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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Kutscher 3 Internet-Draft Ott 4 Expires: August 30, 2002 Bormann 5 TZI, Universitaet Bremen 6 March 01, 2002 8 Session Description and Capability Negotiation 9 draft-ietf-mmusic-sdpng-04.txt 11 Status of this Memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on August 30, 2002. 34 Copyright Notice 36 Copyright (C) The Internet Society (2002). All Rights Reserved. 38 Abstract 40 This document defines a language for describing multimedia sessions 41 with respect to configuration parameters and capabilities of end- 42 systems. 44 This document is a product of the Multiparty Multimedia Session 45 Control (MMUSIC) working group of the Internet Engineering Task 46 Force. Comments are solicited and should be addressed to the working 47 group's mailing list at mmusic@ietf.org and/or the authors. 49 Document Revision 50 $Revision: 4.23 $ 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4 55 2. Terminology and System Model . . . . . . . . . . . . . . . 6 56 3. SDPng . . . . . . . . . . . . . . . . . . . . . . . . . . 9 57 3.1 Conceptual Outline . . . . . . . . . . . . . . . . . . . . 9 58 3.1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . 9 59 3.1.2 Components & Configurations . . . . . . . . . . . . . . . 11 60 3.1.3 Constraints . . . . . . . . . . . . . . . . . . . . . . . 13 61 3.1.4 Session Attributes . . . . . . . . . . . . . . . . . . . . 14 62 3.1.4.1 Owner . . . . . . . . . . . . . . . . . . . . . . . . . . 15 63 3.1.4.2 Session Identification . . . . . . . . . . . . . . . . . . 15 64 3.1.4.3 Time Specification (SDP 't=', 'r=', and 'z=' lines) . . . 16 65 3.1.4.4 Component Semantic Specification . . . . . . . . . . . . . 17 66 3.2 Syntax Definition Mechanisms . . . . . . . . . . . . . . . 18 67 3.3 Referencing Definitions . . . . . . . . . . . . . . . . . 20 68 3.3.1 The sdpng:use Element Type . . . . . . . . . . . . . . . . 21 69 3.3.2 Properties . . . . . . . . . . . . . . . . . . . . . . . . 22 70 3.3.3 Definition Groups . . . . . . . . . . . . . . . . . . . . 23 71 3.3.4 Usage of Child Elements and Attributes of sdpng:use 72 Elements . . . . . . . . . . . . . . . . . . . . . . . . . 26 73 3.4 External Definition Packages . . . . . . . . . . . . . . . 28 74 3.4.1 Profile Definitions . . . . . . . . . . . . . . . . . . . 28 75 3.4.2 Library Definitions . . . . . . . . . . . . . . . . . . . 29 76 3.5 Mappings . . . . . . . . . . . . . . . . . . . . . . . . . 30 77 4. Capability Negotiation . . . . . . . . . . . . . . . . . . 32 78 4.1 Outline of the Negotiation Process . . . . . . . . . . . . 32 79 4.2 The Collapsing Algorithm . . . . . . . . . . . . . . . . . 34 80 4.2.1 Collapsing Two Configurations . . . . . . . . . . . . . . 35 81 4.2.1.1 Collapsing of Attributes . . . . . . . . . . . . . . . . . 35 82 4.2.1.2 Collapsing two Elements . . . . . . . . . . . . . . . . . 38 83 4.2.1.3 Collapsing nested Elements . . . . . . . . . . . . . . . . 39 84 4.2.2 Deriving an actual Configuration . . . . . . . . . . . . . 41 85 5. Formal Specification . . . . . . . . . . . . . . . . . . . 42 86 5.1 XML Schema as a Definition Mechanism . . . . . . . . . . . 42 87 5.2 SDPng Schema . . . . . . . . . . . . . . . . . . . . . . . 43 88 5.3 Profiles . . . . . . . . . . . . . . . . . . . . . . . . . 44 89 5.4 SDPng Documents . . . . . . . . . . . . . . . . . . . . . 45 90 5.5 Libraries . . . . . . . . . . . . . . . . . . . . . . . . 46 91 5.6 Details on the use of specific XML Mechanisms . . . . . . 47 92 5.6.1 Default Namespace . . . . . . . . . . . . . . . . . . . . 47 93 5.6.2 Qualified Locals . . . . . . . . . . . . . . . . . . . . . 47 94 5.6.3 Fixed Namespace Prefixes . . . . . . . . . . . . . . . . . 48 95 5.7 SDPng Schema Definitions . . . . . . . . . . . . . . . . . 48 96 5.7.1 SDPng Base Definition . . . . . . . . . . . . . . . . . . 48 97 5.7.2 Audio Codec Profile . . . . . . . . . . . . . . . . . . . 55 98 5.7.3 RTP profile . . . . . . . . . . . . . . . . . . . . . . . 56 99 5.8 Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 59 100 6. Use of SDPng in conjunction with other IETF Signaling 101 Protocols . . . . . . . . . . . . . . . . . . . . . . . . 60 102 6.1 The Session Announcement Protocol (SAP) . . . . . . . . . 60 103 6.2 Session Initiation Protocol (SIP) . . . . . . . . . . . . 61 104 6.3 Real-Time Streaming Protocol (RTSP) . . . . . . . . . . . 67 105 6.4 Media Gateway Control Protocol (MEGACOP) . . . . . . . . . 68 106 7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 69 107 References . . . . . . . . . . . . . . . . . . . . . . . . 70 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . 71 109 A. Base SDPng Specifications for Audio Codec Descriptions . . 72 110 A.1 DVI4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 111 A.2 G.722 . . . . . . . . . . . . . . . . . . . . . . . . . . 73 112 A.3 G.726 . . . . . . . . . . . . . . . . . . . . . . . . . . 73 113 A.4 G.728 . . . . . . . . . . . . . . . . . . . . . . . . . . 73 114 A.5 G.729 . . . . . . . . . . . . . . . . . . . . . . . . . . 73 115 A.6 G.729 Annex D and E . . . . . . . . . . . . . . . . . . . 74 116 A.7 GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 117 A.7.1 GSM Full Rate . . . . . . . . . . . . . . . . . . . . . . 74 118 A.7.2 GSM Half Rate . . . . . . . . . . . . . . . . . . . . . . 74 119 A.7.3 GSM Enhanced Full Rate . . . . . . . . . . . . . . . . . . 74 120 A.8 L8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 121 A.9 L16 . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 122 A.10 LPC . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 123 A.11 MPA . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 124 A.12 PCMA and PCMU . . . . . . . . . . . . . . . . . . . . . . 75 125 A.13 QCELP . . . . . . . . . . . . . . . . . . . . . . . . . . 75 126 A.14 VDVI . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 127 B. SDPng Library for Audio Codec Definitions . . . . . . . . 76 128 C. SDPng Library for RTP Payload Format Definitions . . . . . 77 129 D. Change History . . . . . . . . . . . . . . . . . . . . . . 78 130 Full Copyright Statement . . . . . . . . . . . . . . . . . 79 132 1. Introduction 134 Multiparty multimedia conferencing is one of the applications that 135 require dynamic interchange of end-system capabilities and the 136 negotiation of a parameter set that is appropriate for all sending 137 and receiving end-systems in a conference. For some applications, 138 e.g. for loosely coupled conferences or for broadcast scenarios, it 139 may be sufficient to simply have session parameters be fixed by the 140 initiator of a conference. In such a scenario no negotiation is 141 required because only those participants with media tools that 142 support the predefined settings can join a media session and/or a 143 conference. 145 This approach is applicable for conferences that are announced some 146 time ahead of the actual start date of the conference. Potential 147 participants can check the availability of media tools in advance and 148 tools such as session directories can configure media tools upon 149 startup. This procedure however fails to work for conferences 150 initiated spontaneously including Internet phone calls or ad-hoc 151 multiparty conferences. Fixed settings for parameters such as media 152 types, their encoding etc. can easily inhibit the initiation of 153 conferences, for example in situations where a caller insists on a 154 fixed audio encoding that is not available at the callee's end- 155 system. 157 To allow for spontaneous conferences, the process of defining a 158 conference's parameter set must therefore be performed either at 159 conference start (for closed conferences) or maybe (potentially) even 160 repeatedly every time a new participant joins an active conference. 161 The latter approach may not be appropriate for every type of 162 conference without applying certain policies: For conferences with 163 TV-broadcast or lecture characteristics (one main active source) it 164 is usually not desired to re-negotiate parameters every time a new 165 participant with an exotic configuration joins because it may 166 inconvenience existing participants or even exclude the main source 167 from media sessions. But conferences with equal "rights" for 168 participants that are open for new participants on the other hand 169 would need a different model of dynamic capability negotiation, for 170 example a telephone call that is extended to a 3-parties conference 171 at some time during the session. 173 SDP [2] allows to specify multimedia sessions (i.e. conferences, 174 "session" as used here is not to be confused with "RTP session"!) by 175 providing general information about the session as a whole and 176 specifications for all the media streams (RTP sessions and others) to 177 be used to exchange information within the multimedia session. 179 Currently, media descriptions in SDP are used for two purposes: 181 o to describe session parameters for announcements and invitations 182 (the original purpose of SDP) and 184 o to describe the capabilities of a system and possibly provide a 185 choice between a number of alternatives (which SDP was not 186 designed for). 188 A distinction between these two "sets of semantics" is only made 189 implicitly. 191 This document is based upon a set of requirements specified in a 192 companion document [1]. In the following, we first introduce a model 193 for session description and capability negotiation as well as the 194 basic terms used throughout this specification (section 2). Next, we 195 outline the concept for the concepts underlying SDPng and introduce 196 the syntactical components step by step in section 3. In section 4, 197 we provide a formal definition of the SDPng session description 198 language. Finally, we overview aspects of using SDPng with various 199 IETF signaling protocols in section 5. In Appendix A, we provide 200 basic audio codec and payload type definitions that are subsumed in 201 SDPng libraries in Appendix B and Appendix C. 203 The next version of this draft will only contain the formal 204 specification of the language itself. Requirements and the 205 description of the system model will be moved to a separate document. 207 2. Terminology and System Model 209 Any (computer) system has, at a time, a number of rather fixed 210 hardware as well as software resources. These resources ultimately 211 define the limitations on what can be captured, displayed, rendered, 212 replayed, etc. with this particular device. We term features enabled 213 and restricted by these resources "system capabilities". 215 Example: System capabilities may include: a limitation of the 216 screen resolution for true color by the graphics board; available 217 audio hardware or software may offer only certain media encodings 218 (e.g. G.711 and G.723.1 but not GSM); and CPU processing power and 219 quality of implementation may constrain the possible video 220 encoding algorithms. 222 In multiparty multimedia conferences, participants employ different 223 "components" in conducting the conference. 225 Example: In lecture multicast conferences one component might be 226 the voice transmission for the lecturer, another the transmission 227 of video pictures showing the lecturer and the third the 228 transmission of presentation material. 230 Depending on system capabilities, user preferences and other 231 technical and political constraints, different configurations can be 232 chosen to accomplish the use of these components in a conference. 234 Each component can be characterized at least by (a) its intended use 235 (i.e. the function it shall provide) and (b) one or more possible 236 ways to realize this function. Each way of realizing a particular 237 function is referred to as a "configuration". 239 Example: A conference component's intended use may be to make 240 transparencies of a presentation visible to the audience on the 241 Mbone. This can be achieved either by a video camera capturing 242 the image and transmitting a video stream via some video tool or 243 by loading a copy of the slides into a distributed electronic 244 white-board. For each of these cases, additional parameters may 245 exist, variations of which lead to additional configurations (see 246 below). 248 Two configurations are considered different regardless of whether 249 they employ entirely different mechanisms and protocols (as in the 250 previous example) or they choose the same and differ only in a single 251 parameter. 253 Example: In case of video transmission, a JPEG-based still image 254 protocol may be used, H.261 encoded CIF images could be sent, as 255 could H.261 encoded QCIF images. All three cases constitute 256 different configurations. Of course there are many more detailed 257 protocol parameters. 259 Each component's configurations are limited by the participating 260 system's capabilities. In addition, the intended use of a component 261 may constrain the possible configurations further to a subset 262 suitable for the particular component's purpose. 264 Example: In a system for highly interactive audio communication 265 the component responsible for audio may decide not to use the 266 available G.723.1 audio codec to avoid the additional latency but 267 only use G.711. This would be reflected in this component only 268 showing configurations based upon G.711. Still, multiple 269 configurations are possible, e.g. depending on the use of A-law 270 or u-Law, packetization and redundancy parameters, etc. 272 In modelling multimedia sessions, we distinguish two types of 273 configurations: 275 o potential configurations 276 (a set of any number of configurations per component) indicating a 277 system's functional capabilities as constrained by the intended 278 use of the various components; 280 o actual configurations 281 (exactly one per instance of a component) reflecting the mode of 282 operation of this component's particular instantiation. 