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Alvestrand 3 Internet-Draft Google 4 Intended status: Standards Track April 28, 2015 5 Expires: October 30, 2015 7 WebRTC MediaStream Identification in the Session Description Protocol 8 draft-ietf-mmusic-msid-10 10 Abstract 12 This document specifies a Session Description Protocol (SDP) Grouping 13 mechanism for RTP media streams that can be used to specify relations 14 between media streams. 16 This mechanism is used to signal the association between the SDP 17 concept of "media description" and the WebRTC concept of 18 "MediaStream" / "MediaStreamTrack" using SDP signaling. 20 This document is a work item of the MMUSIC WG, whose discussion list 21 is mmusic@ietf.org. 23 Requirements Language 25 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 26 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 27 document are to be interpreted as described in RFC 2119 [RFC2119]. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on October 30, 2015. 46 Copyright Notice 48 Copyright (c) 2015 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.1. Structure Of This Document . . . . . . . . . . . . . . . 3 65 1.2. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 3 66 1.3. The WEBRTC MediaStream . . . . . . . . . . . . . . . . . 4 67 2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . 4 68 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 3.1. Handling of non-signalled tracks . . . . . . . . . . . . 6 70 3.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 8 71 3.2.1. Generating the initial offer . . . . . . . . . . . . 8 72 3.2.2. Answerer processing of the Offer . . . . . . . . . . 8 73 3.2.3. Generating the answer . . . . . . . . . . . . . . . . 8 74 3.2.4. Offerer processing of the answer . . . . . . . . . . 8 75 3.2.5. Modifying the session . . . . . . . . . . . . . . . . 9 76 3.3. Example SDP description . . . . . . . . . . . . . . . . . 9 77 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 78 4.1. Attribute registration in existing registries . . . . . . 10 79 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 80 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 81 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 82 7.1. Normative References . . . . . . . . . . . . . . . . . . 11 83 7.2. Informative References . . . . . . . . . . . . . . . . . 11 84 Appendix A. Design considerations, rejected alternatives . . . . 12 85 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 12 86 B.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 12 87 B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 12 88 B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 12 89 B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 13 90 B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 13 91 B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 13 92 B.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 13 93 B.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 13 94 B.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 14 95 B.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 14 96 B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 14 97 B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 14 98 B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 15 99 B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 15 100 B.15. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 15 101 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 103 1. Introduction 105 1.1. Structure Of This Document 107 This document adds a new Session Description Protocol (SDP) [RFC4566] 108 mechanism that can associate application layer identifiers with the 109 binding between media streams, attaching identifiers to the media 110 streams and attaching identifiers to the groupings they form. It is 111 designed for use with WebRTC[I-D.ietf-rtcweb-overview] . 113 Section 1.2 gives the background on why a new mechanism is needed. 115 Section 2 gives the definition of the new mechanism. 117 Section 3 gives the necessary semantic information and procedures for 118 using the msid attribute to signal the association of 119 MediaStreamTracks to MediaStreams in support of the WebRTC API 120 [W3C.WD-webrtc-20150210]. 122 1.2. Why A New Mechanism Is Needed 124 When media is carried by RTP [RFC3550], each RTP media stream is 125 distinguished inside an RTP session by its SSRC; each RTP session is 126 distinguished from all other RTP sessions by being on a different 127 transport association (strictly speaking, 2 transport associations, 128 one used for RTP and one used for RTCP, unless RTP/RTCP multiplexing 129 [RFC5761] is used). 