configuration is selected, the SDP answer will look 1259 like 1261 v=0 1262 o=- 25678 753849 IN IP4 192.0.2.1 1263 s= 1264 c=IN IP4 192.0.2.1 1265 t=0 0 1266 a=csup:med-v0 1267 m=audio 3456 RTP/AVP 18 100 1268 a=rtpmap:100 telephone-event/8000 1269 a=fmtp:100 0-15 1270 a=acfg:1 m=2,3 pt=1:0,2:18,3:100 1272 3.3.7. Substitution of Media Payload Type Numbers in Capability 1273 Attribute Parameters 1275 In some cases, for example, when an RFC 2198 [RFC2198] redundancy 1276 audio subtype (RED) capability is defined in an mfcap attribute, the 1277 parameters to an attribute may contain payload type numbers. Two 1278 options are available for specifying such payload type numbers. They 1279 may be expressed explicitly, in which case they are bound to actual 1280 payload types by means of the payload type number parameter (pt=) in 1281 the appropriate potential or latent configuration. For example, the 1282 following SDP fragment defines a potential configuration with 1283 redundant G.711 mu-law: 1285 m=audio 45678 RTP/AVP 0 1286 a=rtpmap:0 PCMU/8000 1287 a=rmcap:1 PCMU/8000 1288 a=rmcap:2 RED/8000 1289 a=mfcap:2 0/0 1290 a=pcfg:1 m=2,1 pt=2:98,1:0 1292 The potential configuration is then equivalent to 1294 m=audio 45678 RTP/AVP 98 0 1295 a=rtpmap:0 PCMU/8000 1296 a=rtpmap:98 RED/8000 1297 a=fmtp:98 0/0 1299 A more general mechanism is provided via the parameter substitution 1300 rule. When an mfcap, mscap, or acap attribute is processed, its 1301 arguments will be scanned for a payload type number escape sequences 1302 of the following form (in ABNF): 1304 ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1 1306 If the sequence is found, the sequence is replaced by the payload 1307 type number assigned to the media capability number, as specified by 1308 the pt= parameter in the selected potential configuration; only 1309 actual payload type numbers are supported - wildcards are excluded. 1310 The sequence "%%" (null digit string) is replaced by a single percent 1311 sign and processing continues with the next character, if any. 1313 For example, the above offer sequence could have been written as 1315 m=audio 45678 RTP/AVP 0 1316 a=rtpmap:0 PCMU/8000 1317 a=rmcap:1 PCMU/8000 1318 a=rmcap:2 RED/8000 1319 a=mfcap:2 %m=1%/%m=1% 1320 a=pcfg:1 m=2,1 pt=2:98,1:0 1322 and the equivalent SDP is the same as above. 1324 3.3.8. The Session Capability Attribute 1326 Potential and latent configurations enable offerers and answerers to 1327 express a wide range of alternative configurations for current and 1328 future negotiation. However in practice, it may not be possible to 1329 support all combinations of these configurations. 1331 The session capability attribute provides a means for the offerer 1332 and/or the answerer to specify combinations of specific media stream 1333 configurations which it is willing and able to support. Each session 1334 capability in an offer or answer MAY be expressed as a list of 1335 required potential configurations, and MAY include a list of optional 1336 potential and/or latent configurations. 1338 The choices of session capabilities may be based on processing load, 1339 total bandwidth, or any other criteria of importance to the 1340 communicating parties. If the answerer supports media capabilities 1341 negotiation, and session configurations are offered, it MUST accept 1342 one of the offered configurations, or it MUST refuse the session. 1343 Therefore, if the offer includes any session capabilities, it SHOULD 1344 include all the session capabilities the offerer is willing to 1345 support. 1347 The session capability attribute is a session-level attribute 1348 described by: 1350 "a=sescap:" 1352 which corresponds to the standard value attribute definition with 1354 att-field = "sescap" 1355 att-value = session-num 1*WSP list-of-configs 1356 [1*WSP optional-configs] 1357 session-num = NonZeroDigit *9(DIGIT) ; DIGIT defined 1358 ; in RFC5234 1359 list-of-configs = alt-config *("," alt-config) 1360 optional-configs = "[" list-of-configs "]" 1361 alt-config = config-number *("|" config-number) 1363 The session-num identifies the session: a lower-number session is 1364 preferred over a higher-number session, and leading zeroes are not 1365 permitted. Each alt-config list specifies alternative media 1366 configurations within the session; preference is based on config-num 1367 as specified in RFC 5939 [RFC5939]. Note that the session preference 1368 order, when present, takes precedence over the individual media 1369 stream configuration preference order. 1371 Use of session capability attributes requires that configuration 1372 numbers assigned to potential and latent configurations MUST be 1373 unique across the entire session; RFC 5939 [RFC5939] requires only 1374 that pcfg configuration numbers be unique within a media description. 1375 Also, leading zeroes are not permitted. 1377 As an example, consider an endpoint that is capable of supporting an 1378 audio stream with either one H.264 video stream or two H.263 video 1379 streams with a floor control stream. In the latter case, the second 1380 video stream is optional. The SDP offer might look like the 1381 following (offering audio, an H.263 video streams, BFCP and another 1382 optional H.263 video stream)- the empty lines are added for 1383 readability only (not part of valid SDP): 1385 v=0 1386 o=- 25678 753849 IN IP4 192.0.2.1 1387 s= 1388 c=IN IP4 192.0.2.1 1389 t=0 0 1390 a=creq:med-v0 1391 a=sescap:2 1,2,5,[3] 1392 a=sescap:1 1,4 1394 m=audio 54322 RTP/AVP 0 1395 a=rtpmap:0 PCMU/8000 1396 a=pcfg:1 1398 m=video 22344 RTP/AVP 102 1399 a=rtpmap:102 H263-1998/90000 1400 a=fmtp:102 CIF=4;QCIF=2;F=1;K=1 1401 i=main video stream 1402 a=label:11 1403 a=pcfg:2 1404 a=rmcap:1 H264/90000 1405 a=mfcap:1 profile-level-id=42A01E; packetization-mode=2 1406 a=acap:1 label:13 1407 a=pcfg:4 m=1 a=1 pt=1:104 1409 m=video 33444 RTP/AVP 103 1410 a=rtpmap:103 H263-1998/90000 1411 a=fmtp:103 CIF=4;QCIF=2;F=1;K=1 1412 i=secondary video (slides) 1413 a=label:12 1414 a=pcfg:3 1415 m=application 33002 TCP/BFCP * 1416 a=setup:passive 1417 a=connection:new 1418 a=floorid:1 m-stream:11 12 1419 a=floor-control:s-only 1420 a=confid:4321 1421 a=userid:1234 1422 a=pcfg:5 1424 If the answerer understands MediaCapNeg, but cannot support the 1425 Binary Floor Control Protocol, then it would respond with (invalid 1426 empty lines in SDP included again for readability): 1428 v=0 1429 o=- 25678 753849 IN IP4 192.0.2.1 1430 s= 1431 c=IN IP4 192.0.2.22 1432 t=0 0 1433 a=csup:med-v0 1434 a=sescap:1 1,4 1436 m=audio 23456 RTP/AVP 0 1437 a=rtpmap:0 PCMU/8000 1438 a=acfg:1 1440 m=video 41234 RTP/AVP 104 1441 a=rtpmap:104 H264/90000 1442 a=fmtp:104 profile-level-id=42A01E; packetization-mode=2 1443 a=acfg:4 m=1 a=1 pt=1:104 1445 m=video 0 RTP/AVP 103 1446 a=acfg:3 1448 m=application 0 TCP/BFCP * 1449 a=acfg:5 1451 An endpoint that doesn't support Media capabilities negotiation, but 1452 does support H.263 video, would respond with one or two H.263 video 1453 streams. In the latter case, the answerer may issue a second offer 1454 to reconfigure the session to one audio and one video channel using 1455 H.264 or H.263. 1457 Session capabilities can include latent capabilities as well. Here's 1458 a similar example in which the offerer wishes to initially establish 1459 an audio stream, and prefers to later establish two video streams 1460 with chair control. If the answerer doesn't understand Media CapNeg, 1461 or cannot support the dual video streams or flow control, then it may 1462 support a single H.264 video stream. Note that establishment of the 1463 most favored configuration will require two offer/answer exchanges. 