idnits 2.17.1 draft-ietf-mmusic-msid-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (March 6, 2014) is 3704 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-05 Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group H. Alvestrand 3 Internet-Draft Google 4 Intended status: Standards Track March 6, 2014 5 Expires: September 7, 2014 7 WebRTC MediaStream Identification in the Session Description Protocol 8 draft-ietf-mmusic-msid-05 10 Abstract 12 This document specifies a grouping mechanism for RTP media streams 13 that can be used to specify relations between media streams. 15 This mechanism is used to signal the association between the SDP 16 concept of "m-line" and the WebRTC concept of "MediaStream" / 17 "MediaStreamTrack" using SDP signaling. 19 This document is a work item of the MMUSIC WG, whose discussion list 20 is mmusic@ietf.org. 22 Requirements Language 24 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 25 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 26 document are to be interpreted as described in RFC 2119 [RFC2119]. 28 Status of this Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on September 7, 2014. 45 Copyright Notice 47 Copyright (c) 2014 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. Structure Of This Document . . . . . . . . . . . . . . . . 3 64 1.2. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 3 65 1.3. Application to the WEBRTC MediaStream . . . . . . . . . . 4 66 2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . . 4 67 3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 5 68 4. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . . 6 69 4.1. Handling of non-signalled tracks . . . . . . . . . . . . . 7 70 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 71 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 72 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 73 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 75 8.2. Informative References . . . . . . . . . . . . . . . . . . 10 76 Appendix A. Design considerations, rejected alternatives . . . . 11 77 Appendix B. Usage with multiple MediaStreams per M-line . . . . . 11 78 B.1. Mechanism design with multiple SSRCs . . . . . . . . . . . 12 79 B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 13 80 Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 13 81 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 13 82 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 13 83 C.3. Changes from alvestrand-rtcweb-msid-02 to 84 mmusic-msid-00 . . . . . . . . . . . . . . . . . . . . . . 13 85 C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 13 86 C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 14 87 C.6. Changes from alvestrand-mmusic-msid-02 to 88 ietf-mmusic-00 . . . . . . . . . . . . . . . . . . . . . . 14 89 C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . . 14 90 C.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . . 14 91 C.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . . 14 92 C.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . . 15 93 C.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 15 94 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 96 1. Introduction 98 1.1. Structure Of This Document 100 This document adds a new grouping relation between M-lines that can 101 associate application layer identifiers with the binding between 102 media streams, attaching identifiers to the media streams and 103 attaching identifiers to the groupings they form. 105 Section 1.2 gives the background on why a new mechanism is needed. 107 Section 2 gives the definition of the new mechanism. 109 Section 4 gives the application of the new mechanism for providing 110 necessary semantic information for the association of 111 MediaStreamTracks to MediaStreams in the WebRTC API . 113 1.2. Why A New Mechanism Is Needed 115 When media is carried by RTP [RFC3550], each RTP media stream is 116 distinguished inside an RTP session by its SSRC; each RTP session is 117 distinguished from all other RTP sessions by being on a different 118 transport association (strictly speaking, 2 transport associations, 119 one used for RTP and one used for RTCP, unless RTCP multiplexing 120 [RFC5761] is used). 122 SDP gives a description based on m-lines. According to the model 123 used in [I-D.roach-mmusic-unified-plan], each m-line describes 124 exactly one media source, and if mulitple media sources are carried 125 in an RTP session, this is signalled using BUNDLE 126 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each 127 media source is carried in its own RTP session. 129 There exist cases where an application using RTP and SDP needs to 130 signal some relationship between RTP media streams that may be 131 carried in either the same RTP session or different RTP sessions. 132 For instance, there may be a need to signal a relationship between a 133 video track and an audio track, and where the generator of the SDP 134 does not yet know if they will be carried in the same RTP session or 135 different RTP sessions. 137 The SDP grouping framework [RFC5888] can be used to group m-lines. 138 However, there is sometimes the need for an application to specify 139 some application-level information about the association between the 140 m-line and the group. This is not possible using the SDP grouping 141 framework. 