131 SDP gives a description based on media descriptions. According to 132 the model used in [I-D.ietf-rtcweb-jsep], each media description 133 describes exactly one media source, and if mulitple media sources are 134 carried in an RTP session, this is signalled using BUNDLE 135 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each 136 media source is carried in its own RTP session. 138 The SDP grouping framework [RFC5888] can be used to group media 139 descriptions. However, for the use case of WebRTC, there is the need 140 for an application to specify some application-level information 141 about the association between the media description and the group. 142 This is not possible using the SDP grouping framework. 144 1.3. The WEBRTC MediaStream 146 The W3C WebRTC API specification [W3C.WD-webrtc-20150210] specifies 147 that communication between WebRTC entities is done via MediaStreams, 148 which contain MediaStreamTracks. A MediaStreamTrack is generally 149 carried using a single SSRC in an RTP session (forming an RTP media 150 stream. The collision of terminology is unfortunate.) There might 151 possibly be additional SSRCs, possibly within additional RTP 152 sessions, in order to support functionality like forward error 153 correction or simulcast. This complication is ignored below. 155 In the RTP specification, media streams are identified using the SSRC 156 field. Streams are grouped into RTP Sessions, and also carry a 157 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 158 Therefore, the association of an RTP media stream to MediaStreams 159 need to be explicitly signaled. 161 WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where 162 one SDP media description is used to describe each MediaStreamTrack, 163 and that the BUNDLE mechanism 164 [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to group 165 MediaStreamTracks into RTP sessions. Therefore, the need is to 166 specify the ID of a MediaStreamTrack and its associated MediaStream 167 for each media description, which can be accomplished with a media- 168 level SDP attribute. 170 This usage is described in Section 3. 172 2. The Msid Mechanism 174 This document defines a new SDP [RFC4566] media-level "msid" 175 attribute. This new attribute allows endpoints to associate RTP 176 media streams that are described in different media descriptions. 177 The attribute also allows application-specific information to the 178 association. 180 The value of the "msid" attribute consists of an identifier and 181 optional application-specific data. 183 The name of the attribute is "msid". 185 The value of the attribute is specified by the following ABNF 186 [RFC5234] grammar: 188 msid-value = msid-id [ SP msid-appdata ] 189 msid-id = 1*64token-char ; see RFC 4566 190 msid-appdata = 1*64token-char ; see RFC 4566 192 An example msid value for a group with the identifier "examplefoo" 193 and application data "examplebar" might look like this: 195 msid:examplefoo examplebar 197 The identifier is a string of ASCII characters that are legal in a 198 "token", consisting of between 1 and 64 characters. 200 Application data is carried on the same line as the identifier, 201 separated from the identifier by a space. 203 The identifier uniquely identifies a group within the scope of an SDP 204 description. 206 There may be multiple msid attributes in a single media description. 207 There may also be multiple media descriptions that have the same 208 value for identifier and application data. 210 Endpoints can update the associations between RTP media streams as 211 expressed by msid attributes at any time; the semantics and 212 restrictions of such grouping and ungrouping are application 213 dependent. 215 3. Procedures 217 This section describes how to use the msid-semantic attribute for 218 associating media descriptions representing MediaStreamTracks within 219 MediaStreams as defined in [W3C.WD-webrtc-20150210]. 221 In the Javascript API, each MediaStream and MediaStreamTrack has an 222 "id" attribute, which is a DOMString. 224 The value of the "msid-id" field in the msid consists of the "id" 225 attribute of a MediaStream, as defined in its WebIDL specification. 227 The value of the "msid-appdata" field in the msid consists of the 228 "id" attribute of a MediaStreamTrack, as defined in its WebIDL 229 specification. 