1465 v=0 1466 o=- 25678 753849 IN IP4 192.0.2.1 1467 s= 1468 c=IN IP4 192.0.2.1 1469 t=0 0 1470 a=creq:med-v0 1471 a=sescap:1 1,3,4,5 1472 a=sescap:2 1,2 1473 a=sescap:3 1 1474 a=rmcap:1 H263-1998/90000 1475 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 1476 a=tcap:1 RTP/AVP TCP/BFCP 1477 m=audio 54322 RTP/AVP 0 1478 a=rtpmap:0 PCMU/8000 1479 a=pcfg:1 1480 m=video 22344 RTP/AVP 102 1481 a=rtpmap:102 H264/90000 1482 a=fmtp:102 profile-level-id=42A01E; packetization-mode=2 1483 a=label:11 1484 a=content:main 1485 a=pcfg:2 1486 a=lcfg:3 mt=video t=1 m=1 a=31,32 1487 a=acap:31 label:12 1488 a=acap:32 content:main 1489 a=lcfg:4 mt=video t=1 m=1 a=41,42 1490 a=acap:41 label:13 1491 a=acap:42 content:slides 1492 a=lcfg:5 mt=application m=51 t=51 1493 a=tcap:51 TCP/BFCP 1494 a=omcap:51 * 1495 a=acap:51 setup:passive 1496 a=acap:52 connection:new 1497 a=acap:53 floorid:1 m-stream:12 13 1498 a=acap:54 floor-control:s-only 1499 a=acap:55 confid:4321 1500 a=acap:56 userid:1234 1502 In this example, the default offer, as seen by endpoints which do not 1503 understand capabilities negotiation, proposes a PCMU audio stream and 1504 an H.264 video stream. Note that the offered lcfg lines for the 1505 video streams don't carry pt= parameters because they're not needed 1506 (payload type numbers will be assigned in the offer/answer exchange 1507 that establishes the streams). Note also that the three rmcap, 1508 mfcap, and tcap attributes used by lcfg:3 and lcfg:4 are included at 1509 the session level so they may be referenced by both latent 1510 configurations. As per Section 3.3, the media attributes generated 1511 from the rmcap, mfcap, and tcap attributes are always media-level 1512 attributes. If the answerer supports Media CapNeg, and supports the 1513 most desired configuration, it would return the following SDP: 1515 v=0 1516 o=- 25678 753849 IN IP4 192.0.2.1 1517 s= 1518 c=IN IP4 192.0.2.22 1519 t=0 0 1520 a=csup:med-v0 1521 a=sescap:1 1,3,4,5 1522 a=sescap:2 1,2 1523 a=sescap:3 1 1524 m=audio 23456 RTP/AVP 0 1525 a=rtpmap:0 PCMU/8000 1526 a=acfg:1 1527 m=video 0 RTP/AVP 102 1528 a=pcfg:2 1529 a=lcfg:3 mt=video t=1 m=1 a=31,32 1530 a=lcfg:4 mt=video t=1 m=1 a=41,42 1531 a=lcfg:5 mt=application t=2 1533 This exchange supports immediate establishment of an audio stream for 1534 preliminary conversation. This exchange would presumably be followed 1535 at the appropriate time with a "reconfiguration" offer/answer 1536 exchange to add the video and chair control streams. 1538 3.4. Offer/Answer Model Extensions 1540 In this section, we define extensions to the offer/answer model 1541 defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for 1542 media format and associated parameter capabilities, latent 1543 configurations and acceptable combinations of media stream 1544 configurations to be used with the SDP Capability Negotiation 1545 framework. Note that the procedures defined in this section extend 1546 the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6; 1547 those procedures form a baseline set of capability negotiation offer/ 1548 answer procedures that MUST be followed, subject to the extensions 1549 defined here. 1551 SDP Capability Negotiation [RFC5939] provides a relatively compact 1552 means to offer the equivalent of an ordered list of alternative 1553 configurations for offered media streams (as would be described by 1554 separate m= lines and associated attributes). The attributes acap, 1555 mscap, mfcap, omcap and rmcap are designed to map somewhat 1556 straightforwardly into equivalent m= lines and conventional 1557 attributes when invoked by a pcfg, lcfg, or acfg attribute with 1558 appropriate parameters. The a=pcfg: lines, along with the m= line 1559 itself, represent offered media configurations. The a=lcfg: lines 1560 represent alternative capabilities for future use. 1562 3.4.1. Generating the Initial Offer 1564 The Media Capabilities negotiation extensions defined in this 1565 document cover the following categories of features: 1567 o Media Format Capabilities and associated parameters (rmcap, omcap, 1568 mfcap, and mscap attributes) 1570 o Potential configurations using those media format capabilities and 1571 associated parameters 1573 o Latent media streams (lcfg attribute) 1575 o Acceptable combinations of media stream configurations (sescap 1576 attribute). 1578 The high-level description of the operation is as follows: 1580 When an endpoint generates an initial offer and wants to use the 1581 functionality described in the current document, it SHOULD identify 1582 and define the media formats and associated parameters it can support 1583 via the rmcap, omcap, mfcap and mscap attributes. The SDP media 1584 line(s) ("m=") should be made up with the actual configuration to be 1585 used if the other party does not understand capability negotiations 1586 (by default, this is the least preferred configuration). Typically, 1587 the media line configuration will contain the minimum acceptable 1588 configuration from the offerer's point of view. 1590 Preferred configurations for each media stream are identified 1591 following the media line. The present offer may also include latent 1592 configuration (lcfg) attributes, at the media level, describing media 1593 streams and/or configurations the offerer is not now offering, but 1594 which it is willing to support in a future offer/answer exchange. A 1595 simple example might be the inclusion of a latent video configuration 1596 in an offer for an audio stream. 1598 Lastly, if the offerer wishes to impose restrictions on the 1599 combinations of potential configurations to be used, it will include 1600 session capability (sescap) attributes indicating those. 1602 If the offerer requires the answerer to understand the media 1603 capability extensions, the offerer MUST include a creq attribute 1604 containing the value "med-v0". If media capability negotiation is 1605 required only for specific media descriptions, the "med-v0" value 1606 MUST be provided only in creq attributes within those media 1607 descriptions, as described in RFC 5939 [RFC5939]. 1609 Below, we provide a more detailed description of how to construct the 1610 offer SDP. 1612 3.4.1.1. Offer with Media Capabilities 1614 For each RTP-based media format the offerer wants to include as a 1615 media format capability, the offer MUST include an "rmcap" attribute 1616 for the media format as defined in Section 3.3.1. 1618 For each non RTP-based media format the offer wants to include as a 1619 media format capability, the offer MUST include an "omcap" attribute 1620 for the media format as defined in Section 3.3.1. 1622 Since the media capability number space is shared between the rmcap 1623 and omcap attributes, each media capability number provided 1624 (including ranges) MUST be unique in the entire SDP. 1626 If an "fmtp" parameter value is needed for a media format (whether 1627 RTP-based or not) in a media capability, then the offer MUST include 1628 one or more "mfcap" parameters with the relevant fmtp parameter 1629 values for that media format as defined in Section 3.3.2. When 1630 multiple "mfcap" parameters are provided for a given media 1631 capability, they MUST be provided in accordance with the 1632 concatenation rules in Section 3.3.2.1. 1634 For each of the media format capabilities above, the offer MAY 1635 include one or more "mscap" parameters with attributes needed for 1636 those specific media formats as defined in Section 3.3.3. Such 1637 attributes will be instantiated at the media-level, and hence 1638 session-level only attributes MUST NOT be used in the "mscap" 1639 parameter. The "mscap" parameter MUST NOT include an "rtpmap" or 1640 "fmtp" attribute (rmcap and mfcap are used instead). 