143 1.3. Application to the WEBRTC MediaStream 145 The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies 146 that communication between WebRTC entities is done via MediaStreams, 147 which contain MediaStreamTracks. A MediaStreamTrack is generally 148 carried using a single SSRC in an RTP session (forming an RTP media 149 stream. The collision of terminology is unfortunate.) There might 150 possibly be additional SSRCs, possibly within additional RTP 151 sessions, in order to support functionality like forward error 152 correction or simulcast. This complication is ignored below. 154 In the RTP specification, media streams are identified using the SSRC 155 field. Streams are grouped into RTP Sessions, and also carry a 156 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 157 Therefore, the association of an RTP media stream to MediaStreams 158 need to be explicitly signaled. 160 The WebRTC work has come to agreement (documented in 161 [I-D.roach-mmusic-unified-plan]) that one M-line is used to describe 162 each MediaStreamTrack, and that the BUNDLE mechanism 163 [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to group 164 MediaStreamTracks into RTP sessions. Therefore, the need is to 165 specify the ID of a MediaStreamTrack and its containing MediaStream 166 for each M-line, which can be accomplished with a media-level 167 attribute. 169 This usage is described in Section 4. 171 2. The Msid Mechanism 173 This document registers a new SDP [RFC4566] media-level "msid" 174 attribute. This new attribute allows endpoints to associate RTP 175 media streams that are carried in the same or different m-lines, as 176 well as allowing application-specific information to the association. 178 The value of the "msid" attribute consists of an identifier and 179 optional application-specific data, according to the following ABNF 180 [RFC5234] grammar: 182 ; "attribute" is defined in RFC 4566. 183 attribute =/ msid-attr 184 msid-attr = "msid:" identifier [ " " appdata ] 185 identifier = token 186 appdata = token 188 An example MSID value for a group with the identifier "examplefoo" 189 and application data "examplebar" might look like this: 191 msid:examplefoo examplebar 193 The identifier is a string of ASCII characters chosen from 0-9, a-z, 194 A-Z and - (hyphen), consisting of between 1 and 64 characters. It 195 MUST be unique among the identifier values used in the same SDP 196 session. It is RECOMMENDED that is generated using a random-number 197 generator. 199 Application data is carried on the same line as the identifier, 200 separated from the identifier by a space. 202 The identifier uniquely identifies a group within the scope of an SDP 203 description. 205 There may be multiple msid attributes on a single m-line. There may 206 also be multiple m-lines that have the same value for identifier and 207 application data. 209 Endpoints can update the associations between RTP media streams as 210 expressed by msid attributes at any time; the semantics and 211 restrictions of such grouping and ungrouping are application 212 dependent. 214 3. The Msid-Semantic Attribute 216 A session-level attribute is defined for signaling the semantics 217 associated with an msid grouping. This allows msid groupings with 218 different semantics to coexist. 220 This OPTIONAL attribute gives the group identifier and its group 221 semantic; it carries the same meaning as the ssrc-group-attr of RFC 222 5576 section 4.2, but uses the identifier of the group rather than a 223 list of SSRC values. 225 This attribute MUST be present if "a=msid" is used. 227 An empty list of identifiers is an indication that the sender 228 understands the indicated semantic, but has no msid groupings of the 229 given type in the present SDP. 231 An identifier of "*" is an indication that all "a=msid" lines in the 232 SDP have this specific semantic. 234 The ABNF of msid-semantic is: 236 attribute =/ msid-semantic-attr 237 msid-semantic-attr = "msid-semantic:" token identifier-list 238 identifier-list = (" " identifier)* / " *" 239 token = 241 The semantic field may hold values from the IANA registriy "Semantics 242 for the msid-semantic SDP attribute" (which is defined by this memo). 244 An example msid-semantic might look like this, if a semantic LS was 245 registered by IANA for the same purpose as the existing LS grouping 246 semantic: 248 a=msid-semantic:LS xyzzy forolow 250 This means that the SDP description has two lip sync groups, with the 251 group identifiers xyzzy and forolow, respectively. 253 4. Applying Msid to WebRTC MediaStreams 255 This section creates a new semantic for use with the framework 256 defined in Section 2, to be used for associating m-lines representing 257 MediaStreamTracks within MediaStreams as defined in 258 [W3C.WD-webrtc-20120209]. 260 The semantic token for this semantic is "WMS" (short for WebRTC Media 261 Stream). 263 The value of the msid corresponds to the "id" attribute of a 264 MediaStream. 266 The appdata for a WebRTC MediaStreamTrack consists of the "id" 267 attribute of a MediaStreamTrack. 269 If two different m-lines have MSID attributes with the same value for 270 identifier and appdata, it means that these two m-lines are both 271 intended for the same MediaStreamTrack. So far, no semantic for such 272 a mixture have been defined, but this specification does not forbid 273 the practice. 