284 Example: The potential configuration of the aforementioned video 285 component may indicate support for JPEG, H.261/CIF, and 286 H.261/QCIF. A particular instantiation for a video conference may 287 use the actual configuration of H.261/CIF for exchanging video 288 streams. 290 In summary, the key terms of this model are: 292 o A multimedia session (streaming or conference) consists of one or 293 more conference components for multimedia "interaction". 295 o A component describes a particular type of interaction (e.g. audio 296 conversation, slide presentation) that can be realized by means of 297 different applications (possibly using different protocols). 299 o A configuration is a set of parameters that are required to 300 implement a certain variation (realization) of a certain 301 component. There are actual and potential configurations. 303 * Potential configurations describe possible configurations that 304 are supported by an end-system. 306 * An actual configuration is an "instantiation" of one of the 307 potential configurations, i.e. a decision how to realize a 308 certain component. 310 In less abstract words, potential configurations describe what a 311 system can do ("capabilities") and actual configurations describe 312 how a system is configured to operate at a certain point in time 313 (media stream spec). 315 To decide on a certain actual configuration, a negotiation process 316 needs to take place between the involved peers: 318 1. to determine which potential configuration(s) they have in 319 common, and 321 2. to select one of this shared set of common potential 322 configurations to be used for information exchange (e.g. based 323 upon preferences, external constraints, etc.). 325 In SAP-based [9] session announcements on the Mbone, for which SDP 326 was originally developed, the negotiation procedure is non-existent. 327 Instead, the announcement contains the media stream description sent 328 out (i.e. the actual configurations) which implicitly describe what a 329 receiver must understand to participate. 331 In point-to-point scenarios, the negotiation procedure is typically 332 carried out implicitly: each party informs the other about what it 333 can receive and the respective sender chooses from this set a 334 configuration that it can transmit. 336 Capability negotiation must not only work for 2-party conferences but 337 is also required for multi-party conferences. Especially for the 338 latter case it is required that the process to determine the subset 339 of allowable potential configurations is deterministic to reduce the 340 number of required round trips before a session can be established. 341 For instance, in order to be used with SIP, the capability 342 negotiation is required to work with the offer/answer model that is 343 for session initiation with SIP -- limiting the negotiation to 344 exactly one round trip. 346 The requirements for the SDPng specification, subdivided into general 347 requirements and requirements for session descriptions, potential and 348 actual configurations as well as negotiation rules, are captured in a 349 companion document [1]. 351 3. SDPng 353 This section introduces the underlying concepts of the Session 354 Description Protocol - next generation (SDPng). The focus of this 355 section is on the concepts of the capability description and 356 negotiation language with a stepwise introduction of the various 357 syntactical elements. Note that this section does only examples 358 accompanied by explanations -- a full formal specification is 359 provided in section 4. 361 3.1 Conceptual Outline 363 The description language follows the system model introduced in the 364 beginning of this document. We use a rather abstract language to 365 avoid misinterpretations due to different intuitive understanding of 366 terms as far as possible. 368 The concept of a capability description language addresses various 369 pieces of a full description of system and application capabilities 370 in four separate "sections": 372 Definitions (elementary and compound); see Section 3.1.1. 374 Potential or Actual Configurations; see Section 3.1.2. 376 Constraints; see Section 3.1.3. 378 Session attributes; see Section 3.1.4. 380 3.1.1 Definitions 382 The "Definitions" section specifies a number of basic abstractions 383 that are later referenced to avoid repetitions in more complex 384 specifications and allow for a concise representation. Definition 385 elements are labelled with an identifier by which they may be 386 referenced. They may be elementary or compound (i.e. combinations 387 of elementary entities). Examples of definitions that could occur in 388 "Definitions" sections include (but are not limited to) codec 389 definitions, redundancy schemes, transport mechanisms and payload 390 formats. 392 Elementary definition elements do not reference other elements. Each 393 elementary entity only consists of one of more attributes and their 394 values. Default values specified in the definition section may be 395 overridden in descriptions for potential (and later actual) 396 configurations. A mechanisms for overriding definitions is specified 397 below. 399 For the moment, elementary abstractions are defined for media types 400 (i.e. codecs) and for media transports mechanisms. For each 401 transport and for each codec to be used, the respective attributes 402 need to be defined. This definition may either be provided within 403 the "Definitions" section itself or in an external document (similar 404 to the audio-video profile or an IANA registry that defines payload 405 types and media stream identifiers). 407 It is not required to define all codecs and transport mechanisms in a 408 "Definitions" sections and reference them when specifying potential 409 and actual configurations. Instead, a syntactic mechanism is defined 410 that allows to give some definitions directly in a configurations 411 section. 413 Examples for elementary definitions: 415 418 421 The element type "audio:codec" is used in these examples to define 422 audio codec configurations. The configuration parameters are given 423 as attribute values. 425 Definitions may have default values specified along with them for 426 each attribute (as well as for their contents). Some of these 427 default values may be overridden so that a codec definition can 428 easily be re-used in a different context (e.g. by specifying a 429 different sampling rate) without the need for a large number of base 430 specifications. In the following example the definition of audio- 431 L16-mono is re-used for the defintion of the corresponding stereo 432 codec. Appendix A provides a complete set of corresponding 433 audio:codec definitions of the codecs used in RFC 1890 [4]. 435 438 The example shows how existing definitions can be referenced in new 439 definitions. This approach allows to create simple as well as more 440 complex definitions in an extensible set of reference documents. 441 Section 3.4 specifies the mechanisms for external references. 443 Besides definitions of audio codecs other definitions such as RTP 444 payload formats and specific transport mechanisms are suitable to be 445 defined in a definition section for later referencing. The following 446 example shows how RTP payload types are defined using a pre-defined 447 codec. 449 450 452 In this example, the payload type "rtp-avp-11" is defined with 453 payload type number 11, referencing the codec "audio-L16-mono". 454 Instead of referencing an existing definition it is also possible to 455 define the format "inline": 457 458 459 461 Note: For negotiation between endpoints, it may be helpful to define 462 two modes of operation: explicit and implicit. Implicit 463 specifications may refer to externally defined entities to minimize 464 traffic volume, explicit specifications would list all external 465 definitions used in a description in the "Definitions" section. 466 Again, see Section 3.4 for complete discussion of external 467 definitions. 469 The "Definitions" section may be empty if all transport, codecs, and 470 other pieces needed to the specify Potential and Actual 471 Configurations (as detailed below) are either included by referencing 472 external definitions or are explicitly described within the 473 Configurations themselves. 475 3.1.2 Components & Configurations 477 The "Configurations" section contains all the components that 478 constitute the multimedia application (IP telephone call, real-time 479 streaming application, multi-player gaming session etc.). For each 480 of these components, the potential and, later, the actual 481 configurations are given. Potential configurations are used during 482 capability exchange and/or negotiation, actual configurations to 483 configure media streams after negotiation (e.g. with RTSP) or in 484 session announcements (e.g. via SAP). A potential and the actual 485 configuration of a component may be identical. 487 Each component is labelled with an identifier so that it can be 488 referenced, e.g. to associate semantics with a particular media 489 stream. For such a component, any number of configurations may be 490 given with each configuration describing an alternative way to 491 realize the functionality of the respective component. 493 Each configuration (potential as well as actual) is labelled with an 494 identifier. A configuration combines one or more (elementary and/or 495 compound) entities from the "Definitions" section to describe a 496 potential or an actual configuration. Within the specification of 497 the configuration, default values from the referenced entities may be 498 overwritten. In addition, it is also possible to provide definition 499 elements inline, inside the definition of a configuration. 501 Note: Not all protocol environments and their respective operation 502 allow to explicitly distinguish between Potential and Actual 503 Configurations. Therefore, SDPng so far does not provide for 504 syntactical identification of a Configurations as being a Potential 505 or an Actual one. The semantics of configurations are to be 506 determined from the requirements of the specific protocol that uses 507 SDPng to express capabilities and configurations. 509 The following example shows how RTP sessions can be described by 510 referencing payload definitions. 512 513 514 515 516 517 518 520 521 522 523 524 525 526 528 For example, an IP telephone call may require just a single component 529 "name=interactive-audio" with two possible ways of implementing it. 530 The two corresponding configurations are "AVP-audio-0" without 531 modification, the other ("AVP-audio-11") uses linear 16-bit encoding. 532 Typically, transport address parameters such as the port number would 533 also be provided. In this example, this information is given by the 534 "rtp:udp" element. Of course, it must be possible to specify other 535 transport mechanisms as well. See Section 3.2 for a discussion of 536 extension mechanisms that allow applications to use non-standard 537 transport (or other) specifications. 539 During/after the negotiation phase, an actual configuration is chosen 540 out of a number of alternative potential configurations, the actual 541 configuration may refer to the potential configuration just by its 542 "id", possibly allowing for some parameter modifications. 543 Alternatively, the full actual configuration may be given. 545 Instead of referencing existing payload type definitions it is also 546 possible to provide the required information "inline". The following 547 example illustrates this: 549 550 551 552 553 554 556 557 558 559 560 561 563 The UDP/IPv4 multicast transport that is used in the examples is a 564 simple variant of a transport specification. More complex ones are 565 conceivable. For example, it must also be possible to specify the 566 usage of source filters (inclusion and exclusion), Source Specific 567 Multicast, the usage of multi-unicast, or other parameters such as 568 QoS parameters. Therefore it is possible to extend the definition of 569 transport mechanisms by providing the required information in the 570 element content. An example: 572 573 574 575 576 577 579 580 581 582 584 Additional transport mechanisms and options will be defined in future 585 versions of this document. 587 3.1.3 Constraints 589 Definitions specify media, transport, and other capabilities, whereas 590 configurations indicate which combinations of these could be used to 591 provide the desired functionality in a certain setting. 593 There may, however, be further constraints within a system (such as 594 CPU cycles, DSP resources available, dedicated hardware, etc.) that 595 limit which of these configurations can be instantiated in parallel 596 (and how many instances of these may exist). We deliberately do not 597 couple this aspect of system resource limitations to the various 598 application semantics as the constraints may exist across application 599 boundaries. Also, in many cases, expressing such constraints is 600 simply not necessary (as many uses of the current SDP show), so 601 additional overhead can be avoided where this is not needed. 