1642 If the offerer wants to limit the relevance (and use) of a media 1643 format capability or parameter to a particular media stream, the 1644 media format capability or parameter MUST be provided within the 1645 corresponding media description. Otherwise, the media format 1646 capabilities and parameters MUST be provided at the session level. 1647 Note however, that the attribute or parameter embedded in these will 1648 always be instantiated at the media-level. 1650 This is due to those parameters being effectively media-level 1651 parameters. If session-level attributes are needed, the "acap" 1652 attribute defined in RFC 5939 [RFC5939] can be used, however it 1653 does not provide for media format-specific instantiation. 1655 Inclusion of the above does not constitute an offer to use the 1656 capabilities; a potential configuration is needed for that. If the 1657 offerer wants to offer one or more of the media capabilities above, 1658 they MUST be included as part of a potential configuration (pcfg) 1659 attribute as defined in Section 3.3.4. Each potential configuration 1660 MUST include a config-number, and each config-number MUST be unique 1661 in the entire SDP (note that this differs from RFC 5939 [RFC5939], 1662 which only requires uniqueness within a media description). Also, 1663 the config-number MUST NOT overlap with any config-number used by a 1664 latent configuration in the SDP. As described in RFC 5939 [RFC5939], 1665 lower config-numbers indicate a higher preference; the ordering still 1666 applies within a given media description only though. 1668 For a media capability to be included in a potential configuration, 1669 there MUST be an "m=" parameter in the pcfg attribute referencing the 1670 media capability number in question. When one or more media 1671 capabilities are included in an offered potential configuration 1672 (pcfg), they completely replace the list of media formats offered in 1673 the actual configuration (m= line). Any attributes included for 1674 those formats remain in the SDP though (e.g., rtpmap, fmtp, etc.). 1675 For non-RTP based media formats, the format-name (from the "omcap" 1676 media capability) is simply added to the "m=" line as a media format 1677 (e.g. t38). For RTP-based media, payload type mappings MUST be 1678 provided by use of the "pt" parameter in the potential configuration 1679 (see Section 3.3.4.2); payload type escaping may be used in mfcap, 1680 mscap, and acap attributes as defined in Section 3.3.7. 1682 Note that the "mt" parameter MUST NOT be used with the pcfg attribute 1683 (since it is defined for the lcfg attribute only); the media type in 1684 a potential configuration cannot be changed from that of the 1685 encompassing media description. 1687 3.4.1.2. Offer with Latent Configuration 1689 If the offerer wishes to offer one or more latent configurations for 1690 future use, the offer MUST include a latent configuration attribute 1691 (lcfg) for each as defined in Section 3.3.5. 1693 Each lcfg attribute 1695 o MUST be specified at the media level 1697 o MUST include a config-number that is unique in the entire SDP 1698 (incl. for any potential configuration attributes). Note that 1699 config-numbers in latent configurations do not indicate any 1700 preference order 1702 o MUST include a media type ("mt") 1704 o MUST reference a valid transport capability ("t") 1706 Each lcfg attribute MAY include additional capability references, 1707 which may refer to capabilities anywhere in the session description, 1708 subject to any restrictions normally associated with such 1709 capabilities. For example, a media-level attribute capability must 1710 be present at the media-level in some media description in the SDP. 1711 Note that this differs from the potential configuration attribute, 1712 which cannot validly refer to media-level capabilities in another 1713 media description (per RFC 5939 [RFC5939], Section 3.5.1). 1715 Potential configurations constitute an actual offer and hence may 1716 instantiate a referenced capability. Latent configurations are 1717 not actual offers and hence cannot instantiate a referenced 1718 capability; it is therefore safe for those to refer to 1719 capabilities in another media description. 1721 3.4.1.3. Offer with Configuration Combination Restrictions 1723 If the offerer wants to indicate restrictions or preferences among 1724 combinations of potential and/or latent configuration, a session 1725 capability (sescap) attribute MUST be provided at the session-level 1726 for each such combination as described in Section 3.3.8. Each sescap 1727 attribute MUST include a session-num that is unique in the entire 1728 SDP; the lower the session-num the more preferred that combination 1729 is. Furthermore, sescap preference order takes precedence over any 1730 order specified in individual pcfg attributes. 1732 For example, if we have pcfg-1 and pcfg-2, and sescap-1 references 1733 pcfg-2, whereas sescap-2 references pcfg-1, then pcfg-2 will be 1734 the most preferred potential configuration. Without the sescap, 1735 pcfg-1 would be the most preferred. 1737 3.4.2. Generating the Answer 1739 When receiving an offer, the answerer MUST check the offer for creq 1740 attributes containing the value "med-v0"; answerers compliant with 1741 this specification will support this value in accordance with the 1742 procedures specified in RFC 5939 [RFC5939]. 1744 The SDP MAY contain 1746 o Media format capabilities and associated parameters (rmcap, omcap, 1747 mfcap, and mscap attributes) 1749 o Potential configurations using those media format capabilities and 1750 associated parameters 1752 o Latent media streams (lcfg attribute) 1754 o Acceptable combinations of media stream configurations (sescap 1755 attribute) 1757 The high-level informative description of the operation is as 1758 follows: 1760 When the answering party receives the offer and if it supports the 1761 required capability negotiation extensions, it should select the 1762 most-preferred configuration it can support for each media stream, 1763 and build its answer accordingly. The configuration selected for 1764 each accepted media stream is placed into the answer as a media line 1765 with associated parameters and attributes. If a proposed 1766 configuration is chosen for a given media stream, the answer must 1767 contain an actual configuration (acfg) attribute for that media 1768 stream to indicate which offered pcfg attribute was used to build the 1769 answer. The answer should also include any potential or latent 1770 configurations the answerer can support, especially any 1771 configurations compatible with other potential or latent 1772 configurations received in the offer. The answerer should make note 1773 of those configurations it might wish to offer in the future. 1775 Below we provide a more detailed normative description of how the 1776 answerer processes the offer SDP and generates an answer SDP. 1778 3.4.2.1. Processing Media Capabilities and Potential Configurations 1780 The answerer MUST first determine if it needs to perform media 1781 capability negotiation by examining the SDP for valid and preferred 1782 potential configuration attributes that include media configuration 1783 parameters (i.e., an "m" parameter in the pcfg attribute). 