231 If two different media descriptions have MSID attributes with the 232 same values for "msid-id" and "msid-appdata", it means that these two 233 media descriptions are both intended for the same MediaStreamTrack. 235 So far, no semantic for such a mixture have been defined, but this 236 specification does not forbid the practice. 238 When an SDP session description is updated, a specific "msid-id" 239 continues to refer to the same MediaStream, and a specific "msid- 240 appdata" to the same MediaStreamTrack. There is no memory apart from 241 the currently valid SDP descriptions; if an msid "identifier" value 242 disappears from the SDP and appears in a later negotiation, it will 243 be taken to refer to a new MediaStream. 245 The following is a high level description of the rules for handling 246 SDP updates. Detailed procedures are in Section 3.2. 248 o When a new msid "identifier" value occurs in a session 249 description, the recipient can signal to its application that a 250 new MediaStream has been added. 252 o When a session description is updated to have media descriptions 253 with an msid "identifier" value, with one or more different 254 "appdata" values, the recipient can signal to its application that 255 new MediaStreamTracks have been added to the MediaStream. This is 256 done for each different msid "identifier" value. 258 o When a session description is updated to no longer list any msid 259 attribute on a specific media description, the recipient can 260 signal to its application that the corresponding MediaStreamTrack 261 has ended. 263 In addition to signaling that the track is closed when its msid 264 attribute disappears from the SDP, the track will also be signaled as 265 being closed when all associated SSRCs have disappeared by the rules 266 of [RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout), 267 and when the corresponding media description is disabled by setting 268 the port number to zero. Changing the direction of the media 269 description (by setting "sendonly", "recvonly" or "inactive" 270 attributes) will not close the MediaStreamTrack. (This mechanism may 271 be used to signal that a particular MediaStreamTrack should be put on 272 temporary hold, but that usage is not specified in this memo.) 274 The association between SSRCs and media descriptions is specified in 275 [I-D.ietf-rtcweb-jsep]. 277 3.1. Handling of non-signalled tracks 279 Entities that do not use msid will not send msid. This means that 280 there will be some incoming RTP packets that the recipient has no 281 predefined MediaStream id value for. 283 Note that this handling is triggered by incoming RTP packets, not by 284 SDP negotiation. 286 When MSID is used, the only time this can happen is when, at a time 287 subsequent to the initial negotiation, a negotiation is performed 288 where the answerer adds a MediaStreamTrack to an already established 289 connection and starts sending data before the answer is received by 290 the offerer. For initial negotiation, packets won't flow until the 291 ICE candidates and fingerprints have been exchanged, so this is not 292 an issue. 294 The recipient of those packets will perform the following steps: 296 o When RTP packets are initially received, it will create an 297 appropriate MediaStreamTrack based on the type of the media 298 (carried in PayloadType), and use the mid RTP attribute (if 299 present) to associate the RTP packets with a specific media 300 section. If the connection is not in the RTCSignalingState 301 "stable", it will wait at this point. 303 o When the connection is in the RTCSignalingState "stable", it will 304 look at the relevant media section to find the msid attribute. 306 o If there is an msid attribute, it will use that attribute to 307 populate the "id" field of the MediaStreamTrack and associated 308 MediaStreams, as described above. 310 o If there is no msid attribute, the identifier of the 311 MediaStreamTrack will be set to a randomly generated string, and 312 it will be signalled as being part of a MediaStream with the 313 WebIDL "label" attribute set to "Non-WebRTC stream". 315 o After deciding on the "id" field to be applied to the 316 MediaStreamTrack, the track will be signalled to the user. 318 The process above may involve a considerable amount of buffering 319 before the stable state is entered, If the implementation wishes to 320 limit this buffering, it MUST signal to the user that media has been 321 discarded. 