603 Therefore, we introduce a "Constraints" section to contain these 604 additional limitations. Constraints refer to potential 605 configurations and to entity definitions and express and use simple 606 logic to express mutual exclusion, limit the number of 607 instantiations, and allow only certain combinations. The following 608 example shows the definition of a constraints that restricts the 609 maximum number of instantiation of two alternatives (that would have 610 to be defined in the configuration section before) when they are used 611 in parallel: 613 614 615 616 617 618 620 As the example shows, constraints are defined by defining limits on 621 simultaneous instantiations of alternatives. They are not defined by 622 expressing abstract end-system resources, such as CPU speed or memory 623 size. 625 By default, the "Constraints" section is empty (or missing) which 626 means that no further restrictions apply. 628 3.1.4 Session Attributes 630 The fourth and final section of the SDPng syntax addresses session 631 layer attributes. These attributes largely include those defined by 632 SDP [RFC2327] (which are explicitly indicated in the following 633 specification) to describe originator, purpose, and timing of a 634 multimedia session among other characteristics. Furthermore, SDPng 635 includes attributes indicating the semantics of the various 636 Components in a teleconference or other session. This part of the 637 specification is open ended with an IANA registry to be set up to 638 register further types of components; only a few of the examples are 639 listed here. 641 A session-level specification for connection information (SDP "c=" 642 line), bandwidth information (SDP "b=" line), and encryption keys 643 (SDP "k=" lines) is deliberately not provided for in SDPng. The 644 relevant information can be specified directly in the Configuration 645 section for individual alternatives. 647 Session level attributes as defined by SDP still have to be examined 648 and adopted for SDPng in a future revision of this specification. 650 3.1.4.1 Owner 652 The owner refers to the creator of a session as defined in RFC2327 653 ("o=" line). The syntax is as follows: 655 658 The owner element must be present if SDPng is used with SAP. For all 659 other protocols, the owner element is not necessarily required. The 660 attributes listed above match those from the SDP specification; all 661 attributes must be present and they are used following the rules of 662 RFC2327. 664 The owner element is an empty element. 666 3.1.4.2 Session Identification 668 The "session" element is used to identify the session and to provide 669 a description and possible further references. It provides the 670 following attributes: 672 name: The session name as it is to appear e.g. in a session 673 directory. This is equivalent to the SDP "s=" line. 675 The session element can contain arbitrary text of any length (but 676 authors are encouraged to keep the inline description brief and 677 provide additional information via URLs using the info element 678 described below. This text is used to provide a description of the 679 session; it is the equivalent of the SDP "i=" lines. 681 Additionally, the session element can contain other elements of the 682 following types to provide further information about the session and 683 its creator: 685 info: The info element is intended to provide a pointer to further 686 information on the session itself. It is an empty element and 687 provides the attribute xlink:href that is used to specify an URI. 688 Info elements are optional, they may occur any number of times. 690 contact: The contact element provides contact information on the 691 creator of the session. It is an empty element and provides an 692 attribute xlink:href that is used to specify an URI. Any URI 693 scheme suitable to reach a person or a group of persons is 694 acceptable (e.g. sip:, mailto:, tel:). Contact elements are 695 optional, they may occur any number of times. 697 698 And here comes a long description of the seminar indicating what 699 this might be about and so forth. But we also include further 700 information -- as additional elements: 701 702 703 704 705 706 707 709 3.1.4.3 Time Specification (SDP 't=', 'r=', and 'z=' lines) 711 The time specification for a session follows the same rules as in 712 SDP. Time specifications are usually only meaningful when used in 713 conjunction with SAP and are optional. SDPng uses the following 714 elements and attributes to specify timing: 716 The element "time" is used to indicate a schedule for the session; 717 time has two optional attributes: 719 start: The starting time of the first occurrence of the session as 720 defined in RFC2327. 722 end: The ending time of the last occurrence of the session as defined 723 in RFC2327. 725 The time element can contain the following elements: 727 repeat: This element specifies the repetition pattern for the 728 schedule. There may be zero or more occurrences of this element 729 within the time element. "repeat" has two mandatory and one 730 optional attribute and is an empty element; the attributes are as 731 defined in SDP: 733 interval: The duration between two start times of the session. 734 This attribute is always present. 736 duration: The duration for which the session will be active 737 starting at each repetition interval. This attribute is always 738 present. 740 offset: The offset relative to "start" attribute at which this 741 repetition of the session is start. This attribute is 742 optional; if it is absent, a default value of "0" is assumed. 744 Formatting of the attribute values follows the rules defined in 745 RFC2327. 747 zone: The zone element specifies one or more time zone adjustments as 748 defined in RFC2327. This element has zero or more occurrences in 749 the time element. It is an empty element and has two attributes 750 as defined in SDP: 752 adjtime: The time at which the next adjustment will take place. 754 delta: The adjustment offset (typically +/- 1 hours). 756 The example from RFC2327, page 16, expressed in SDPng: 758 763 The time element can occur multiple times. 765 3.1.4.4 Component Semantic Specification 767 Another important session parameter is to specify - ideally in a 768 machine-readable way but at least understandable for humans - the 769 function of the various components in a session. Typically, the 770 semantics of the streams are implicitly assumed (e.g. a video stream 771 goes together with the only audio stream in a session). There are, 772 however, scenarios in which such intuitive understanding is not 773 sufficient and the semantics must be made explicit. 775 776 Audio stream for the different speakers 777 779 The above example shows a simple definition of the semantics for the 780 component "audio-interactive". Further options may be added to 781 provide additional information, e.g. language, and other functions 782 may be specified (e.g. "panel", "audience", "chair", etc.). 784 3.2 Syntax Definition Mechanisms 786 In order to allow for the possibility to validate session 787 descriptions and in order to allow for structured extensibility, 788 SDPng relies on a syntax framework that provides concepts as well as 789 concrete procedures for document validation and extending the set of 790 allowed syntax elements. 792 SGML/XML technologies allow for the creation of Document Type 793 Definitions (DTDs) that can define the allowed content models for the 794 elements of conforming documents. Documents can be formally 795 validated against a given DTD to check their conformance and 796 correctness. XML DTDs however, cannot easily be extended. It is not 797 possible to alter to content models of element types or to add new 798 element types after the DTD has been specified. 800 For SDPng, a mechanism is needed that allows the specification of a 801 base syntax -- for example basic elements for the high level 802 structure of description documents -- while allowing extensions, for 803 example elements and attributes for new transport mechanisms, new 804 media types etc. to be added on demand. Still, it has to be ensured 805 that extensions do not result in name collisions. Furthermore, it 806 must be possible for applications that process descriptions documents 807 to distinguish extensions from base definitions. 809 For XML, mechanisms have been defined that allow for structured 810 extensibility of a model of allowed syntax: XML Namespace and XML 811 Schema. 813 XML Schema mechanisms allows to constrain the allowed document 814 content, e.g. for documents that contain structured data and also 815 provide the possibility that document instances can conform to 816 several XML Schema definitions at the same time, while allowing 817 Schema validators to check the conformance of these documents. 819 Extensions of the session description language, say for allowing to 820 express the parameters of a new media type, would require the 821 creation of a corresponding XML schema definition that contains the 822 specification of element types that can be used to describe 823 configurations of components for the new media type. Session 824 description documents have to reference the non-standard Schema 825 module, thus enabling parsers and validators to identify the elements 826 of the new extension module and to either ignore them (if they are 827 not supported) or to consider them for processing the 828 session/capability description. 830 It is important to note that the functionality of validating 831 capability and session description documents is not necessarily 832 required to generate or process them. For example, endpoints would 833 be configured to understand only those parts of description documents 834 that are conforming to the baseline specification and simply ignore 835 extensions they cannot support. The usage of XML and XML Schema is 836 thus rather motivated by the need to allow for extensions being 837 defined and added to the language in a structured way that does not 838 preclude the possibility to have applications to identify and process 839 the extensions elements they might support. The baseline 840 specification of XML Schema definitions and profiles must be well- 841 defined and targeted to the set of parameters that are relevant for 842 the protocols and algorithms of the Internet Multimedia Conferencing 843 Architecture, i.e. transport over RTP/UDP/IP, the audio video profile 844 of RFC1890 etc. 846 Section 3.4 describes profile definitions and library definition. A 847 detailed definition of how the formal SDPng syntax and the 848 corresponding extension mechanisms is provided in Section 5. 850 The example below shows how the definition of codecs, transport- 851 variants and configuration of components as well as a conference 852 description are realized in SDPng. 854 855 858 861 862 864 866 867 868 869 870 871 872 874 875 876 877 878 879 880 882 883 884 885 886 888 889 891 892 This seminar is about SDPng... 893 894 895 896 898 903 904 Audio stream for the different speakers 905 907 909 Section 5 specifies the formal Schema definition that this example is 910 conforming to. 912 A real-world capability description would likely be shorter than the 913 presented example because the codec and transport definitions can be 914 factored-out to profile definition documents that would only be 915 referenced in capability description documents. 917 3.3 Referencing Definitions 919 This section specifies some generic mechanisms for referencing 920 existing definitions. Referencing existing definition allows to 921 contruct definitions without having to include all parameters inline. 922 By using these mechanisms, complex definitions can be derived by 923 combining multiple basic mechanisms. Common parameters that occur in 924 different configurations do not have to be repeated but can be 925 defined once and then be referenced as often as they are needed. 927 3.3.1 The sdpng:use Element Type 929 The element type "sdpng:use" is a generic reference mechanisms that 930 allows to refer to arbitrary definition within another definition or 931 configuration element. "sdpng:use" is an element type with one 932 mandatory attribute called "href". The value of that attribute is 933 the name of the definition to be referenced. An example: 935 936 937 938 939 940 941 942 943 944 945 946 948 In this example, an element "rtp:udp" is used in the definitions 949 section to define some transport parameters that should later be re- 950 used by referencing this definition using the specified name 951 "endpoint-addr-1". Within the element "rtp:session" in the 952 configurations section the definition is referenced using the "use" 953 element. 955 An implementation that processes this SDPng document and wants to 956 evaluate the configuration for the alternative "rtp-avp-10" MUST 957 replace the "use" element by the referenced element. If the 958 referenced element contains "use" elements itself, those MUST also be 959 dereferenced. 961 When applying this algorithm to the sample SDPng document, the 962 following result SDPng document is generated: 964 965 966 967 968 969 970 971 972 973 974 975 977 For the purpose of comparing configurations, both SDPng documents are 978 equal. 980 3.3.2 Properties 982 The element type "sdpng:prop" can be used to add properties to 983 definitions. "sdpng:prop" has two attributes: 985 name: the name of the property 987 value: the value for the named property 989 For example: 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1007 For comparing and collapsing elements, all sdpng:prop element that 1008 are contained in a parent element (like alt in the example above) 1009 MUST be transformed to attributes of the containing element. If the 1010 parent element already provides a corresponding attribute its value 1011 MUST be overwritten. 