1785 Such a potential configuration is valid if: 1787 1. It is valid according to the rules defined in RFC 5939 [RFC5939] 1789 2. It contains a config-number that is unique in the entire SDP and 1790 does not overlap with any latent configuration config-numbers 1792 3. All media format capabilities (rmcap or omcap), media format 1793 parameter capabilities (mfcap), and media-specific capabilities 1794 (mscap) referenced by the potential configuration ("m" parameter) 1795 are valid themselves (as defined in Section 3.3.1, 3.3.2, and 1796 3.3.3) and each of them is provided either at the session level 1797 or within this particular media description. 1799 4. All RTP-based media format capabilities (rmcap) have a 1800 corresponding payload type ("pt") parameter in the potential 1801 configuration that result in mapping to a valid payload type that 1802 is unique within the resulting SDP. 1804 5. Any concatenation (see Section 3.3.2.1) and substitution (see 1805 Section 3.3.7) applied to any capability (mfcap, mscap, or acap) 1806 referenced by this potential configuration results in a valid 1807 SDP. 1809 Note that, since SDP does not interpret the value of fmtp parameters, 1810 any resulting fmtp parameter value will be considered valid. 1812 Secondly, the answerer MUST determine the order in which potential 1813 configurations are to be negotiated. In the absence of any Session 1814 Capability ("sescap") attributes, this simply follows the rules of 1815 RFC 5939 [RFC5939], with a lower config-number within a media 1816 description being preferred over a higher one. If a valid "sescap" 1817 attribute is present, the preference order provided in the "sescap" 1818 attribute MUST take precedence. A "sescap" attribute is considered 1819 valid if: 1821 1. It adheres to the rules provided in Section 3.3.8. 1823 2. All the configurations referenced by the "sescap" attribute are 1824 valid themselves (note that this can include the actual, 1825 potential and latent configurations). 1827 The answerer MUST now process the offer for each media stream based 1828 on the most preferred valid potential configuration in accordance 1829 with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2, 1830 and further extended below: 1832 o If one or more media format capabilities are included in the 1833 potential configuration, then they replace all media formats 1834 provided in the "m=" line for that media description. For non-RTP 1835 based media formats (omcap), the format-name is added. For RTP- 1836 based media formats (rmcap), the payload-type specified in the 1837 payload-type mapping ("pt") is added and a corresponding "rtpmap" 1838 attribute is added to the media description. 1840 o If one or more media format parameter capabilities are included in 1841 the potential configuration, then the corresponding "fmtp" 1842 attributes are added to the media description. Note that this 1843 inclusion is done indirectly via the media format capability. 1845 o If one or more media-specific capabilities are included in the 1846 potential configuration, then the corresponding attributes are 1847 added to the media description. Note that this inclusion is done 1848 indirectly via the media format capability. 1850 o When checking to see if the answerer supports a given potential 1851 configuration that includes one or more media format capabilities, 1852 the answerer MUST support at least one of the media formats 1853 offered. If he does not, the answerer MUST proceed to the next 1854 potential configuration based on the preference order that 1855 applies. 1857 o If Session Capability ("sescap") preference ordering is included, 1858 then the potential configuration selection process MUST adhere to 1859 the ordering provided. Note that this may involve coordinated 1860 selection of potential configurations between media descriptions. 1861 The answerer MUST accept one of the offered "sescap" combinations 1862 (i.e. all the required potential configurations specified) or it 1863 MUST reject the entire session. 1865 Once the answerer has selected a valid and supported offered 1866 potential configuration for all of the media streams (or has fallen 1867 back to the actual configuration plus any added session attributes), 1868 the answerer MUST generate a valid answer SDP as described in RFC 1869 5939 [RFC5939], Section 3.6.2, and further extended below: 1871 o Additional answer capabilities and potential configurations MAY be 1872 returned in accordance with Section 3.3.6.1. Capability numbers 1873 and configuration numbers for those MUST be distinct from the ones 1874 used in the offer SDP. 1876 o Latent configuration processing and answer generation MUST be 1877 performed, as specified below. 1879 o Session capability specification for the potential and latent 1880 configurations in the answer MAY be included (see Section 3.3.8). 1882 3.4.2.2. Latent Configuration Processing 1884 The answerer MUST determine if it needs to perform any latent 1885 configuration processing by examining the SDP for valid latent 1886 configuration attributes (lcfg). An lcfg attribute is considered 1887 valid if: 1889 o It adheres to the description in Section 3.3.5. 1891 o It includes a config-number that is unique in the entire SDP and 1892 does not overlap with any potential configuration config-number 1894 o It includes a valid media type ("mt=") 1896 o It references a valid transport capability ("t=") 1898 o All other capabilities referenced by it are valid. 1900 For each such valid latent configuration in the offer, the answerer 1901 checks to see if it could support the latent configuration in a 1902 subsequent offer/answer exchange. If so, it includes the latent 1903 configuration with the same configuration number in the answer, 1904 similar to the way potential configurations are processed and the 1905 selected one returned in an actual configuration attribute (see RFC 1906 5939 [RFC5939]). If the answerer supports only a (non-mandatory) 1907 subset of the parameters offered in a latent configuration, the 1908 answer latent configuration will include only those parameters 1909 supported (similar to "acfg" processing). Note that latent 1910 configurations do not constitute an actual offer at this point in 1911 time; they merely indicate additional configurations that could be 1912 supported. 1914 If a Session Capability ("sescap") attribute is included and it 1915 references a latent configuration, then the answerer processing of 1916 that latent configuration must be done within the constraints 1917 specified by that Session Capability, i.e. it must be possible to 1918 support it at the same time as any required (i.e. non-optional) 1919 potential configurations in the session capability. The answerer may 1920 in turn add his own "sescap" indications in the answer as well. 1922 3.4.3. Offerer Processing of the Answer 1924 The offerer MUST process the answer in accordance with RFC 5939 1925 [RFC5939] Section 3.6.3, and further explained below. 1927 When the offerer processes the answer SDP based on a valid actual 1928 configuration attribute in the answer, and that valid configuration 1929 includes one or more media capabilities, the processing MUST 1930 furthermore be done as if the offer was sent using those media 1931 capabilities instead of the actual configuration. In particular, the 1932 media formats in the "m=" line, and any associated payload type 1933 mappings (rtpmap), fmtp parameters (mfcap) and media-specific 1934 attributes (mscap) MUST be used. Note that this may involve use of 1935 concatenation and substitution rules (see Section 3.3.2.1 and 3.3.7). 