275 When an SDP description is updated, a specific msid continues to 276 refer to the same MediaStream. Once negotiation has completed on a 277 session, there is no memory; an msid value that appears in a later 278 negotiation will be taken to refer to a new MediaStream. 280 The following are the rules for handling updates of the list of 281 m-lines and their msid values. 283 o When a new msid value occurs in the description, the recipient can 284 signal to its application that a new MediaStream has been added. 286 o When a description is updated to have more m-lines with the same 287 msid value, but different appdata values, the recipient can signal 288 to its application that new MediaStreamTracks have been added to 289 the media stream. 291 o When a description is updated to no longer list the msid value on 292 a specific m-line, the recipient can signal to its application 293 that the corresponding media stream track has been closed. 295 o When a description is updated to no longer list the msid value on 296 any m-line, the recipient can signal to its application that the 297 media stream has been closed. 299 In addition to signaling that the track is closed when it disappears 300 from the SDP, the track will also be signaled as being closed when 301 all associated SSRCs have disappeared by the rules of [RFC3550] 302 section 6.3.4 (BYE packet received) and 6.3.5 (timeout). 304 The association between SSRCs and m-lines is specified in 305 [I-D.roach-mmusic-unified-plan]. 307 4.1. Handling of non-signalled tracks 309 Non-WebRTC entities will not send msid. This means that there will 310 be some incoming RTP packets that the recipient has no predefined 311 MediaStream id value for. 313 Handling will depend on whether or not any MSIDs are signaled in the 314 relevant m-line(s). There are two cases: 316 o No msid-semantic:WMS attribute is present. The SDP session is 317 assumed to be a backwards-compatible session. All incoming media, 318 on all m-lines that are part of the SDP session, are assumed to 319 belong to independent media streams, each with one track. The 320 identifier of this media stream and of the media stream track is a 321 randomly generated string; the label of this media stream will be 322 set to "Non-WMS stream". 324 o An msid-semantic:WMS attribute is present. In this case, the 325 session is WebRTC compatible, and the packets are either caused by 326 a bug or by timing skew between the arrival of the media packets 327 and the SDP description. These packets MAY be discarded, or they 328 MAY be buffered for a while in order to allow immediate startup of 329 the media stream when the SDP description is updated. The arrival 330 of media packets MUST NOT cause a new MediaStreamTrack to be 331 signaled. 333 If a WebRTC entity sends a description, it MUST include the msid- 334 semantic:WMS attribute, even if no media streams are sent. This 335 allows us to distinguish between the case of no media streams at the 336 moment and the case of legacy SDP generation. 338 It follows from the above that the WebRTC entity must have the SDP of 339 the other party before it can decide correctly whether or not a 340 "default" MediaStream should be created. RTP media packets that 341 arrive before the remote party's SDP MUST be buffered or discarded, 342 and MUST NOT cause a new MediaStreamTrack to be signalled. 344 It follows from the above that media stream tracks in the "default" 345 media stream cannot be closed by signaling; the application must 346 instead signal these as closed when the SSRC disappears according to 347 the rules of RFC 3550 section 6.3.4 and 6.3.5. 349 NOTE IN DRAFT: Previous versions of this memo suggested adding all 350 incoming SSRCs to a single MediaStream. This is problematic because 351 we do not know if the SSRCs are synchronized or not before we learn 352 the CNAME of the SSRCs, which only happens when an RTCP packet 353 arrives. How to identify a non-WMS stream is still open for 354 discussion - including whether it's necessary to do so. Using the 355 stream label seems like an easy thing to do for debuggability - it's 356 not signalled, and is intended for human consumption anyway. 358 Another alternative is to group the incoming media streams based on 359 CNAME; this preseerves the synchronization semantics of CNAME, but 360 means that one cannot signal the MediaStreamTrack before the CNAME of 361 the SSRC is known (which will happen only on arrival of the relevant 362 RTCP packet). 364 5. IANA Considerations 366 This document requests IANA to register the "msid" attribute and the 367 "msid-control" attribute in the "att-field (media level only)" 368 registry within the SDP parameters registry, according to the 369 procedures of [RFC4566] 371 The required information for "msid" is: 373 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 374 successor address designated by IESG 376 o Attribute name: msid 378 o Long-form attribute name: Media stream group Identifier 380 o The attribute value contains only ASCII characters, and is 381 therefore not subject to the charset attribute. 383 o The attribute gives an association over a set of m-lines. It can 384 be used to signal the relationship between a WebRTC MediaStream 385 and a set of m-lines. 387 o The details of appropriate values are given in RFC XXXX. 389 This document requests IANA to create a new registry called 390 "Semantics for the msid-semantic SDP attribute", which should have 391 exactly the same rules as for the "Semantics for the ssrc-group SDP 392 attribute" registry (Expert Review), and to register the "WMS" 393 semantic within this new registry. 