1013 The example above would thus be transformed to: 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1030 The main purpose of the sdpng:prop element type is to provide a 1031 mechanism by which attributes of referenced elements can be modified 1032 by the referring element. An application for this is described in 1033 Section 3.3.4. 1035 3.3.3 Definition Groups 1037 Using the sdpng:group element arbitrary definition can be combined 1038 and defined as a group with a specific name. Using this name, the 1039 definitions contained in the group can be referenced with the 1040 sdpng:use element and embedded into other elements. 1042 An example for the use of the sdpng:group element: 1044 1045 1046 1047 1049 1050 1051 1052 1054 1055 1056 1058 1059 1060 1061 1062 1063 1064 1065 1067 This example shows how a group that has been defined in the 1068 definitions section is referenced using the sdpng:use element. The 1069 group element contains two sdpng:prop elements. 1071 For comparing and collapsing elements, all references to sdpng:group 1072 element MUST be replaced by the content of the corresponding 1073 sdpng:group element. The example above would thus be transformed to: 1075 1076 1077 1078 1080 1081 1082 1083 1085 1086 1087 1089 1090 1091 1092 1093 1094 1095 1096 1097 1099 In this example the content of the sdpng:group element named g1 has 1100 been embedded into the alt element that contained the sdpng:use 1101 element referencing the group element. 1103 According to the rules in Section 3.3.2 the sdpng:prop elements are 1104 transformed in a second step to yield the following final decription: 1106 1107 1108 1109 1111 1112 1113 1114 1116 1117 1118 1120 1121 1122 1123 1124 1125 1126 1128 As a general rule, all references MUST be resolved before sdpng:prop 1129 elements are processed and transformed into attribute values. 1131 3.3.4 Usage of Child Elements and Attributes of sdpng:use Elements 1133 It is also possible to provide arbitrary other elements within a 1134 sdpng:use element (depending on the specific application). All 1135 elements that occur in a sdpng:use element MUST be transfomed to 1136 child elements of the referenced element when resolving a sdpng:use 1137 reference. If the reference already provides child elements, the 1138 child elements of the sdpng:use element are added to the list of 1139 child elements of the referenced element. 1141 Any existing elements of a referenced element with the same GI as an 1142 element in the corresponding sdpng:use element MUST be replaced by 1143 the element of the sdpng:use element. This mechanism allows to 1144 extend and to change referenced elements in a simple way. 1146 In the following we give an example of using an sdpng:prop element 1147 within a sdpng:use element which has the semantics of adding 1148 properties to the referenced element. The semantics and processing 1149 requirements for the sdpng:prop element are specified in Section 1150 3.3.2. 1152 Example for the usage of an sdpng:use element containing an 1153 sdpng:prop element: 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1170 This will be transformed to: 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1187 In a second step, the sdpng:prop element would be transformed to an 1188 attribute of its parent element (rtp:udp in this case) according to 1189 the rules specified in Section 3.3.2. 1191 As an abbreviation, the properties for the referenced element do not 1192 have to be specified using sdpng:prop elements within the sdpng:use 1193 element but can also specified directly as attributes of the 1194 sdpng:use element, as shown in the following example: 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1209 In this example, the sdp:use element has no child element sdpng:prop 1210 but provides the property "foo" directly as an attribute. All 1211 attributes of a sdpng:use element other than href MUST be transformed 1212 to attributes of the referenced elements. 1214 If the referenced element is a definition group (see Section 3.3.3), 1215 any child elements of an sdpng:use element MUST be transformed to 1216 child elements of the parent element of the sdpng:use element. Any 1217 properties (either explicit sdpng:prop elements or attributes of the 1218 sdpng:use element) MUST be transformed to properties of the parent 1219 element of the sdpng:use element. 1221 3.4 External Definition Packages 1223 There are two types of external definitions: 1225 Profile Definitions (Section 3.4.1) define rules for specifying 1226 parameters that are not covered by the base SDPng specification. 1228 Library Definitions (Section 3.4.2) contain definitions that can be 1229 referenced in SDPng documents. 1231 3.4.1 Profile Definitions 1233 In order to allow for extensibility it must be possible to define 1234 extensions to the basic SDPng configuration options. 1236 For example, if some application requires the use of a new transport 1237 protocol, endpoints must be able to describe their configuration with 1238 respect to the parameters of that transport protocol. The mandatory 1239 and optional parameters that can be configured and negotiated when 1240 using the transport protocol will be specified in a definition 1241 document. Such a definition document is called a "profile". 1243 A profile contains rules that specify how SDPng is used to describe 1244 conferences or end-system capabilities with respect to the parameters 1245 of the profile. The concrete properties of the profile definitions 1246 mechanism are still to be defined. 1248 An example of such a profile would be the RTP profile that defines 1249 how to specify RTP parameters. Another example would be the audio 1250 codec profiles that defines how specify audio codec parameters. 1252 SDPng documents can reference profiles and provide concrete 1253 definitions, for example the definition for the GSM audio codec. 1254 (This would be done in the "Definitions" section of an SDPng 1255 document.) An SDPng document that references a profile and provides 1256 concrete definitions of configurations can be validated against the 1257 profile definition. 1259 3.4.2 Library Definitions 1261 While profile definitions specify the allowed parameters for a given 1262 profile, SDPng "Definitions" sections refer to profile definitions 1263 and define concrete configurations based on a specific profile. 1265 In order for such definitions to be imported into SDPng documents, 1266 "SDPng libraries" may be defined and referenced in SDPng documents. 1267 A library is a set of definitions that is conforming to one or more 1268 profile definitions. 1270 The purpose of the library concept is to allow certain common 1271 definitions to be factored-out so that not every SDPng document has 1272 to include the basic definitions, for example the PCMU codec 1273 definition. SDP [2] uses a similar concept by relying on the well 1274 known static payload types (defined in RFC1890 [4]) that are also 1275 just referenced but never defined in SDP documents. 1277 An SPDng document that references definitions from an external 1278 library has to declare the use of the external library. The external 1279 library, being a set of configuration definitions for a given 1280 profile, again needs to declare the use of the profile that it is 1281 conforming to. A library itself can make reference to other external 1282 libraries. 1284 There are different possibilities of how profiles definitions and 1285 libraries can be used in SDPng documents: 1287 o In an SPDng document, a profile definition can be referenced and 1288 all the configuration definitions are provided within the document 1289 itself. The SDPng document is self-contained with respect to the 1290 definitions it uses. 1292 o In an SPDng document, the use of an external library can be 1293 declared. The library references a profile definition and the 1294 SDPng document references the library. There are two alternatives 1295 how external libraries can be referenced: 1297 by name: Referencing libraries by names implies the use of a 1298 registration authority where definitions and reference names 1299 can be registered with. It is conceivable that the most common 1300 SDPng definitions be registered that way and that there will be 1301 a baseline set of definitions that minimal implementations must 1302 understand. Secondly, a registration procedure will be 1303 defined, that allows vendors to register frequently used 1304 definitions with a registration authority (e.g., IANA) and to 1305 declare the use of registered definition packages in conforming 1306 SDPng documents. Of course, care should be taken not to make 1307 the external references too complex and thus require too much a 1308 priori knowledge in a protocol engine implementing SDPng. 1309 Relying on this mechanism in general is also problematic 1310 because it impedes the extensibility, as it requires 1311 implementors to provide support for new extensions in their 1312 products before they can inter-operate. Registration is not 1313 useful for spontaneous or experimental extensions that are 1314 defined in an SDPng library. 1316 by address: An alternative to referencing libraries by name is to 1317 declare the use of an external library by providing an address, 1318 i.e., an URL, that specifies where the library can be obtained. 1319 While this allows the use of arbitrary third-party libraries 1320 that can extend the basic SDPng set of configuration options in 1321 many ways, in introduces additional complexity that could 1322 result in in higher latency for the processing of a description 1323 document with references to external libraries. In addition, 1324 there are problems if the referenced libraries cannot be 1325 accessed by all communication partners. 1327 o Because of these problematic properties of external libraries, the 1328 final SDPng specification will have to provide a set of 1329 recommendations under which circumstances the different mechanisms 1330 of referring to external definitions should be used. 1332 3.5 Mappings 1334 A mapping needs to be defined in particular to SDP that allows to 1335 translate final session descriptions (i.e. the result of capability 1336 negotiation processes) to SDP documents. In principle, this can be 1337 done in a rather schematic fashion for the basic definitions. 1339 In addition, mappings to H.245 will be defined in order to support 1340 applications like SIP-H.323 gateways. 1342 4. Capability Negotiation 1344 SDPng is a description language for both potential configurations 1345 (i.e. capabilities) of participants in multimedia conferencers and 1346 for actual configurations (i.e. final specifications of parameters). 1347 Capability negotiation is the process of generating a usable set of 1348 potential configurations and finally an actual configuration from a 1349 set of potential configurations provided by each potential 1350 participant in a multimedia conference. 1352 SDPng supports the specification of endpoint capabilities and defines 1353 a negotiation process: In a negotiation process, capability 1354 descriptions are exchanged between participants. These descriptions 1355 are processed in a "collapsing" step which results in a set of 1356 commonly supported potential configurations. In a second step, the 1357 final actual configuration is determined that is used for a 1358 conference. This section specifies the usage of SDPng for capability 1359 negotiation. It defines the collapsing algorithm and the procedures 1360 for exchanging SDPng documents in a negotiation phase. 1362 The description language and the rules for the negotiation phase that 1363 are defined here are (in general) independent of the means by which 1364 descriptions are conveyed during a negotiation phase (a reliable 1365 transport service with causal ordering is assumed). There are 1366 however properties and requirements of call signalling protocols that 1367 have been considered to allow for a seamless integration of the 1368 negotiation into the call setup process. For example, in order to be 1369 usable with SIP, it must be possible to negotiate the conference 1370 configuration within the three-way-handshake of the call setup phase. 1371 In order to use SDPng instead of SDP according to the offer/answer 1372 model defined in [15] it must be able to determine an actual 1373 configuration in a single request/response cycle. 1375 4.1 Outline of the Negotiation Process 1377 Conceptually, the negotiation process comprises the following 1378 individual steps (considering two parties, A and B, where A tries to 1379 invite B to a conference). Please note that is describes the steps 1380 of the negotiation process conceptually -- it does not specify 1381 requirements for implementations. Specific procedures that MUST be 1382 followed by implementations are given below. 1384 1. A determines its potential configurations for the components that 1385 should be used in the conference (e.g. "interactive audio" and 1386 "shared whiteboard") and sends a corresponding SDPng instance to 1387 B. This SDPng instances is denoted "CAP(A)". 1389 2. B receives A's SDPng instance and analyzes the set of components 1390 (sdpng:c elements) in the description. For each component that B 1391 wishes to support it generates a list of potential configurations 1392 corresponding to B's capabilities, denoted "CAP(B)". 1394 3. B applies the collapsing function and obtains a list of potential 1395 configurations that both A and B can support, denoted 1396 "CAP(A)xCAP(B) = CAP(AB)". 