1936 The actual configuration attribute may also be used to infer the lack 1937 of acceptability of higher-preference configurations that were not 1938 chosen, subject to any constraints provided by a Session Capability 1939 attribute ("sescap") in the offer. Note that the SDP Capability 1940 Negotiation base specification [RFC5939] requires the answerer to 1941 choose the highest preference configuration it can support, subject 1942 to local policies. 1944 When the offerer receives the answer, it SHOULD furthermore make note 1945 of any capabilities and/or latent configurations included for future 1946 use, and any constraints on how those may be combined. 1948 3.4.4. Modifying the Session 1950 If, at a later time, one of the parties wishes to modify the 1951 operating parameters of a session, e.g., by adding a new media 1952 stream, or by changing the properties used on an existing stream, it 1953 can do so via the mechanisms defined for offer/answer [RFC3264]. If 1954 the initiating party has remembered the codecs, potential 1955 configurations, latent configurations and session capabilities 1956 provided by the other party in the earlier negotiation, it MAY use 1957 this knowledge to maximize the likelihood of a successful 1958 modification of the session. Alternatively, the initiator MAY 1959 perform a new capabilities exchange as part of the reconfiguration. 1960 In such a case, the new capabilities will replace the previously- 1961 negotiated capabilities. This may be useful if conditions change on 1962 the endpoint. 1964 4. Examples 1966 In this section, we provide examples showing how to use the Media 1967 Capabilities with the SDP Capability Negotiation. 1969 4.1. Alternative Codecs 1971 This example provides a choice of one of six variations of the 1972 adaptive multirate codec. In this example, the default configuration 1973 as specified by the media line is the same as the most preferred 1974 configuration. Each configuration uses a different payload type 1975 number so the offerer can interpret early media. 1977 v=0 1978 o=- 25678 753849 IN IP4 192.0.2.1 1979 s= 1980 c=IN IP4 192.0.2.1 1981 t=0 0 1982 a=creq:med-v0 1983 m=audio 54322 RTP/AVP 96 1984 a=rtpmap:96 AMR-WB/16000/1 1985 a=fmtp:96 mode-change-capability=1; max-red=220; \ 1986 mode-set=0,2,4,7 1987 a=rmcap:1,3,5 audio AMR-WB/16000/1 1988 a=rmcap:2,4,6 audio AMR/8000/1 1989 a=mfcap:1,2,3,4 mode-change-capability=1 1990 a=mfcap:5,6 mode-change-capability=2 1991 a=mfcap:1,2,3,5 max-red=220 1992 a=mfcap:3,4,5,6 octet-align=1 1993 a=mfcap:1,3,5 mode-set=0,2,4,7 1994 a=mfcap:2,4,6 mode-set=0,3,5,6 1995 a=pcfg:1 m=1 pt=1:96 1996 a=pcfg:2 m=2 pt=2:97 1997 a=pcfg:3 m=3 pt=3:98 1998 a=pcfg:4 m=4 pt=4:99 1999 a=pcfg:5 m=5 pt=5:100 2000 a=pcfg:6 m=6 pt=6:101 2002 In the above example, media capability 1 could have been excluded 2003 from the first rmcap declaration and from the corresponding mfcap 2004 attributes, and the pcfg:1 attribute line could have been simply 2005 "pcfg:1". 2007 The next example offers a video stream with three options of H.264 2008 and 4 transports. It also includes an audio stream with different 2009 audio qualities: four variations of AMR, or AC3. The offer looks 2010 something like: 2012 v=0 2013 o=- 25678 753849 IN IP4 192.0.2.1 2014 s=An SDP Media NEG example 2015 c=IN IP4 192.0.2.1 2016 t=0 0 2017 a=creq:med-v0 2018 a=ice-pwd:speEc3QGZiNWpVLFJhQX 2019 m=video 49170 RTP/AVP 100 2020 c=IN IP4 192.0.2.56 2021 a=maxprate:1000 2022 a=rtcp:51540 2023 a=sendonly 2024 a=candidate 12345 1 UDP 9 192.0.2.56 49170 host 2025 a=candidate 23456 2 UDP 9 192.0.2.56 51540 host 2026 a=candidate 34567 1 UDP 7 198.51.100.1 41345 srflx raddr \ 2027 192.0.2.56 rport 49170 2028 a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \ 2029 192.0.2.56 rport 51540 2030 a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \ 2031 192.0.2.56 rport 49170 2032 a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \ 2033 192.0.2.56 rport 51540 2034 b=AS:10000 2035 b=TIAS:10000000 2036 b=RR:4000 2037 b=RS:3000 2038 a=rtpmap:100 H264/90000 2039 a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \ 2040 sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \ 2041 sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \ 2042 sprop-init-buf-time=102478; deint-buf-cap=128000 2043 a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF 2044 a=rmcap:1-3,7-9 H264/90000 2045 a=rmcap:4-6 rtx/90000 2046 a=mfcap:1-9 profile-level-id=42A01E 2047 a=mfcap:1-9 aMljiA== 2048 a=mfcap:1,4,7 packetization-mode=0 2049 a=mfcap:2,5,8 packetization-mode=1 2050 a=mfcap:3,6,9 packetization-mode=2 2051 a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI 2052 a=mfcap:1,7 sprop-interleaving-depth=45; \ 2053 sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \ 2054 deint-buf-cap=128000 2055 a=mfcap:4 apt=100 2056 a=mfcap:5 apt=99 2057 a=mfcap:6 apt=98 2058 a=mfcap:4-6 rtx-time=3000 2059 a=mscap:1-6 rtcp-fb nack 2060 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \ 2061 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 2062 a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97 2063 a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96 2064 a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95 2065 a=pcfg:4 t=2 m=7 a=1 pt=7:100 2066 a=pcfg:5 t=2 m=8 a=1 pt=8:99 2067 a=pcfg:6 t=2 m=9 a=1 pt=9:98 2068 a=pcfg:7 t=3 m=1,3 pt=1:100,4:97 2069 a=pcfg:8 t=3 m=2,4 pt=2:99,4:96 2070 a=pcfg:9 t=3 m=3,6 pt=3:98,6:95 2071 m=audio 49176 RTP/AVP 101 100 99 98 2072 c=IN IP4 192.0.2.56 2073 a=ptime:60 2074 a=maxptime:200 2075 a=rtcp:51534 2076 a=sendonly 2077 a=candidate 12345 1 UDP 9 192.0.2.56 49176 host 2078 a=candidate 23456 2 UDP 9 192.0.2.56 51534 host 2079 a=candidate 34567 1 UDP 7 198.51.100.1 41348 srflx \ 2080 raddr 192.0.2.56 rport 49176 2081 a=candidate 45678 2 UDP 7 198.51.100.1 52569 srflx \ 2082 raddr 192.0.2.56 rport 51534 2083 a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \ 2084 raddr 192.0.2.56 rport 49176 2085 a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \ 2086 raddr 192.0.2.56 rport 51534 2087 b=AS:512 2088 b=TIAS:512000 2089 b=RR:4000 2090 b=RS:3000 2091 a=maxprate:120 2092 a=rtpmap:98 AMR-WB/16000 2093 a=fmtp:98 octet-align=1; mode-change-capability=2 2094 a=rtpmap:99 AMR-WB/16000 2095 a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2 2096 a=rtpmap:100 AMR-WB/16000/2 2097 a=fmtp:100 octet-align=1; interleaving=30 2098 a=rtpmap:101 AMR-WB+/72000/2 2099 a=fmtp:101 interleaving=50; int-delay=160000; 2100 a=rmcap:14 ac3/48000/6 2101 a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \ 2102 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 2103 a=tcap:4 RTP/SAVP 2104 a=pcfg:10 t=4 a=23 2105 a=pcfg:11 t=4 m=14 a=23 pt=14:102 2107 This offer illustrates the advantage in compactness that arises if 2108 one can avoid deleting the base configuration attributes and 2109 recreating them in acap attributes for the potential configurations. 