323 It follows from the above that media stream tracks in the "default" 324 media stream cannot be closed by removing the msid attribute; the 325 application must instead signal these as closed when the SSRC 326 disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5 327 or by disabling the media description by setting its port to zero. 329 3.2. Detailed Offer/Answer Procedures 331 These procedures are given in terms of RFC 3264-recommended sections. 332 They describe the actions to be taken in terms of MediaStreams and 333 MediaStreamTracks; they do not include event signalling inside the 334 application, which is described in JSEP. 336 3.2.1. Generating the initial offer 338 For each media description in the offer, if there is an associated 339 outgoing MediaStreamTrack, the offerer adds one "a=msid" attribute to 340 the section for each MediaStream with which the MediaStreamTrack is 341 associated. The "identifier" field of the attribute is set to the 342 WebIDL "id" attribute of the MediaStream, and the "appdata" field is 343 set to the WebIDL "id" attribute of the MediaStreamTrack. 345 3.2.2. Answerer processing of the Offer 347 For each media description in the offer, and for each "a=msid" 348 attribute in the media description, the receiver of the offer will 349 perform the following steps: 351 o Extract the "appdata" field of the "a=msid" attribute 353 o Check if a MediaStreamTrack with the same WebIDL "id" attribute as 354 the "appdata" field already exists, and is not in the "ended" 355 state. If it is not found, create it. 357 o Extract the "identifier" field of the "a=msid" attribte. 359 o Check if a MediaStream with the same WebIDL "id" attribute already 360 exists. If not, create it. 362 o Add the MediaStreamTrack to the MediaStream 364 o Signal to the user that a new MediaStreamTrack is available. 366 3.2.3. Generating the answer 368 The answer is generated in exactly the same manner as the offer. 369 "a=msid" values in the offer do not influence the answer. 371 3.2.4. Offerer processing of the answer 373 The answer is processed in exactly the same manner as the offer. 375 3.2.5. Modifying the session 377 On subsequent exchanges, precisely the same procedure as for the 378 initial offer/answer is followed, but with one additional step in the 379 parsing of the offer and answer: 381 o For each MediaStreamTrack that has been created as a result of 382 previous offer/answer exchanges, and is not in the "ended" state, 383 check to see if there is still an "a=msid" attribute in the 384 present SDP whose "appdata" field is the same as the WebIDL "id" 385 attribute of the track. 387 o If no such attribute is found, stop the MediaStreamTrack. This 388 will set its state to "ended". 390 3.3. Example SDP description 392 The following SDP description shows the representation of a WebRTC 393 PeerConnection with two MediaStreams, each of which has one audio and 394 one video track. Only the parts relevant to the MSID are shown. 396 Line wrapping, empty lines and comments are added for clarity. They 397 are not part of the SDP. 399 # First MediaStream - id is 4701... 400 m=audio 56500 UDP/TLS/RTP/SAVPF 96 0 8 97 98 401 a=msid:47017fee-b6c1-4162-929c-a25110252400 402 f83006c5-a0ff-4e0a-9ed9-d3e6747be7d9 404 m=video 56502 UDP/TLS/RTP/SAVPF 100 101 405 a=msid:47017fee-b6c1-4162-929c-a25110252400 406 b47bdb4a-5db8-49b5-bcdc-e0c9a23172e0 408 # Second MediaStream - id is 6131.... 409 m=audio 56503 UDP/TLS/RTP/SAVPF 96 0 8 97 98 410 a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae 411 b94006c5-cade-4e0a-9ed9-d3e6747be7d9 413 m=video 56504 UDP/TLS/RTP/SAVPF 100 101 414 a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae 415 f30bdb4a-1497-49b5-3198-e0c9a23172e0 417 4. IANA Considerations 419 4.1. Attribute registration in existing registries 421 This document requests IANA to register the "msid" attribute in the 422 "att-field (media level only)" registry within the SDP parameters 423 registry, according to the procedures of [RFC4566] 425 The required information for "msid" is: 427 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 428 successor address designated by IESG 430 o Attribute name: msid 432 o Long-form attribute name: Media stream group Identifier 434 o Subject to charset: The attribute value contains only ASCII 435 characters, and is therefore not subject to the charset attribute. 