1398 4. B sends CAP(B) to A. 1400 5. A also applies the collapsing function and obtains "CAP(AB)". At 1401 this step, both A and B know each other capabilities and the 1402 potential configurations that both can support. 1404 6. In order to obtain an actual configuration from the potential 1405 configuration that have been obtained, both particpants have to 1406 pick a subset of the potential configurations should actually be 1407 used in the conference and generate the actual configuration. It 1408 should be noted that it depends on the specific application 1409 whether each component must be assigned exactly one actual 1410 configuration (one sdpng:alt element) or whether it is allowed to 1411 list multiple actual configurations. In this model we assume 1412 that A selects the actual configuration, denoted CFG(AB). 1414 7. A augments CFG(AB) with the transport parameters it intends to 1415 use, e.g., on which endpoint addresses A wishes to receive data, 1416 obtaining CFG_T(A). A sends CFG_T(A) to A. 1418 8. B receives CFG_T(A) and adds its own transport parameters, 1419 resulting in CFG_T(AB). CFG_T(AB) contains the selected actual 1420 configurations and the transport parameters of both A and B (plus 1421 any other SDPng data, e.g., meta-information on the conference). 1422 CFG_T(AB) is the complete conference description. Both A and B 1423 now have the following information: 1425 CAP(A) A's supported potential configurations 1427 CAP(B) B's supported potential configurations 1429 CAP(AB) The set of potential configurations supported by both A 1430 and B. 1432 CFG(AB) The set of actual configurations to be used. 1434 CFG_T(AB) The set of actual configurations to be used augmented 1435 with all required parameters. 1437 In this model, the capability negotiation and configuration exchange 1438 process leads to a description that represents a global view of the 1439 configuration that should be used. This means, it contains the 1440 complete configuration for all participants including per-participant 1441 information like transport parameters. 1443 Note that the model presented here results in four SDPng exchanges. 1444 As an optimization, this procedure can be abbreviated to two 1445 exchanges by including the transport (and other) parameters into the 1446 potential configurations. A embeds its desired transport parameters 1447 into the list of potential configurations and B also sends all 1448 required parameters in the response together with B's potential 1449 configurations. Both A and B can then derive CFG_T(AB). Transport 1450 parameters are usually not negotiable, therefor they have to be 1451 distingiushed them from other configuration information. 1453 Specific procedures for re-negotiation and multi-party negotiation 1454 will be defined in a future version of this document. 1456 4.2 The Collapsing Algorithm 1458 The following procedure MUST be used for the collapsing of two SDPng 1459 document instances into one: 1461 CAP(A) and CAP(B) are the two SDPng description document instances. 1462 For each component (sdpng:c element) in CAP(A) there is a 1463 corresponding component in CAP(B). Components MAY be empty 1464 (containing no sdpng:alt elements) which means that there is no 1465 potential configuration and the component should not be used in the 1466 conference. 1468 Let cfg_AB be the result configuration element, initialized to an 1469 empty sdpng:cfg element. 1471 1. For each component (sdpng:c element) in CAP(A) named c_A 1473 * Let c_AB be the current result component, initialized to an 1474 empty sdpng:c element. 1476 * For each alternative (sdpng:alt element) in c_A named a_A 1478 + For each session element (name depends on the profile being 1479 used) in a_A named s_A 1481 - Resolve any reference to definition elements recursively 1482 and obtain s1_A, the standalone media session 1483 description. (Refer to Section 4.2.1 for a description 1484 of how to resolve references.) 1486 - Locate the component element that matches c_A in CAP(B) 1487 named (c_B). 1489 - Let a_AB be the current result alternative, initialized 1490 to an empty sdpng:alt element. 1492 - For each alternative (sdpng:alt element) in c_B named 1493 a_B 1495 o For each session element (name depends on the profile 1496 being used) in a_B named s_B 1498 * Let s1_AB be the computed result media session 1499 configuration. 1501 * Resolve any reference to definition elements 1502 recursively and obtain s1_B, the standalone media 1503 session description. 1505 * Apply collapse(s1_A,s2_B) to compute s1_AB, the 1506 collapsed media session configuration. 1508 * If s1_AB is not empty, add s1_AB to a_AB, the set 1509 of sessions for the current result alternative. 1511 - If a_AB is not empty, add a_AB to c_AB. 1513 * If c_AB is not empty, add c_AB to cfg_AB. 1515 The collapsing function for collapsing two elements is specified in 1516 Section 4.2.1. 1518 4.2.1 Collapsing Two Configurations 1520 Before two media session configuration element can be collapsed as 1521 described in Section 4.2 all references to definitions MUST be 1522 resolved. This MUST be performed recursively, i.e. references in 1523 definitions MUST also be resolved. For resolving references, the 1524 algorithm specified in Section 3.3 MUST be used. 1526 By resolving all references two intermediate session configuration 1527 elements are obtained that can then be collapsed according to the 1528 algorithm specified in the following sections. 1530 4.2.1.1 Collapsing of Attributes 1532 In SDPng, capabilities are specified in attributs of XML elements. 1533 Three different types of capabilities with different collapsing rules 1534 are defined. The type of a capability is encoded in the attribute 1535 value. 1537 Set of symbols: 1538 An attribute can specify a set of symbols. When two attributes 1539 are collapsed the result is the intersection of the two sets. 1541 The following examples shows how two elements (with one attribute 1542 representing a set of symbols) originated from two capability 1543 descriptions from participants A and B are collapsed: 1545 Element x in A's capability description: 1546 1548 Element x in B's capability description: 1549 1551 Result: 1552 1554 If the intersection result in an empty set the collapsing process 1555 has failed and there is no common set of values. If the 1556 collapsing of one of an element's attributes with the type "set of 1557 symbols" has failed, the collapsing process of the element itself 1558 MUST be considered to have failed as well. 1560 Numerical ranges: 1561 An attribute can also specify a numercial range. When two 1562 attributes are collapsed the result is the range of values that 1563 represents the intersection of the set of values that is included 1564 in both ranges. 1566 The following examples shows how two elements (with one attribute 1567 representing a numerical range) originated from two capability 1568 descriptions from participants A and B are collapsed: 1570 Element x in A's capability description: 1571 1573 Element x in B's capability description: 1574 1576 Result: 1577 1579 A numerical range is represented by a tuple of comma-separated 1580 numbers in brackets. The first number represents the lower bound 1581 of the range and the second number represents the upper bound. 1582 Let MIN(a,b) be a function that returns the minimum of a and b and 1583 MAX(a,b) be a function that returns the maximum of a and b. Given 1584 two ranges (minA, maxA) and (minB, maxB), the collapsed new range 1585 MUST be calculated using this algorithm: 1587 (MAX(minA, minB), MIN(maxA, maxB)) 1589 If this process results in a range with a smaller first value, 1590 the range is invalid and the collapsing has failed since there is 1591 no common range. If the collapsing of one of an element's 1592 attributes with the type "numerical range" has failed, the 1593 collapsing process of the element itself MUST be considered to 1594 have failed as well. 1596 Optional parameters: 1597 A failure of collapsing attributes of the types "set of symbols" 1598 and "numerical range" results in a failure of collapsing the 1599 corresponding element. There is a third type named "optional 1600 parameter" defined, that provides different collapsing rules. An 1601 optional parameter is an attribute with an arbitrary value. When 1602 collapsing two attributes of this type, their values MUST be 1603 tested for equality. If they are equal, the collapsing has been 1604 successful and the attribute MUST appear as is in the result 1605 description. If the attributes' values are different, the 1606 collapsing is considered to have failed and the attribute MUST not 1607 appear in the result description. However, a failure in 1608 collapsing an attribute of type "optional parameter" does not 1609 affect the collapsing of the containing element. 1611 An example for a successful collapsing: 1613 Element x in A's capability description: 1614 1616 Element x in B's capability description: 1617 1619 Result: 1620 1622 An example for an unsuccessful collapsing: 1624 Element x in A's capability description: 1625 1627 Element x in B's capability description: 1628 1630 Result: 1631 1633 4.2.1.2 Collapsing two Elements 1635 In order to collapse two elements with multiple attributes, the 1636 following algorithm specified below MUST be applied. In general, the 1637 collapsing of two elements (if successful) yields a result element 1638 that contains the collapsed attributes. If the collapsing of two 1639 elements has failed, no result element is generated. 1641 1. For each attribute, determine the type and collapse the attribute 1642 by applying the algorithm for the corresponding attribute type. 1644 2. If an attribute with a different type than "optional parameter" 1645 does not occur in both elements, the collapsing for this element 1646 MUST be considered to have failed. 1648 3. If the collapsing of any attribute with a different type than 1649 "optional parameter" has failed, the collapsing of the element 1650 itself MUST be considered to have failed. 1652 4. If the collapsing has been successful, obtain the result element 1653 by using the same element name (GI) and the attributes with their 1654 collapsed values. Exclude any attribute of type "optional 1655 parameter" that has failed to collapse. 1657 An example: 1659 Element x in A's capability description: 1660 1662 Element x in B's capability description: 1663 1665 Result: 1666 1668 4.2.1.3 Collapsing nested Elements 1670 In order to collapse nested elements the following algorithm MUST be 1671 applied: 1673 In analogy to attributes representing optional parameters there is 1674 also the possibility to mark elements as optional for the negotiation 1675 process. Elements MAY provide an attribute names "status" that 1676 contains a symbol or a comma-separated list of symbols as its value. 1677 If the value "opt" occurs in the list of a "status" attribute of both 1678 elements to be collapsed, the elements to be collapsed are treated as 1679 optional. This means, if the collapsing of the attributes has failed 1680 (according to the rules specified in Section 4.2.1.2), the collapsing 1681 process does not yield a result element but is still treated as 1682 "successful", i.e., further collapsing operation on other elements 1683 can continue. The semantics of optional elements are that they 1684 describe optional features that may be supported and selected during 1685 a negotiation phase but do not neccessarily have to be supported by 1686 all participants in order to achieve interoperability. The example 1687 below shows how to generate a result element in the presence of 1688 optional child elements that have failed to collapse. 1690 The collapsing algorithm for nested elements: 1692 1. Let x be an element that occurs in the capability description of 1693 two participants A and B and that should be collapsed. 1695 2. Collapse the attributes of the element x using the algorithm 1696 specified in Section 4.2.1.2. If the collapsing has failed 1697 according to the rules of Section 4.2.1.2 and if the elements to 1698 be collapsed are not marked as optional, the collapsing of the 1699 element and all of its children MUST be considered to have 1700 failed. The collapsing MUST be stopped. If the collapsing has 1701 failed and both elements have been marked as optional, the child 1702 elements MUST NOT be processed. In this case, the collapsing 1703 process does not yield a result element but the collapsing of 1704 other elements (sibling or parent elements) MUST be continued. 1706 3. If the collapsing has been successful according to the rules of 1707 Section 4.2.1.2, the child elements of A's and B's x element MUST 1708 be processed. If there are no child elements in both A's and B's 1709 content the collapsing has been successful and can be terminated. 1710 If either A's or B's x element provides child elements, apply the 1711 following algorithm to each child element named c of participant 1712 A's element x: 1714 1. Find a corresponding element (same GI) in the set of 1715 participant B's child elements. If no matching element has 1716 been found, the collapsing of element x MUST be considered to 1717 have failed. 1719 2. If a matching element has been found, apply the collapsing 1720 algorithm recursively. As long as the collapsing is 1721 successful, the result of collapsing each element is 1722 transferred to the result element, such that the resulting 1723 element tree is isomorphic to both A's and B's element tree. 1725 If there are elements in B's x element that have not been 1726 processed (because there is no corresponding element in A's x 1727 element), the collapsing MUST be considered to have failed and 1728 MUST be stopped. 