2111 4.2. Alternative Combinations of Codecs (Session Configurations) 2113 If an endpoint has limited signal processing capacity, it might be 2114 capable of supporting, say, a G.711 mu-law audio stream in 2115 combination with an H.264 video stream, or a G.729B audio stream in 2116 combination with an H.263-1998 video stream. It might then issue an 2117 offer like the following: 2119 v=0 2120 o=- 25678 753849 IN IP4 192.0.2.1 2121 s= 2122 c=IN IP4 192.0.2.1 2123 t=0 0 2124 a=creq:med-v0 2125 a=sescap:1 2,4 2126 a=sescap:2 1,3 2127 m=audio 54322 RTP/AVP 18 2128 a=rtpmap:18 G729/8000 2129 a=fmtp:18 annexb=yes 2130 a=rmcap:1 PCMU/8000 2131 a=pcfg:1 m=1 pt=1:0 2132 a=pcfg:2 2133 m=video 54344 RTP/AVP 100 2134 a=rtpmap:100 H263-1998/90000 2135 a=rmcap:2 H264/90000 2136 a=mfcap:2 profile-level-id=42A01E; packetization-mode=2 2137 a=pcfg:3 m=2 pt=2:101 2138 a=pcfg:4 2140 Note that the preferred session configuration (and the default as 2141 well) is G.729B with H.263. This overrides the individual media 2142 stream preferences which are PCMU and H.264 by the potential 2143 configuration numbering rule. 2145 4.3. Latent Media Streams 2147 Consider a case in which the offerer can support either G.711 mu-law, 2148 or G.729B, along with DTMF telephony events for the 12 common 2149 touchtone signals, but is willing to support simple G.711 mu-law 2150 audio as a last resort. In addition, the offerer wishes to announce 2151 its ability to support video and MSRP in the future, but does not 2152 wish to offer a video stream or an MSRP stream at present. The offer 2153 might look like the following: 2155 v=0 2156 o=- 25678 753849 IN IP4 192.0.2.1 2157 s= 2158 c=IN IP4 192.0.2.1 2159 t=0 0 2160 a=creq:med-v0 2161 m=audio 23456 RTP/AVP 0 2162 a=rtpmap:0 PCMU/8000 2163 a=rmcap:1 PCMU/8000 2164 a=rmcap:2 G729/8000 2165 a=rmcap:3 telephone-event/8000 2166 a=mfcap:3 0-11 2167 a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 2168 a=lcfg:2 mt=video t=1 m=10|11 2169 a=rmcap:10 H263-1998/90000 2170 a=rmcap:11 H264/90000 2171 a=tcap:1 RTP/AVP 2172 a=lcfg:3 mt=message t=2 m=20 2173 a=tcap:2 TCP/MSRP 2174 a=omcap:20 * 2176 The first lcfg attribute line ("lcfg:2") announces support for H.263 2177 and H.264 video (H.263 preferred) for future negotiation. The second 2178 lcfg attribute line ("lcfg:3") announces support for MSRP for future 2179 negotiation. The m-line and the rtpmap attribute offer an audio 2180 stream and provide the lowest precedence configuration (PCMU without 2181 any DTMF encoding). The rmcap lines define the RTP-based media 2182 format capabilities (PCMU, G729, telephone-event, H263-1998 and H264) 2183 and the omcap line defines the non-RTP based media format capability 2184 (wildcard). The mfcap attribute provides the format parameters for 2185 telephone-event, specifying the 12 commercial DTMF 'digits'. The 2186 pcfg attribute line defines the most-preferred media configuration as 2187 PCMU plus DTMF events and the next-most-preferred configuration as 2188 G.729B plus DTMF events. 2190 If the answerer is able to support all the potential configurations, 2191 and also support H.263 video (but not H.264), it would reply with an 2192 answer like: 2194 v=0 2195 o=- 24351 621814 IN IP4 192.0.2.2 2196 s= 2197 c=IN IP4 192.0.2.2 2198 t=0 0 2199 a=csup:med-v0 2200 m=audio 54322 RTP/AVP 0 100 2201 a=rtpmap:0 PCMU/8000 2202 a=rtpmap:100 telephone-event/8000 2203 a=fmtp:100 0-11 2204 a=acfg:1 m=1,3 pt=1:0,3:100 2205 a=pcfg:1 m=2,3 pt=2:18,3:100 2206 a=lcfg:2 mt=video t=1 m=10 2208 The lcfg attribute line announces the capability to support H.263 2209 video at a later time. The media line and subsequent rtpmap and fmtp 2210 attribute lines present the selected configuration for the media 2211 stream. The acfg attribute line identifies the potential 2212 configuration from which it was taken, and the pcfg attribute line 2213 announces the potential capability to support G.729 with DTMF events 2214 as well. If, at some later time, congestion becomes a problem in the 2215 network, either party may, with expectation of success, offer a 2216 reconfiguration of the media stream to use G.729 in order to reduce 2217 packet sizes. 2219 5. IANA Considerations 2221 5.1. New SDP Attributes 2223 The IANA is hereby requested to register the following new SDP 2224 attributes: 2226 Attribute name: rmcap 2227 Long form name: RTP-based media format capability 2228 Type of attribute: session-level and media-level 2229 Subject to charset: no 2230 Purpose: associate RTP-based media capability number(s) with 2231 media subtype and encoding parameters 2232 Appropriate Values: see Section 3.3.1 2233 Contact name: Flemming Andreasen, fandres@cisco.com 2235 Attribute name: omcap 2236 Long form name: Non RTP-based media format capability 2237 Type of attribute: session-level and media-level 2238 Subject to charset: no 2239 Purpose: associate non RTP-based media capability number(s) with 2240 media subtype and encoding parameters 2241 Appropriate Values: see Section 3.3.1 2242 Contact name: Flemming Andreasen, fandreas@cisco.com 2244 Attribute name: mfcap 2245 Long form name: media format parameter capability 2246 Type of attribute: session-level and media-level 2247 Subject to charset: no 2248 Purpose: associate media format attributes and 2249 parameters with media format capabilities 2250 Appropriate Values: see Section 3.3.2 2251 Contact name: Flemming Andreasen, fandreas@cisco.com 2253 Attribute name: mscap 2254 Long form name: media-specific capability 2255 Type of attribute: session-level and media-level 2256 Subject to charset: no 2257 Purpose: associate media-specific attributes and 2258 parameters with media capabilities 2259 Appropriate Values: see Section 3.3.3 2260 Contact name: Flemming Andreasen, fandreas@cisco.com 2262 Attribute name: lcfg 2263 Long form name: latent configuration 2264 Type of attribute: media-level 2265 Subject to charset: no 2266 Purpose: to announce supportable media streams 2267 without offering them for immediate use. 2268 Appropriate Values: see Section 3.3.5 2269 Contact name: Flemming Andreasen, fandreas@cisco.com 2271 Attribute name: sescap 2272 Long form name: session capability 2273 Type of attribute: session-level 2274 Subject to charset: no 2275 Purpose: to specify and prioritize acceptable 2276 combinations of media stream configurations. 2277 Appropriate Values: see Section 3.3.8 2278 Contact name: Flemming Andreasen, fandreas@cisco.com 2280 5.2. New SDP Capability Negotiation Option Tag 2282 The IANA is hereby requested to add the new option tag "med-v0", 2283 defined in this document, to the SDP Capability Negotiation Option 2284 Capability registry created for RFC 5939 [RFC5939]. 2286 5.3. SDP Capability Negotiation Configuration Parameters Registry 2288 The IANA is hereby requested to change the "SDP Capability 2289 Negotiation Potential Configuration Parameters" registry currently 2290 registered and defined by RFC 5939 [RFC5939] as follows: 2292 The name of the registry should be "SDP Capability Negotiation 2293 Configuration Parameters Registry" and it should contain a table with 2294 the following column headings: 2296 o Encoding Name: The syntactical value used for the capability 2297 negotiation configuration parameter, as defined in RFC 5939 2298 [RFC5939], Section 3.5. 2300 o Descriptive Name: The name commonly used to refer to the 2301 capability negotiation configuration parameter. 2303 o Potential Configuration Definition: A reference to the RFC that 2304 defines the configuration parameter in the context of a potential 2305 configuration attribute. If the configuration parameter is not 2306 defined for potential configurations, the string "N/A" (Not 2307 Applicable) MUST be present instead. 2309 o Actual Configuration Definition: A reference to the RFC that 2310 defines the configuration parameter in the context of an actual 2311 configuration attribute. If the configuration parameter is not 2312 defined for actual configurations, the string "N/A" (Not 2313 Applicable) MUST be present instead. 2315 o Latent Configuration Definition: A reference to the RFC that 2316 defines the configuration parameter in the context of a latent 2317 configuration attribute. If the configuration parameter is not 2318 defined for latent configurations, the string "N/A" (Not 2319 Applicable) MUST be present instead. 