437 o Purpose: The attribute can be used to signal the relationship 438 between a WebRTC MediaStream and a set of media descriptions. 440 o Appropriate values: The details of appropriate values are given in 441 RFC XXXX. 443 5. Security Considerations 445 An adversary with the ability to modify SDP descriptions has the 446 ability to switch around tracks between media streams. This is a 447 special case of the general security consideration that modification 448 of SDP descriptions needs to be confined to entities trusted by the 449 application. 451 If implementing buffering as mentioned in Section 3.1, the amount of 452 buffering should be limited to avoid memory exhaustion attacks. 454 No other attacks have been identified that depend on this mechanism. 456 6. Acknowledgements 458 This note is based on sketches from, among others, Justin Uberti and 459 Cullen Jennings. 461 Special thanks to Flemming Andreassen, Miguel Garcia, Martin Thomson, 462 Ted Hardie, Adam Roach and Paul Kyzivat for their work in reviewing 463 this draft, with many specific language suggestions. 465 7. References 467 7.1. Normative References 469 [I-D.ietf-rtcweb-jsep] 470 Uberti, J., Jennings, C., and E. Rescorla, "Javascript 471 Session Establishment Protocol", draft-ietf-rtcweb-jsep-09 472 (work in progress), March 2015. 474 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 475 Requirement Levels", BCP 14, RFC 2119, March 1997. 477 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 478 Jacobson, "RTP: A Transport Protocol for Real-Time 479 Applications", STD 64, RFC 3550, July 2003. 481 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 482 Description Protocol", RFC 4566, July 2006. 484 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 485 Specifications: ABNF", STD 68, RFC 5234, January 2008. 487 [W3C.WD-webrtc-20150210] 488 Bergkvist, A., Burnett, D., Jennings, C., and A. 489 Narayanan, "WebRTC 1.0: Real-time Communication Between 490 Browsers", World Wide Web Consortium WD WD-webrtc- 491 20150210, February 2015, 492 . 494 7.2. Informative References 496 [I-D.ietf-mmusic-sdp-bundle-negotiation] 497 Holmberg, C., Alvestrand, H., and C. Jennings, 498 "Negotiating Media Multiplexing Using the Session 499 Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- 500 negotiation-19 (work in progress), March 2015. 502 [I-D.ietf-rtcweb-overview] 503 Alvestrand, H., "Overview: Real Time Protocols for 504 Browser-based Applications", draft-ietf-rtcweb-overview-13 505 (work in progress), November 2014. 507 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 508 Control Packets on a Single Port", RFC 5761, April 2010. 510 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 511 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 513 Appendix A. Design considerations, rejected alternatives 515 One suggested mechanism has been to use CNAME instead of a new 516 attribute. This was abandoned because CNAME identifies a 517 synchronization context; one can imagine both wanting to have tracks 518 from the same synchronization context in multiple MediaStreams and 519 wanting to have tracks from multiple synchronization contexts within 520 one MediaStream (but the latter is impossible, since a MediaStream is 521 defined to impose synchronization on its members). 523 Another suggestion has been to put the msid value within an attribute 524 of RTCP SR (sender report) packets. This doesn't offer the ability 525 to know that you have seen all the tracks currently configured for a 526 media stream. 528 A suggestion that survived for a number of drafts was to define 529 "msid" as a generic mechanism, where the particular semantics of this 530 usage of the mechanism would be defined by an "a=wms-semantic" 531 attribute. This was removed in April 2015. 533 Appendix B. Change log 535 This appendix should be deleted before publication as an RFC. 537 B.1. Changes from alvestrand-rtcweb-msid-00 to -01 539 Added track identifier. 541 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 542 for track muting. 544 Some rewording. 546 B.2. Changes from alvestrand-rtcweb-msid-01 to -02 548 Split document into sections describing a generic grouping mechanism 549 and sections describing the application of this grouping mechanism to 550 the WebRTC MediaStream concept. 552 Removed the mechanism for muting tracks, since this is not central to 553 the MSID mechanism. 555 B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 557 Changed the draft name according to the wishes of the MMUSIC group 558 chairs. 