1730 An example: 1732 Element x in A's capability description: 1733 1734 1735 1737 Element x in B's capability description: 1738 1739 1740 1742 Result: 1743 1744 1745 1747 An example for collapsing optional elements: 1749 Element x in A's capability description: 1750 1751 1752 1754 Element x in B's capability description: 1755 1756 1757 1759 Result: 1760 1762 4.2.2 Deriving an actual Configuration 1764 The result of a capability negotiation process is a potential 1765 configuration, i.e., a description potentially containing multiple 1766 alternatives per component. The alternative themselves may provide 1767 elements that represent collapsed capabilities. In order to derive 1768 an actual configuration, the following problems must be addressed: 1770 1. For each component (sdpng:c element) an appropriate alternative 1771 (sdpng:alt element) has to be selected. It is conceivable that 1772 the order of the alternatives in the description is used as a 1773 preference indicator. More details have to be specified in a 1774 future version of this document. 1776 2. If the description of the selected alternatives contains 1777 attributes with numerical ranges or sets of symbols with more 1778 than one entry, those attributes either have to be transformed 1779 that they represent a single value or participants have to agree 1780 that an actual configuration may contain ranges and sets of 1781 symbols. The semantics of these variable actual configurations 1782 will have to specified in later versions of this document. For 1783 example, for certain applications it may be desireable to agree 1784 on ranges of values for certain attributes during a capability 1785 negotiating meaning that any of the values of the range are 1786 supported (and have to be supported). 1788 The specific procedures to determine an actual configuration have to 1789 be defined in a later version on this document. 1791 5. Formal Specification 1793 This section defines the SDPng syntax and the use of XML mechanisms, 1794 such as XML Namespace and XML Schema. Section 5.1 defines the 1795 relation between SDPng and XML Schema, Section 5.2 specifies general 1796 requirements for documents and profile definitions that are 1797 conforming to the SDPng schema, Section 5.3 list requirements for 1798 profile definitions, Section 5.4 specifies specific requirements for 1799 conforming documents and Section 5.5 lists requirements for the 1800 definition of SDPng libraries. 1802 Section 5.7 defines the SDPng base schema, Section 5.7.2 defines the 1803 profile for audio codec definitions and Section 5.7.3 defines the 1804 profile for RTP payload type definitions. 1806 5.1 XML Schema as a Definition Mechanism 1808 SDPng documents reference profile schema definitions and libraries. 1809 Profile schema definitions contain schema definitions of SDPng 1810 document elements. For example, the general structure is specified 1811 by a schema definition and extensions to SDPng for specific 1812 applications are specified as schema definitions as well. 1814 The baseline SDPng specification consists of a profile (a schema 1815 definition) and a library of commonly used definitions. 1817 SDPng uses XML-Schema [13][14] for defining the possible logical 1818 structures of SDPng documents for the following reasons: 1820 Extensibility: XML-Schema provides mechanisms that allow to extend 1821 existing definitions allowing to uniquely identify element types 1822 (by relying on XML namespaces [11]). 1824 Modularity: XML-Schema provide mechanisms that allow to organize 1825 schema definitions in multiple components. 1827 Expressiveness: XML-Schema provides many data types, that can be 1828 refined by user-supplied definitions. 1830 SDPng documents MUST be schema instances of the SDPng schema as 1831 defined in Section 5.7. The following example shows how a Schema 1832 definition can be referenced in a document instance. 1834 Beginning of an SDPng-document: 1836 1837 1841 XML-Schema specifies that documents can assign a namespace when 1842 referencing a schema definition. A SDPng namespace is defined for 1843 this purpose. The name of this namespace is 1844 "http://www.iana.org/sdpng". A well-known namespace prefix is used 1845 for the SDPng schema definition, in order to allow for very simple 1846 implementations. The well-known SDPng namespace prefix is "sdpng". 1847 Conforming Documents, profile definition and libraries MUST use this 1848 namespace name and this namespace prefix. 1850 For SDPng documents, this initial declaration can be added implicitly 1851 by applications, so that declarations like the one above do not have 1852 to be included in every description document. Details are to be 1853 defined in a later version of this document. 1855 5.2 SDPng Schema 1857 The basic SDPng schema definitions specifies the general document 1858 structures, e.g., one "Definitions" section followed by one 1859 "Configurations" sections, followed by one "Constraints" sections 1860 followed by a "Conference" section (for meta-information). Each 1861 document MUST provide the elements for definitions, configurations, 1862 constraints and conference information in exactly this order, whereby 1863 only the configurations section is MANDATORY. Refer to Section 5.7 1864 for a formal definition of the SDPng base schema and the specific 1865 element types for definitions, configurations, constraints and 1866 conference information. 1868 The SDPng base schema also specifies "abstract" base data types (by 1869 means of XML-Schema type definitions) for elements that MUST be used 1870 by documents in the corresponding sections. The base data types 1871 provide common required attributes, e.g. a "name" attribute for 1872 naming definition elements. 1874 Example: 1875 The following example shows the definition of the base type for 1876 definition elements: 1878 1879 1880 1882 Profiles can then define specific types that augment the base type 1883 definitions. Common attributes or content models, that have been 1884 defined by this base definition, do not have to be provided by those 1885 concrete type definitions. The type definitions can be identified as 1886 allowed element types for the content models that are specified in 1887 the base SDPng schema definition. This allows for automatic 1888 validation of profile definitions and facilitates the extension of 1889 SDPng. 1891 5.3 Profiles 1893 The baseline SDPng specification consists of a profile (a schema 1894 definition) and a library of commonly used definitions. 1896 The library of commonly used definitions provides data types for IP 1897 (and other) addresses. 1899 A profile definition MUST import (using the XML-Schema import 1900 mechanism) the base SDPng schema definition and MUST provide an 1901 extension definition, e.g., specializations of base element types. A 1902 profile definition MUST also provide a target namespace name for its 1903 definitions. For well-known (registered) profiles, the namespace 1904 name will be registered by IANA. Proprietary profiles will use other 1905 namespace names, for example, based on domain names, that are 1906 registered by vendors providing a profile. 1908 Example: 1909 The following example shows such a declaration at the beginning of a 1910 profile definition: 1912 1917 1920 In this example, the namespace prefix "audio" is defined and later 1921 used in schema definitions. (The example profile provides definition 1922 mechanisms for audio codecs.) 1924 The following example shows, how a derived type for "definition" 1925 elements can be specified with XML-Schema mechanisms. In this case, 1926 the abstract type "Definition" (fully qualified as 1927 "sdpng:Definition") is augmented by three attributes that are useful 1928 for defining audio codecs. 1930 Example: 1932 1933 1934 1935 1936 1937 1938 1939 1940 1942 This type definition is then used to define an XML element type 1943 called "codec". 1945 Example: 1947 1949 When used by SDPng documents, the general identifier is qualified 1950 with a namespace prefix, for example as in: "audio:codec". 1952 5.4 SDPng Documents 1954 SDPng documents MUST reference the employed profiles and provide 1955 namespace prefixes for the namespace names of the profiles as shown 1956 in the following example. 1958 Example: 1960 1966 For well-known registered profiles, the namespace name AND the used 1967 namespace prefix SHOULD be registered to allow for simple basic 1968 implementations that can match identifiers by using fixed fully 1969 qualified names without having to interpret namespace declarations 1970 (see Section 5.6.3). There is one issue with declaring used XML- 1971 Schema definitions in documents (see Section 7 below). 1973 The general structure of an SDPng documents MUST conform to the basic 1974 SDPng schema definition and MAY provide a "def" element for 1975 definitions; it MUST provide a "cfg" element for the configuration 1976 section; it MAY provide a "constraints" and a "conf" element. 1978 Example: 1979 The following example shows a sample definition section where the 1980 element "codec" of the "audio codec profile" is used (plus the 1981 element type "pt" of an "RTP profile"): 1983 1984 1986 1988 1991 1992 1994 It can be seen how the attribute name (provided by the base type for 1995 definition elements) and the profile specific attributes "encoding", 1996 "channels" and "sampling" are used together. 1998 The element "rtp:pt" is used to defined a payload type. "rtp:pt" 1999 would have been defined in another profile, again using a type 2000 derived from the base definition type. "rtp:pt" provides attribute 2001 for referencing other definitions, e.g., the definition of audio- 2002 codes as seen before. 2004 5.5 Libraries 2006 SDPng libraries are collections of definitions that are referenced by 2007 documents. Libraries are thus independent, valid SDPng documents. 2009 For example, the definition of the different audio codecs as shown in 2010 the previous example could be provided by a library that can be 2011 referenced by documents without having to define such common codecs 2012 in every document. 2014 The XML mechanism XInclude [12] is used for referencing libraries in 2015 SDPng documents. XInlcude works at the XML Information Set 2016 ("infoset") level, i.e. the mechanisms allows to have an integrating 2017 document reference fragment documents, while these fragments are 2018 well-formed (and, if applicable, valid) documents themselves. By 2019 resolving XInclude directives in integrating documents the documents' 2020 infosets are "merged" together, enabling applications to operate on 2021 the resulting infosets as if it had been generated by parsing a 2022 single, monolithic document. 2024 Inclusion at the XML infoset level has the advantage that documents 2025 are standalone -- they can be validated independently. Another 2026 advantage is that is relatively easy to generate a "merged" infoset 2027 for applications that are not able to resolve references to libraries 2028 themselves. 2030 An alternative for XInclude would be to use references that are 2031 resolved by applications. For XML, this would probably mean to use 2032 an XLink-based approach. This solution would require the definition 2033 of an SDPng link element type and require applications to support 2034 XLink (or at least the SDPng-relevant subset thereof). The inclusion 2035 at the application level is however problematic, because it does not 2036 result in a common integrated XML document infoset but would require 2037 applications to handle multiple infosets, i.e. multiple documents. 2039 5.6 Details on the use of specific XML Mechanisms 2041 This section specifies the use of specific XML mechanisms for SDPng. 2042 In order to allow for efficient parsing and processing, not all 2043 features of XML Schema are allowed. Some variable information is set 2044 to fixed values to allow the development of simplistic servers. 2046 5.6.1 Default Namespace 2048 SDPng document instances MUST use the SDPng namespace 2049 "http://www.iana.org/sdpng". That means, the general SDPng 2050 identifiers can be used without namespace prefixes. 2052 5.6.2 Qualified Locals 2054 XML Schema allows to specify qualification of elements and 2055 attributes. It is possible to use non-qualified element and 2056 attribute names in Schema definitions and document instances for so- 2057 called "local definitions" (this is the default setting). "Local 2058 Definitions" are contained within "global definitions" in an XML 2059 schema definition. In order to simplify parsing and processing of 2060 SDPng document instances, all elements MUST be fully qualified. 2061 Attribute names MUST NOT be fully qualified, they are considered to 2062 have the same namespace as their corresponding elements. 2064 This means, the SDPng Schema definition contains the following 2065 attributes for the "schema" element, that MUST also be used by SDPng 2066 profiles: 2068 o elementFormDefault="qualified" 2069 This means that "locally defined" elements that are used within 2070 the scope of fully-qualified elements MUST always be fully 2071 qualified as well. 2073 o attributeFormDefault="unqualified" 2074 This means that attribute names do not have to be fully qualified. 2075 Implementations MUST infer the namespace for attributes from the 2076 namespace of the element that the attribute belongs to. Note that 2077 the specification of XML Namespaces [11] defines that default 2078 namespaces do not apply to attributes. In profile definitions, 2079 all attributes MUST be defined locally. The same holds for the 2080 base SDPng schema. 