2321 An IANA SDP Capability Negotiation Configuration registration MUST be 2322 documented in an RFC in accordance with the IETF Review policy 2323 [RFC5226]. Furthermore: 2325 o The RFC MUST define the syntax and semantics of each new potential 2326 configuration parameter. 2328 o The syntax MUST adhere to the syntax provided for extension 2329 configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and the 2330 semantics MUST adhere to the semantics provided for extension 2331 configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and 3.5.2. 2333 o Configuration parameters that apply to latent configurations MUST 2334 furthermore adhere to the syntax provided in Section 3.3.5 and the 2335 semantics defined overall in this document. 2337 o Associated with each registration MUST be the encoding name for 2338 the parameter as well as a short descriptive name for it. 2340 o Each registration MUST specify if it applies to 2342 * Potential configurations 2344 * Actual configurations 2346 * Latent configurations 2348 5.4. SDP Capability Negotiation Configuration Parameter Registrations 2350 The IANA is hereby requested to register the following capability 2351 negotiation configuration parameters: 2353 Encoding Name: a 2354 Descriptive Name: Attribute Configuration 2355 Potential Configuration Definition: [RFC5939] 2356 Actual Configuration Definition: [RFC5939] 2357 Latent Configuration Definition: [Note to RFC Editor: This RFC] 2359 Encoding Name: t 2360 Descriptive Name: Transport Protocol Configuration 2361 Potential Configuration Definition: [RFC5939] 2362 Actual Configuration Definition: [RFC5939] 2363 Latent Configuration Definition: [Note to RFC Editor: This RFC] 2365 Encoding Name: m 2366 Descriptive Name: Media Configuration 2367 Potential Configuration Definition: [Note to RFC Editor: This 2368 RFC] 2369 Actual Configuration Definition: [Note to RFC Editor: This RFC] 2370 Latent Configuration Definition: [Note to RFC Editor: This RFC] 2372 Encoding Name: pt 2373 Descriptive Name: Payload Type Number Mapping 2374 Potential Configuration Definition: [Note to RFC Editor: This 2375 RFC] 2376 Actual Configuration Definition: [Note to RFC Editor: This RFC] 2377 Latent Configuration Definition: [Note to RFC Editor: This RFC] 2379 Encoding Name: mt 2380 Descriptive Name: Media Type 2381 Potential Configuration Definition: N/A 2382 Actual Configuration Definition: N/A 2383 Latent Configuration Definition: [Note to RFC Editor: This RFC] 2385 6. Security Considerations 2387 The security considerations of RFC 5939 [RFC5939] apply for this 2388 document. 2390 In RFC 5939 [RFC5939], it was noted that negotiation of transport 2391 protocols (e.g. secure and non-secure) and negotiation of keying 2392 methods and material are potential security issues that warrant 2393 integrity protection to remedy. Latent configuration support 2394 provides hints to the other side about capabilities supported for 2395 further offer/answer exchanges, including transport protocols and 2396 attribute capabilities, e.g. for keying methods. If an attacker can 2397 remove or alter latent configuration information to suggest that only 2398 insecure or less secure alternatives are supported, then he may be 2399 able to force negotiation of a less secure session than would 2400 otherwise have occurred. While the specific attack as described here 2401 differs from those described in RFC 5939 [RFC5939], the 2402 considerations and mitigation strategies are similar to those 2403 described in RFC 5939 [RFC5939]. 2405 Another variation on the above attack involves the Session Capability 2406 ("sescap") attribute defined in this document. The "sescap" enables 2407 a preference order to be specified for all the potential 2408 configurations, and that preference will take precedence over any 2409 preference indication provided in individual potential configuration 2410 attributes. Consequently, an attacker that can insert or modify a 2411 "sescap" attribute may be able to force negotiation of an insecure or 2412 less secure alternative than would otherwise have occurred. Again, 2413 the considerations and mitigation strategies are similar to those 2414 described in RFC 5939 [RFC5939]. 2416 The addition of negotiable media formats and their associated 2417 parameters, defined in this specification can cause problems for 2418 middleboxes which attempt to control bandwidth utilization, media 2419 flows, and/or processing resource consumption as part of network 2420 policy, but which do not understand the media capability negotiation 2421 feature. As for the initial SDP Capability Negotiation work 2422 [RFC5939], the SDP answer is formulated in such a way that it always 2423 carries the selected media encoding for every media stream selected. 2424 Pending an understanding of capabilities negotiation, the middlebox 2425 should examine the answer SDP to obtain the best picture of the media 2426 streams being established. As always, middleboxes can best do their 2427 job if they fully understand media capabilities negotiation. 2429 7. Changes from previous versions 2431 7.1. Changes from version 16 2433 o Changed grammer covering numeric values to forbid use of leading 2434 zeroes, and added text to that effect as well. 2436 o Clarified that all numerical ranges must be strictly increasing 2437 (from leftmost number to rightmost number). 2439 o Now mandating against (rather than recommending against) using 2440 real keying material in a crypto attribute when it is used for a 2441 latent configuration only. 2443 o Reworded Offer section description of config-number use in 2444 potential configurations to make it clear that all config-numbers 2445 must be unique. 2447 7.2. Changes from version 15 2449 o Fixed style of RFC references to be consistent in body of 2450 document. 2452 o Minor updates to address Gen-ART review comments. 2454 7.3. Changes from version 14 2456 o Updated IANA Considerations to fix configuration parameter 2457 registry. Document now updates RFC 5939 [RFC5939] (IANA 2458 considerations only) 2460 o Minor ABNF updates to fix errors. 2462 o Editorial nit fixes to address protocol write-up review. 2464 7.4. Changes from version 13 2466 o Various editorial clarifications and updates to address review 2467 comments. 2469 7.5. Changes from version 12 2471 o Removed "dummy" form in the pcfg payload-type-number, since the 2472 functionality is redundant with the non-RTP media capability 2473 (omcap) and it was inconsistent with other RTP payload type 2474 operation. 2476 o Clarified that latent configuration attribute (lcfg) can only be 2477 used at the media level and hence (technically) as part of a media 2478 description 2480 o Rewrote offer/answer sections and expanded significantly on offer/ 2481 answer operation. 2483 o Updated security considerations 2485 o Various minor editorial clarifications and changes. 2487 7.6. Changes from version 11 2489 o Corrected several statements implying lcfg was a session-level 2490 attribute. 2492 o Added non-RTP based media format capabilities ("a=omcap") and 2493 renamed "mcap" to "rmcap" 2495 7.7. Changes from version 10 2497 o Defined the latent configuration attribute as a media-level 2498 attribute because it specifies a possible future media stream. 2499 Added text to clarify how to specify alternative configurations of 2500 a single latent stream and/or multiple streams. 2502 o Improved the definition of the session capability attribute to 2503 permit both required configurations and optional configurations - 2504 latent configurations cannot be required because they have not yet 2505 been offered. 2507 o Removed the special-case treatment of conflicts between base-level 2508 fmtp attributes and fmtp attributes generated for a configuration 2509 via invoked mcap and mfcap attributes. 