395 The required information is: 397 o Description: WebRTC Media Stream, as given in RFC XXXX. 399 o Token: WMS 401 o Standards track reference: RFC XXXX 403 IANA is requested to replace "RFC XXXX" with the RFC number of this 404 document upon publication. 406 6. Security Considerations 408 An adversary with the ability to modify SDP descriptions has the 409 ability to switch around tracks between media streams. This is a 410 special case of the general security consideration that modification 411 of SDP descriptions needs to be confined to entities trusted by the 412 application. 414 If implementing buffering as mentioned in section Section 4.1, the 415 amount of buffering should be limited to avoid memory exhaustion 416 attacks. 418 No other attacks that are relevant to the browser's security have 419 been identified that depend on this mechanism. 421 7. Acknowledgements 423 This note is based on sketches from, among others, Justin Uberti and 424 Cullen Jennings. 426 Special thanks to Miguel Garcia and Paul Kyzivat for their work in 427 reviewing this draft, with many specific language suggestions. 429 8. References 431 8.1. Normative References 433 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 434 Requirement Levels", BCP 14, RFC 2119, March 1997. 436 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 437 Jacobson, "RTP: A Transport Protocol for Real-Time 438 Applications", STD 64, RFC 3550, July 2003. 440 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 441 Description Protocol", RFC 4566, July 2006. 443 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 444 Specifications: ABNF", STD 68, RFC 5234, January 2008. 446 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 447 Media Attributes in the Session Description Protocol 448 (SDP)", RFC 5576, June 2009. 450 [W3C.WD-webrtc-20120209] 451 Bergkvist, A., Burnett, D., Jennings, C., and A. 452 Narayanan, "WebRTC 1.0: Real-time Communication Between 453 Browsers", World Wide Web Consortium WD WD-webrtc- 454 20120209, February 2012, 455 . 457 8.2. Informative References 459 [I-D.ietf-mmusic-sdp-bundle-negotiation] 460 Holmberg, C., Alvestrand, H., and C. Jennings, 461 "Multiplexing Negotiation Using Session Description 462 Protocol (SDP) Port Numbers", 463 draft-ietf-mmusic-sdp-bundle-negotiation-05 (work in 464 progress), October 2013. 466 [I-D.roach-mmusic-unified-plan] 467 Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for 468 Using SDP with Large Numbers of Media Flows", 469 draft-roach-mmusic-unified-plan-00 (work in progress), 470 July 2013. 472 [I-D.westerlund-avtcore-multiplex-architecture] 473 Westerlund, M., Perkins, C., and H. Alvestrand, 474 "Guidelines for using the Multiplexing Features of RTP", 475 draft-westerlund-avtcore-multiplex-architecture-03 (work 476 in progress), February 2013. 478 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 479 Control Packets on a Single Port", RFC 5761, April 2010. 481 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 482 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 484 Appendix A. Design considerations, rejected alternatives 486 This appendix should be deleted before publication as an RFC. 488 One suggested mechanism has been to use CNAME instead of a new 489 attribute. This was abandoned because CNAME identifies a 490 synchronization context; one can imagine both wanting to have tracks 491 from the same synchronization context in multiple MediaStreams and 492 wanting to have tracks from multiple synchronization contexts within 493 one MediaStream (but the latter is impossible, since a MediaStream is 494 defined to impose synchronization on its members). 496 Another suggestion has been to put the msid value within an attribute 497 of RTCP SR (sender report) packets. This doesn't offer the ability 498 to know that you have seen all the tracks currently configured for a 499 media stream. 501 Appendix B. Usage with multiple MediaStreams per M-line 503 This appendix is included to document the usage of msid as a source- 504 specific attribute. Prior to the acceptance of the Unified Plan 505 document, some implementations used this mechanism to distinguish 506 between multiple MediaStreamTracks that were carried in the same 507 M-line. 509 It reproduces some of the original justification text for this 510 mechanism that is not relevant when Unified Plan is used. 512 B.1. Mechanism design with multiple SSRCs 514 When media is carried by RTP [RFC3550], each RTP media stream is 515 distinguished inside an RTP session by its SSRC; each RTP session is 516 distinguished from all other RTP sessions by being on a different 517 transport association (strictly speaking, 2 transport associations, 518 one used for RTP and one used for RTCP, unless RTCP multiplexing 519 [RFC5761] is used). 521 There exist cases where an application using RTP and SDP needs to 522 signal some relationship between RTP media streams that may be 523 carried in either the same RTP session or different RTP sessions. 524 For instance, there may be a need to signal a relationship between a 525 video track in one RTP session and an audio track in another RTP 526 session. In traditional SDP, it is not possible to signal that these 527 two tracks should be carried in one session, so they are carried in 528 different RTP sessions. 