2082 These rules make SDPng document instances process-able by non-Schema- 2083 aware XML parsers by requiring all element names to be fully 2084 qualified (no "local elements"). 2086 5.6.3 Fixed Namespace Prefixes 2088 In order to facilitate the development of basic implementations, a 2089 few commonly used namespaces names are associated with fixed 2090 prefixes, i.e. document instances and libraries MUST always use these 2091 prefixes. These prefixes MUST NOT be used for namespaces names than 2092 the ones that are assigned to them. In order to ensure the 2093 uniqueness of namespace prefixes, namespace prefixes will be have to 2094 registered together with the corresponding namespace names. 2096 The namespace prefix for the SDPng namespace is "sdpng". 2098 5.7 SDPng Schema Definitions 2100 This section provides the definition of the base SDPng XML Schema. 2102 1. Section 5.7.1 contains the base definition that provides the 2103 general element types for SDPng. 2105 2. Section 5.7.2 contains a profile for audio codecs. 2107 3. Section 5.7.3 contains a profile for RTP payload type 2108 definitions. 2110 5.7.1 SDPng Base Definition 2112 This schema definition defines the general structure of SDPng 2113 document instances. It defines the top-level element type "desc" 2114 that can have a sequence of "def", "cfg", "constraints" and "conf" 2115 elements as element content. 2117 In addition, "extensions hooks" are provided that can be used by 2118 extension profiles providing definitions for specific applications. 2119 In general, these extension are implemented by deriving profile 2120 definitions from SDPng base definitions. The deployed XML Schema 2121 mechanisms are "deriving by extension" and "substitution groups". 2122 The SDPng base definition provides different base types (as 2123 complexType definitions) for elements that are to be used in "def", 2124 "cfg" and "conf" sections. In addition, it also defines specific 2125 element types as "head elements" with assigned types that are used 2126 for defining the content model of, e.g., the "def" element type. 2128 Profiles that provide new element types for specific applications 2129 will define types that are derived from the base types and provide 2130 the additional attributes and element content definitions that are 2131 required for the application. The XML element types that are defined 2132 in a profile are declared as valid substitutes for the base elements 2133 ("head elements") by setting the "substitutionGroup" attribute to the 2134 name of the "head element" type. 2136 For an extension-profile that provides new definition element types, 2137 e.g. for codec definitions, a new complexType would be defined that 2138 extends sdpng:Definition (see below). An element type definition 2139 that assigns that new type must then be declared to be in the 2140 substitutionGroup "sdpng:d". 2142 This mechanism allows common rules for attributes and content models 2143 to be defined in base element definition and re-used by extension 2144 profiles and it also allows validating parsers to identify the 2145 correct type of elements that have been defined by profile 2146 definitions. 2148 The SDPng Base Definition: 2150 2156 2157 2158 This schema definition defines the general structure of SDPng 2159 document instances. It provides base type and base element 2160 definition for elements to occur in the different sections (def, 2161 cfg, constraints, conf) to be derived from in extension-profile 2162 definitions. 2164 For an extension-profile that provides new definition element 2165 types, e.g. for codec definitions, a new complexType would be 2166 defined that extends sdpng:Definition (see below). An element 2167 type definition that assigns that new type must then be declared 2168 to be in the substitutionGroup "sdpng:d". 2169 2170 2171 2172 2173 2174 The top-level element of an SDPng document. It defines the 2175 overall structure of an SPDng document. 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186 2188 2190 2191 2192 The definitions section 2193 2194 2195 2196 2197 2198 2199 2201 2203 2204 2205 The configurations section 2206 2207 2208 2209 2210 2211 2212 2214 2216 2217 2218 The constraints section 2220 2221 2222 2223 2224 2225 2226 2228 2230 2231 2232 The conference section 2233 2234 2236 2238 2239 2240 2241 Placeholder base element for a definition element in the 2242 definitions section. To be derived from by specific definition 2243 element type definitions. 2244 2245 2246 2248 2250 2251 2252 2253 Placeholder base element for a configuration element in the 2254 configurations section. To be derived from by specific 2255 configuration element type definitions. 2256 2257 2258 2260 2262 2263 2264 2265 Placeholder base element for a contraint element in the 2266 contraints section. To be derived from by specific constraint 2267 element type definitions. 2269 2270 2271 2273 2275 2276 2277 2278 The base type for definition. Defines a attribute "name" for 2279 naming definitions. 2280 2281 2282 2283 2285 2287 2288 2289 2290 The specification of a component consists of a sequence of 2291 alternatives. 2292 2293 2294 2295 2296 2297 2298 2299 2301 2302 2303 2304 Each alternative consists of a "sc" (session configuration) 2305 element. The "sc" element is a base element of base type 2306 "sdpng:Session" that is used to derive specific session types 2307 in extension profiles. 2308 2309 2310 2311 2312 2313 2314 2315 2316 2317 2319 2320 2321 2322 The (abstract) base type for session elements. To be derived 2323 from in extension profiles. 2324 2325 2326 2328 2330 2331 2332 2333 The current content model for constraints is a sequence of 2334 "sdpng:par" elements. In each "par" element a sequence of 2335 "use-alt" elements may be used to specific the definitions 2336 that may used in parallel. Each "use-alt" element can define 2337 the number of minimum and maximum instantiations. 2338 2339 2340 2341 2342 2343 2345 2346 2347 2348 2349 2350 2351 2352 2354 2355 2356 2357 2359 2361 2362 2364 2366 2367 2368 2369 2370 2371 2373 2374 2375 2376 The base type for conference meta inforformation 2377 element. Currently, there is no common content model defined. 2378 2379 2380 2382 2384 2385 2386 2387 2388 2389 2390 2391 2392 2393 2394 2395 2396 2397 2398 2399 2401 2403 2404 2405 2406 2407 2408 2409 2410 2411 2412 2413 2414 2415 2416 2417 2418 2419 2420 2421 2422 2423 2424 2425 2426 2427 2429 5.7.2 Audio Codec Profile 2431 The following profile defines an element type that can be used for 2432 specifying audio codec characteristics. The element "audio:codec" is 2433 of type "audio:AudioCodec" which is derived from the SDPng base type 2434 "sdpng:Definition". The element "audio:codec" is declared to have 2435 the substitution group "sdpng:d" (the "head element" of the SDPng 2436 base definition). 2438 This means, "audio:codec" element can be used as child elements in 2439 "sdpng:def" elements. In addition to the attributes specified here 2440 "audio:codec" elements will also have to provide a "name" attribute 2441 as defined by "sdpng:Definition". 2443 2450 2453 2454 2455 2456 2457 2458 2459 2460 2461 2462 2463 2465 2466 2467 2468 2469 2470 2471 2472 2474 5.7.3 RTP profile 2476 The following profile defines element types that can be used for 2477 specifying RTP payload types and RTP session configurations. The 2478 element "rtp:pt" is of type "rtp:PayloadType" which is derived from 2479 the SDPng base type "sdpng:Definition". The element "rtp:pt" is 2480 declared to have the substitution group "sdpng:d" (the "head element" 2481 of the SDPng base definition). 2483 The element "rtp:session" is of type "rtp:Session" which is derived 2484 from the SDPng base type "sdpng:SessionConfig". The element 2485 "rtp:session" is declared to have the substitution group "sdpng:sc" 2486 (the "head element" of the SDPng base definition). 2488 The RTP profile in turn defines base types for the specification of 2489 transport parameters that are to be derived from by profiles that 2490 define rules for elements that can be used to specify parameters for 2491 specific transport mechanisms. 2493 2500 2503 2504 2505 2506 PayloadType, the element for payload type definitions is 2507 derived from "sdpng:Definition". Inside an element of this 2508 type, more definitions may be given (derived from 2509 sdpng:Definition themselves). If no definition is given in the 2510 content, a definition may be referenced using the "format 2511 attribute". 2512 2513 2514 2515 2516 2517 2518 2519 2520 2521 2522 2523 2524 2525 2527 2529 2531 2533 2534 2535 2537 2538 2539 2540 2541 2542 2543 2545 2546 2547 2548 2549 2550 2551 2552 2553 2554 2556 2557 2558 2560 2561 2562 2564 2565 2566 2568 2569 2570 2571 2572 2573 2574 2575 2576 2577 2578 2579 2581 2583 2585 5.8 Issues 2587 o Libraries provide partially specified definitions, i.e. without 2588 transport parameters. How can SDPng documents reference the 2589 definitions and augment them with specific transport parameters? 2591 o Referencing extension profiles: XML-Schema does not support the 2592 declaration of multiple schemas via the schemaLocation attribute. 2593 Conceivable solution: When extension profiles are used, the SDPng 2594 description is a "multi-part" object, that consists of an 2595 integrating schema definition (that references all necessary 2596 profiles and the base definition) and the actual description 2597 document that is a schema instance of the integrating schema. 2599 o Uniqueness of attribute values: When libraries are used they will 2600 contain definition elements with "name" attributes for later 2601 referencing. How to avoid name clashes for those identifiers? 2602 When an SDPng document uses libraries from different sources they 2603 could be incompatible because of name collisions. Possible 2604 solution: Prefix such IDs with a namespace name (either explicitly 2605 or implicitly by interpreting applications). The explicit 2606 prefixes have the advantage that no special knowledge would be 2607 required to resolve links at the cost of very long ID values. 2609 6. Use of SDPng in conjunction with other IETF Signaling Protocols 2611 The SDPng model provides the notion of Components to indicate the 2612 intended types of collaboration between the users in e.g. a 2613 teleconferencing scenario. 2615 Three different abstractions are defined that are used for describing 2616 the properties of a specific Component: 2618 o a Capability refers to the fact that one of the involved parties 2619 supports one particular way of exchanging media -- defined in 2620 terms of transport, codec, and other parameters -- as part of the 2621 media session. 2623 o a Potential Configuration denotes a set of matching Capabilities 2624 from all those involved parties required to successfully realize 2625 one particular Component. 2627 o an Actual Configuration indicates the Potential Configuration 2628 which was chosen by the involved parties to realize a certain 2629 Component at one particular point in time. 2631 As mentioned before, this abstract notion of the interactions between 2632 a number of communicating systems needs to be mapped to the 2633 application scenarios of SDPng in conjunction with the various IETF 2634 signaling protocols: SAP, SIP, RTSP, and MEGACO. 2636 In general, this section provides recommendations and possible 2637 scenarios for the use of SDPng within specific protocols and 2638 applications. Is does not specify normative requirements. 2640 6.1 The Session Announcement Protocol (SAP) 2642 SAP is used to disseminate a previously created (and typically fixed) 2643 session description to a potentially large audience. An interested 2644 member of the audience will use the SDPng description contained in 2645 SAP to join the announced media sessions. 2647 This means that a SAP announcement contains the Actual Configurations 2648 of all Components that are part of the overall teleconference or 2649 broadcast. 2651 A SAP announcement may contain multiple Actual Configurations for the 2652 same Component. In this case, the "same" (i.e. semantically 2653 equivalent) media data from one configuration must be available from 2654 each of the Actual Configurations. In practice, this limits the use 2655 of multiple Actual Configurations to single-source multicast or 2656 broadcast scenarios. 2658 Each receiver of a SAP announcement with SDPng compares its locally 2659 stored Capabilities to realize a certain Component against the Actual 2660 Configurations contained in the announcement. If the intersection 2661 yields one or more Potential Configurations for the receiver, it 2662 chooses the one it sees fit best. If the intersection is empty, the 2663 receiver cannot participate in the announced session. 2665 SAP may be substituted by HTTP (in the general case, at least), SMTP, 2666 NNTP, or other IETF protocols suitable for conveying a media 2667 description from one entity to one or more other without the intend 2668 for further negotiation of the session parameters. 2670 Example from the SAP spec. to be provided. 2672 6.2 Session Initiation Protocol (SIP) 2674 SIP is used to establish and modify multimedia sessions, and SDPng 2675 may be carried at least in SIP INVITE and ACK messages as well as in 2676 a number of responses. From dealing with legacy SDP (and its 2677 essential non-suitability for capability negotiation), a particular 2678 use and interpretation of SDP has been defined for SIP. 2680 One of the important flexibilities introduced by SIP's usage of SDP 2681 is that a sender can change dynamically between all codecs that a 2682 receiver has indicated support (and has provided an address) for. 2683 Codec changes are not signaled out-of-band but only indicated by the 2684 payload type within the media stream. From this arises one important 2685 consequence to the conceptual view of a Component within SDPng. 