2511 o Removed reference to bandwidth capability (bcap) attribute. 2513 o Changed various "must", etc., terms to normative terms ("MUST", 2514 etc.) as appropriate, in Section 3.3.5Section 3.3.6.1 2515 Section 3.3.6.3 and Section 3.3.8 2517 o Attempted to clarify the substitution mechanism in Section 3.3.7 2518 and improve its uniqueness. 2520 o Made various editorial changes, including changing the title in 2521 the header, and removing numbering from some SDP examples. 2523 7.8. Changes from version 09 2525 o Additional corrections to latent media stream example in 2526 Section 4.3 2528 o Fixed up attribute formatting examples and corresponding ABNF. 2530 o Removed preference rule for latent configurations. 2532 o Various spelling and other editorial changes were made. 2534 o updated cross-references. 2536 7.9. Changes from version 08 2538 The major change is in Section 4.3, Latent Media Streams, fixing the 2539 syntax of the answer. All the other changes are editorial. 2541 7.10. Changes from version 04 2543 o The definitions for bcap, ccap, icap, and kcap attributes have 2544 been removed, and are to be defined in another document. 2546 o Corrected formatting of m= and p= configuration parameters to 2547 conform to extension-config-list form defined in RFC 5939 2548 [RFC5939] 2550 o Reorganized definitions of new parameters to make them easier to 2551 find in document. 2553 o Added ability to renegotiate capabilities when modifying the 2554 session (Section 3.4.4). 2556 o Made various editorial changes, clarifications, and typo 2557 corrections. 2559 7.11. Changes from version 03 2561 o A new session capability attribute (sescap) has been added to 2562 permit specification of acceptable media stream combinations. 2564 o Capability attribute definitions corresponding to the i, c, b, and 2565 k SDP line types have been added for completeness. 2567 o Use of the pcfg: attribute in SDP answers has been included in 2568 order to conveniently return information in the answer about 2569 acceptable configurations in the media stream offer. 2571 o The use of the lcfg: attribute(s) in SDP answers has been 2572 restricted to indicate just which latent configuration offers 2573 would be acceptable to the answerer. 2575 o A suggestion for "naive" middleboxes has been added to the 2576 Security Considerations. 2578 o Various editorial changes have been made. 2580 o Several errors/omissions have been corrected. 2582 o The description of the mscap attribute has been modified to make 2583 it clear that it should not be used to generate undefined SDP 2584 attributes, or to "extend" existing attributes. 2586 o are made optional in the mscap attribute 2587 definition. 2589 o "AMR" changed to "AMR-WB" in cases in which the sample rate is 2590 16000. 2592 7.12. Changes from version 02 2594 This version contains several detail changes intended to simplify 2595 capability processing and mapping into conventional SDP media blocks. 2597 o The "mcap" attribute is enhanced to include the role of the "ecap" 2598 attribute; the latter is eliminated. 2600 o The "fcap" attribute has been renamed "mfcap". New replacement 2601 rules vis-a-vis fmtp attributes in the base media specification 2602 have been added. 2604 o A new "mscap" attribute is defined to handle the problem of 2605 attributes (other than rtpmap and fmtp) that are specific to a 2606 particular payload type. 2608 o New rules for processing the mcap, mfcap, and mscap attributes, 2609 and overriding standard rtpmap, fmtp, or other media-specific 2610 attributes, are put forward to reduce the need to use the deletion 2611 option in the a= parameter of the potential configuration (pcfg) 2612 attribute. 2614 o A new parameter, "mt=" is added to the latent configuration 2615 attribute (lcfg) to specify the media stream type (audio, video, 2616 etc.) when the lcfg is declared at the session level. 2618 o The examples are expanded. 2620 o Numerous typos and misspellings have been corrected. 2622 7.13. Changes from version 01 2624 The documents adds a new attribute for specifying bandwidth 2625 capability and a parameter to list in the potential configuration. 2626 Other changes are to align the document with the terminology and 2627 attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07. 2628 The document also clarifies some previous open issues. 2630 7.14. Changes from version 00 2632 The major changes include taking out the "mcap" and "cptmap" 2633 parameter. The mapping of payload type is now in the "pt" parameter 2634 of "pcfg". Media subtype need to explicitly defined in the "cmed" 2635 attribute if referenced in the "pcfg" 2637 8. Acknowledgements 2639 This document is heavily influenced by the discussions and work done 2640 by the SDP Capability Negotiation Design team. The following people 2641 in particular provided useful comments and suggestions to either the 2642 document itself or the overall direction of the solution defined 2643 herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and 2644 Thomas Stach. 2646 We thank Ingemar Johansson and Magnus Westerlund for examples that 2647 stimulated this work, and for critical reading of the document. We 2648 also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for 2649 their review of the document. 2651 9. References 2653 9.1. Normative References 2655 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2656 Requirement Levels", BCP 14, RFC 2119, March 1997. 2658 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2659 with Session Description Protocol (SDP)", RFC 3264, 2660 June 2002. 2662 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2663 Description Protocol", RFC 4566, July 2006. 2665 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2666 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 2667 May 2008. 2669 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2670 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2672 [RFC5939] Andreasen, F., "Session Description Protocol (SDP) 2673 Capability Negotiation", RFC 5939, September 2010. 2675 9.2. Informative References 2677 [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., 2678 Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- 2679 Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, 2680 September 1997. 2682 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 2683 Description Protocol (SDP) Security Descriptions for Media 2684 Streams", RFC 4568, July 2006. 2686 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 2687 "Extended RTP Profile for Real-time Transport Control 2688 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 2689 July 2006. 2691 [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF 2692 Digits, Telephony Tones, and Telephony Signals", RFC 4733, 2693 December 2006. 2695 [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, 2696 "RTP Payload Format and File Storage Format for the 2697 Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband 2698 (AMR-WB) Audio Codecs", RFC 4867, April 2007. 2700 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 2701 "Codec Control Messages in the RTP Audio-Visual Profile 2702 with Feedback (AVPF)", RFC 5104, February 2008. 2704 Authors' Addresses 2706 Robert R Gilman 2707 Independent 2708 3243 W. 11th Ave. Dr. 2709 Broomfield, CO 80020 2710 USA 2712 Email: bob_gilman@comcast.net 2714 Roni Even 2715 Gesher Erove Ltd 2716 14 David Hamelech 2717 Tel Aviv 64953 2718 Israel 2720 Email: ron.even.tlv@gmail.com 2722 Flemming Andreasen 2723 Cisco Systems 2724 Iselin, NJ 2725 USA 2727 Email: fandreas@cisco.com