560 Added text indicting cases where it's appropriate to have the same 561 appdata for multiple SSRCs. 563 Minor textual updates. 565 B.4. Changes from alvestrand-mmusic-msid-00 to -01 567 Increased the amount of explanatory text, much based on a review by 568 Miguel Garcia. 570 Removed references to BUNDLE, since that spec is under active 571 discussion. 573 Removed distinguished values of the MSID identifier. 575 B.5. Changes from alvestrand-mmusic-msid-01 to -02 577 Changed the order of the "msid-semantic: " attribute's value fields 578 and allowed multiple identifiers. This makes the attribute useful as 579 a marker for "I understand this semantic". 581 Changed the syntax for "identifier" and "appdata" to be "token". 583 Changed the registry for the "msid-semantic" attribute values to be a 584 new registry, based on advice given in Atlanta. 586 B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 588 Updated terminology to refer to m-lines rather than RTP sessions when 589 discussing SDP formats and the ability of other linking mechanisms to 590 refer to SSRCs. 592 Changed the "default" mechanism to return independent streams after 593 considering the synchronization problem. 595 Removed the space from between "msid-semantic" and its value, to be 596 consistent with RFC 5576. 598 B.7. Changes from mmusic-msid-00 to -01 600 Reworked msid mechanism to be a per-m-line attribute, to align with 601 draft-roach-mmusic-unified-plan. 603 B.8. Changes from mmusic-msid-01 to -02 605 Corrected several missed cases where the word "ssrc" was not changed 606 to "M-line". 608 Added pointer to unified-plan (which should be moved to point to 609 -jsep) 611 Removed suggestion that ssrc-group attributes can be used with "msid- 612 semantic", it is now only the msid-semantic registry. 614 B.9. Changes from mmusic-msid-02 to -03 616 Corrected even more cases where the word "ssrc" was not changed to 617 "M-line". 619 Added the functionality of using an asterisk (*) in the msid-semantic 620 line, in order to remove the need for listing all msids in the msid- 621 semantic line whne only one msid-semantic is in use. 623 Removed some now-unnecessary text. 625 B.10. Changes from mmusic-msid-03 to -04 627 Changed title to reflect focus on WebRTC MediaStreams 629 Added a section on receiver-side media stream control, using the 630 "msid-control" attribute. 632 B.11. Changes from -04 to -05 634 Removed the msid-control section after WG discussion. 636 Removed some text that seemed only to pertain to resolved issues. 638 B.12. Changes from -05 to -06 640 Addressed issues found in Fleming Andreassen's review 642 Referenced JSEP rather than unified-plan for the M-line mapping model 644 Relaxed MSID definition to allow "token-char" in values rather than 645 a-z 0-9 hyphen; tightened ABNF by adding length description to it. 647 Deleted discussion of abandoned alternatives, as part of preparing 648 for publication. 650 Added a "detailed procedures" section to the WMS semantics 651 description. 653 Added IANA registration of the "msid-semantic" attribute. 655 B.13. Changes from -06 to -07 657 Changed terminology from referring to "WebRTC device" to referring to 658 "entities that implement the WMS semantic". 660 Changed names for ABNF constructions based on a proposal by Paul 661 Kyzivat. 663 Included a section on generic offer/answer semantics. 665 B.14. Changes from -07 to -08 667 Removed Appendix B that described the (now obsolete) ssrc-specific 668 usage of MSID. 670 Adopted a restructuring of the IANA section based on a suggestion 671 from Martin Thomson. 673 A number of text and ABNF clarifications based on suggestions from 674 Ted Hardie, Paul Kyzivat and Adam Roach. 676 Changed the "non-signalled track handling" to create a single stream 677 with multiple tracks again, according to discussions at TPAC in 678 November 2014 680 B.15. Changes from -08 to -09 682 Removed "wms-semantic" and all mention of multiple semantics for 683 msid, as agreed at the Dallas IETF, March 2015. 685 Addressed a number of review comments from Fleming Andresen and 686 others. 688 Changed the term "m-line" to "media description", since that is the 689 term used in RFC 4566. 691 Tried to make sure this document does not describe the API to the 692 application. 694 Author's Address 696 Harald Alvestrand 697 Google 698 Kungsbron 2 699 Stockholm 11122 700 Sweden 702 Email: harald@alvestrand.no