530 Traditionally, SDP was used to describe the RTP sessions, with one 531 m-line being used to describe each RTP session. With the advent of 532 extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this 533 association may be more complex, with multiple m-lines being used to 534 describe one RTP session; the rest of this document therefore talks 535 about m-lines, not RTP sessions, when describing the signalling 536 mechanism. 538 The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to 539 associate RTP media streams when those RTP media streams are 540 described by the same m-line. The semantics of this mechanism 541 prevent the association of RTP media streams that are spread across 542 different m-lines. 544 The SDP grouping framework [RFC5888] can be used to group m-lines. 545 When an m-line describes one and only one RTP media stream, it is 546 possible to associate RTP media streams across different m-lines. 547 However, if an m-line has multiple RTP media streams, using multiple 548 SSRCs, the SDP grouping framework cannot be used for this purpose. 550 There are use cases (some of which are discussed in 551 [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of 552 these approaches is appropriate; In those cases, a new mechanism is 553 needed. 555 In addition, there is sometimes the need for an application to 556 specify some application-level information about the association 557 between the SSRC and the group. This is not possible using either of 558 the frameworks above. 560 B.2. Usage with the SSRC attribute 562 When the MSID attribute was used with the SSRC attribute, it had to 563 be registered in the "Attribute names (source level)" registry rather 564 than the "Attribute names (media level only)" registry, and the msid 565 line was prefixed with "a=ssrc: ". Apart from that, usage of 566 the attribute with SSRC-bound flows was identical with the current 567 proposal. 569 Appendix C. Change log 571 This appendix should be deleted before publication as an RFC. 573 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 575 Added track identifier. 577 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 578 for track muting. 580 Some rewording. 582 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 584 Split document into sections describing a generic grouping mechanism 585 and sections describing the application of this grouping mechanism to 586 the WebRTC MediaStream concept. 588 Removed the mechanism for muting tracks, since this is not central to 589 the MSID mechanism. 591 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 593 Changed the draft name according to the wishes of the MMUSIC group 594 chairs. 596 Added text indicting cases where it's appropriate to have the same 597 appdata for multiple SSRCs. 599 Minor textual updates. 601 C.4. Changes from alvestrand-mmusic-msid-00 to -01 603 Increased the amount of explanatory text, much based on a review by 604 Miguel Garcia. 606 Removed references to BUNDLE, since that spec is under active 607 discussion. 609 Removed distinguished values of the MSID identifier. 611 C.5. Changes from alvestrand-mmusic-msid-01 to -02 613 Changed the order of the "msid-semantic: " attribute's value fields 614 and allowed multiple identifiers. This makes the attribute useful as 615 a marker for "I understand this semantic". 617 Changed the syntax for "identifier" and "appdata" to be "token". 619 Changed the registry for the "msid-semantic" attribute values to be a 620 new registry, based on advice given in Atlanta. 622 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 624 Updated terminology to refer to m-lines rather than RTP sessions when 625 discussing SDP formats and the ability of other linking mechanisms to 626 refer to SSRCs. 628 Changed the "default" mechanism to return independent streams after 629 considering the synchronization problem. 631 Removed the space from between "msid-semantic" and its value, to be 632 consistent with RFC 5576. 634 C.7. Changes from mmusic-msid-00 to -01 636 Reworked msid mechanism to be a per-m-line attribute, to align with 637 [I-D.roach-mmusic-unified-plan] 639 C.8. Changes from mmusic-msid-01 to -02 641 Corrected several missed cases where the word "ssrc" was not changed 642 to "M-line". 644 Added pointer to unified-plan (which should be moved to point to 645 -jsep) 647 Removed suggestion that ssrc-group attributes can be used with "msid- 648 semantic", it is now only the msid-semantic registry. 650 C.9. Changes from mmusic-msid-02 to -03 652 Corrected even more cases where the word "ssrc" was not changed to 653 "M-line". 655 Added the functionality of using an asterisk (*) in the msid-semantic 656 line, in order to remove the need for listing all msids in the msid- 657 semantic line whne only one msid-semantic is in use. 659 Removed some now-unnecessary text. 661 C.10. Changes from mmusic-msid-03 to -04 663 Changed title to reflect focus on WebRTC MediaStreams 665 Added a section on receiver-side media stream control, using the 666 "msid-control" attribute. 668 C.11. Changes from -04 to -05 670 Removed the msid-control section after WG discussion. 672 Removed some text that seemed only to pertain to resolved issues. 674 Author's Address 676 Harald Alvestrand 677 Google 678 Kungsbron 2 679 Stockholm, 11122 680 Sweden 682 Email: harald@alvestrand.no