2687 There is no clear distinction between Potential and Actual 2688 Configurations. There need not be a single Actual Configuration be 2689 chosen at setup time within the SIP signaling. Instead, a number of 2690 Potential Configurations is signaled in SIP (with all transport 2691 parameters required for carrying media streams) and the Actual 2692 Configuration is only identified by the payload type which is 2693 actually being transmitted at any point in time. 2695 Note that since SDPng does not explicitly distinguish between 2696 Potential and Actual Configurations, this has no implications on the 2697 SDPng signaling itself. 2699 SIP relies on an "offer/answer" model for the exchange of capability 2700 and configuration information. Either the caller or the callee sends 2701 an initial session description that is processed by the other side 2702 and returned. For capability negotiation, this means that the 2703 negotiation follows a two-stage-process: The "offerer" sends its 2704 capability description to the receiver. The receiver processes the 2705 offerers capabilities and his own capabilities and generates a result 2706 capability description that is sent back to the offerer. Both sides 2707 now know the commonly supported configurations and can initiate the 2708 media sessions. 2710 Because of this strict "offer/answer" model, the offerer must already 2711 send complete configurations (i.e. include transport addresses) along 2712 with the capability descriptions. The answer must also contain 2713 complete configuration parameters. The following figure shows, how 2714 SDPng content can be used in an INVITE request with a correspong 200 2715 OK message. 2717 Simple description document with only one alternative: 2719 F1 INVITE A -> B 2721 INVITE sip:B@example.com SIP/2.0 2722 Via: SIP/2.0/UDP hostA.example.com:5060 2723 From: A 2724 To: B 2725 Call-ID: 1234@hostA.example.com 2726 CSeq: 1 INVITE 2727 Contact: 2728 Content-Type: application/sdpng 2729 Content-Length: 685 2731 2732 2735 2736 2738 2739 2740 2741 2742 2744 2745 2746 2747 2749 2750 2752 2753 2754 2755 Telephony media stream 2756 2757 2759 ================================================== 2761 F2 (100 Trying) B -> A 2763 SIP/2.0 100 Trying 2764 Via: SIP/2.0/UDP hostA.example.com:5060 2765 From: A 2766 To: B 2767 Call-ID: 1234@hostA.example.com 2768 CSeq: 1 INVITE 2769 Content-Length: 0 2771 ================================================== 2773 F3 180 Ringing B -> A 2775 SIP/2.0 180 Ringing 2776 Via: SIP/2.0/UDP hostA.example.com:5060 2777 From: A 2778 To: B ;tag=987654 2779 Call-ID: 1234@hostA.example.com 2780 CSeq: 1 INVITE 2781 Content-Length: 0 2783 ================================================== 2785 F4 200 OK B -> A 2787 SIP/2.0 200 OK 2788 Via: SIP/2.0/UDP hostA.example.com:5060 2789 From: A 2790 To: B ;tag=987654 2791 Call-ID: 1234@hostA.example.com 2792 CSeq: 1 INVITE 2793 Contact: 2794 Content-Type: application/sdpng 2795 Content-Length: 479 2797 2798 2801 2803 2805 2806 2807 2808 2809 2811 2813 2814 2815 2816 2817 ================================================== 2819 ACK from A to B omitted 2821 In the INVITE message, A sends B a description document, that 2822 specifies exactly one component with one alternative (the PCMU audio 2823 stream). All required transport parameters all already contained in 2824 the description. The rtp:udp element provides an attribute "role" 2825 with a value of "receive", indicating that the specified endpoint 2826 address is used by the endpoint to receive media data. The element 2827 also provides the attribute "endpoint" with a value of "A", 2828 denominating the endpoint that can receive data on the specified 2829 address. This means, the semantics of specified transport addresses 2830 in configuration descriptions are the same as for SDP (when used with 2831 SIP): An endpoint specifies where it wants to receive data. 2833 In the 200 OK message, B sends an updated description document to A. 2834 For the sake of conciseness, the conf element (containing meta 2835 information about the conference) has been omitted. B supports the 2836 payload format that A has offered and adds his own transport 2837 parameters to the configuration information, specifying the endpoint 2838 address where B wants to receive media data. In order to 2839 disambiguate its transport configurations from A's, B sets the 2840 attribute "endpoint" to the value "B". The specific value of the 2841 "endpoint" attribute is not important, the only requirements are that 2842 a party that contributes to the session description, must use a 2843 unique name for the endpoint attribute and that a contributing party 2844 must use the same value for the endpoint attributes of all elements 2845 it adds to the session description. 2847 The following example shows a capability description that provides 2848 two alternatives for the audio component. 2850 Description document with two alternatives: 2852 F1 INVITE A -> B 2854 INVITE sip:B@example.com SIP/2.0 2855 Via: SIP/2.0/UDP hostA.example.com:5060 2856 From: A 2857 To: B 2858 Call-ID: 1234@hostA.example.com 2859 CSeq: 1 INVITE 2860 Contact: 2861 Content-Type: application/sdpng 2862 Content-Length: 935 2864 2865 2868 2870 2871 2873 2875 2877 2878 2879 2880 2881 2882 2883 2884 2885 2886 2888 2889 2891 2892 2893 2894 Telephony media stream 2895 2896 2898 ================================================== 2900 F2 (100 Trying) B -> A 2902 SIP/2.0 100 Trying 2903 Via: SIP/2.0/UDP hostA.example.com:5060 2904 From: A 2905 To: B 2906 Call-ID: 1234@hostA.example.com 2907 CSeq: 1 INVITE 2908 Content-Length: 0 2910 ================================================== 2912 F3 180 Ringing B -> A 2914 SIP/2.0 180 Ringing 2915 Via: SIP/2.0/UDP hostA.example.com:5060 2916 From: A 2917 To: B ;tag=987654 2918 Call-ID: 1234@hostA.example.com 2919 CSeq: 1 INVITE 2920 Content-Length: 0 2922 ================================================== 2924 F4 200 OK B -> A 2926 SIP/2.0 200 OK 2927 Via: SIP/2.0/UDP hostA.example.com:5060 2928 From: A 2929 To: B ;tag=987654 2930 Call-ID: 1234@hostA.example.com 2931 CSeq: 1 INVITE 2932 Contact: 2933 Content-Type: application/sdpng 2934 Content-Length: 479 2936 2937 2940 2941 2942 2944 2947 2949 2951 2952 2953 2954 2955 2956 2957 2958 ================================================== 2960 ACK from A to B omitted 2962 In the INVITE message, A sends B a description document, that 2963 specifies one component with two alternatives for the audio stream 2964 (PCMU and G.729). Since A wants to use the same transport address 2965 for receiving media data regardless of the payload format, A provides 2966 the transport specification in the def element and references this 2967 definition in the rtp:session elements for both alternatives by using 2968 the attribute "transport". 2970 In the 200 OK message, B sends an updated description document to A. 2971 B does only support PCMU, so it removes the alternative for G.729 2972 from the description. B also defines its transport address in the 2973 def element and references this definition by adding "B-rcv" to the 2974 attribute "transport" of the rtp:session element. (B could also have 2975 used the rtp:udp element inside the rtp:session element, but this 2976 example intends to demonstrate how to reference multiple transport 2977 definitions by using the attribute "transport"). 2979 6.3 Real-Time Streaming Protocol (RTSP) 2981 In contrast to SIP, RTSP has, from its intended usage, a clear 2982 distinction between offering Potential Configurations (typically by 2983 the server) and choosing one out of these (by the client), and, in 2984 some cases; some parameters (such as multicast addresses) may be 2985 dictated by the server. Hence with RTSP, there is a clear 2986 distinguish between Potential Configurations during the negotiation 2987 phase and a finally chosen Actual Configuration according to which 2988 streaming will take place. 2990 Example from the RTSP spec to be provided. 2992 6.4 Media Gateway Control Protocol (MEGACOP) 2994 The MEGACO architecture also follows the SDPng model of a clear 2995 separation between Potential and Actual Configurations. Upon 2996 startup, a Media Gateway (MG) will "register" with its Media Gateway 2997 Controller (MGC) and the latter will audit the MG for its 2998 Capabilities. Those will be provided as Potential Configurations, 2999 possibly with extensive Constraints specifications. Whenever a media 3000 path needs to be set up by the MGC between two MGs or an MG needs to 3001 be reconfigured internally, the MGC will use (updated) Actual 3002 Configurations. 3004 Details and examples to be defined. 3006 7. Open Issues 3008 Definition of baseline libraries 3010 A registry (reuse of SDP mechanisms and names etc.) needs to be 3011 set up. 3013 Negotiation mechanisms for multiparty conferencing need to be 3014 formalized. 3016 Mapping to other media description formats (SDP, H.245, ...) 3017 should be provided. For H.245, this is probably a different 3018 document (belonging to the SIP-H.323 inter-working group). 3020 Implicit declaration of SDPng schema and default profiles 3022 References 3024 [1] Kutscher, D., Ott, J., Bormann, C. and I. Curcio, "Requirements 3025 for Session Description and Capability Negotiation", Internet 3026 Draft draft-ietf-mmusic-sdpng-req-01.txt, April 2001. 3028 [2] Handley, M. and V. Jacobsen, "SDP: Session Description 3029 Protocol", RFC 2327, April 1998. 3031 [3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen, 3032 "RTP: A Transport Protocol for Real-Time Applications", RFC 3033 1889, January 1996. 3035 [4] Schulzrinne, H., "RTP Profile for Audio and Video Conferences 3036 with Minimal Control", RFC 1890, January 1996. 3038 [5] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video 3039 Conferences with Minimal Control", draft-ietf-avt-profile-new- 3040 10.txt (work in progress), March 2001. 3042 [6] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, 3043 M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP 3044 Payload for Redundant Audio Data", RFC 2198, September 1997. 3046 [7] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for 3047 Generic Forward Error Correction", RFC 2733, December 1999. 3049 [8] Perkins, C. and O. Hodson, "Options for Repair of Streaming 3050 Media", RFC 2354, June 1998. 3052 [9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement 3053 Protocol", RFC 2974, October 2000. 3055 [10] World Wide Web Consortium (W3C), "Extensible Markup Language 3056 (XML) 1.0 (Second Edition)", Status W3C Recommendation, Version 3057 http://www.w3.org/TR/2000/REC-xml-20001006, October 2000. 3059 [11] World Wide Web Consortium (W3C), "Namespaces in XML", Status 3060 W3C Recommendation, Version http://www.w3.org/TR/1999/REC-xml- 3061 names-19990114, January 1999. 3063 [12] World Wide Web Consortium (W3C), "XML Inclusions (XInclude) 3064 Version 1.0", Status W3C Working Draft, Version 3065 http://www.w3.org/TR/2001/WD-xinclude-20010516, May 2001. 3067 [13] World Wide Web Consortium (W3C), "XML Schema Part 1: 3068 Structures", Version http://www.w3.org/TR/2001/REC-xmlschema-1- 3069 20010502/, Status W3C Recommendation, May 2001. 3071 [14] World Wide Web Consortium (W3C), "XML Schema Part 2: 3072 Datatypes", Version http://www.w3.org/TR/2001/REC-xmlschema-2- 3073 20010502/, Status W3C Recommendation, May 2001. 3075 [15] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 3076 SDP", draft-ietf-mmusic-sdp-offer-answer-01.txt (work in 3077 progress), February 2002. 3079 Authors' Addresses 3081 Dirk Kutscher 3082 TZI, Universitaet Bremen 3083 Bibliothekstr. 1 3084 Bremen 28359 3085 Germany 3087 Phone: +49.421.218-7595, sip:dku@tzi.org 3088 Fax: +49.421.218-7000 3089 EMail: dku@tzi.uni-bremen.de 3091 Joerg Ott 3092 TZI, Universitaet Bremen 3093 Bibliothekstr. 1 3094 Bremen 28359 3095 Germany 3097 Phone: +49.421.201-7028, sip:jo@tzi.org 3098 Fax: +49.421.218-7000 3099 EMail: jo@tzi.uni-bremen.de 3101 Carsten Bormann 3102 TZI, Universitaet Bremen 3103 Bibliothekstr. 1 3104 Bremen 28359 3105 Germany 3107 Phone: +49.421.218-7024, sip:cabo@tzi.org 3108 Fax: +49.421.218-7000 3109 EMail: cabo@tzi.org 3111 Appendix A. Base SDPng Specifications for Audio Codec Descriptions 3113 [5] specifies a number of audio codecs including short name to be 3114 used as reference by session description protocols such as SDP and 3115 SDPng. Those codec names, as listed in the first column of the above 3116 table, are used to identify codecs in SDPng. 3118 The following sections indicate the default values that are assumed 3119 if nothing else than the codec reference is specified. 3121 The following audio:codec attributes are defined for audio codecs: 3123 name: the identifier to be later used for referencing the codec spec 3125 encoding: the RTP/AVP profile identifier as registered with IANA 3127 mime: the MIME type; may alternatively be specified instead of 3128 "encoding" 3130 channels: the number of independent media channels 3132 pattern: the media channel pattern for mapping channels to payload 3134 sampling: the sample rate for the codec (which in most cases equals 3135 the RTP clock) 3137 Furthermore, options may be defined of the following format: 3139 3141 if a value is associated with the option (note that arbitrary complex 3142 values are allowed), or alternatively: 3144