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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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-04 Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--).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 November 5, 2013 5 Expires: May 9, 2014 7 Cross Session Stream Identification in the Session Description Protocol 8 draft-ietf-mmusic-msid-02 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 May 9, 2014. 45 Copyright Notice 47 Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . 9 73 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 75 8.2. Informative References . . . . . . . . . . . . . . . . . . 10 76 Appendix A. Design considerations, open questions and and 77 alternatives . . . . . . . . . . . . . . . . . . . . 11 78 Appendix B. Usage with multiple MediaStreams per M-line . . . . . 11 79 B.1. Mechanism design with multiple SSRCs . . . . . . . . . . . 12 80 B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 13 81 Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 13 82 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 13 83 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 13 84 C.3. Changes from alvestrand-rtcweb-msid-02 to 85 mmusic-msid-00 . . . . . . . . . . . . . . . . . . . . . . 13 86 C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 14 87 C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 14 88 C.6. Changes from alvestrand-mmusic-msid-02 to 89 ietf-mmusic-00 . . . . . . . . . . . . . . . . . . . . . . 14 90 C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . . 14 91 C.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . . 14 92 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 94 1. Introduction 96 1.1. Structure Of This Document 98 This document adds a new grouping relation between M-lines that can 99 associate application layer identifiers with the binding between 100 media streams, attaching identifiers to the media streams and 101 attaching identifiers to the groupings they form. 103 Section 1.2 gives the background on why a new mechanism is needed. 105 Section 2 gives the definition of the new mechanism. 107 Section 4 gives the application of the new mechanism for providing 108 necessary semantic information for the association of 109 MediaStreamTracks to MediaStreams in the WebRTC API . 111 1.2. Why A New Mechanism Is Needed 113 When media is carried by RTP [RFC3550], each RTP media stream is 114 distinguished inside an RTP session by its SSRC; each RTP session is 115 distinguished from all other RTP sessions by being on a different 116 transport association (strictly speaking, 2 transport associations, 117 one used for RTP and one used for RTCP, unless RTCP multiplexing 118 [RFC5761] is used). 120 SDP gives a description based on m-lines. According to the model 121 used in [I-D.roach-mmusic-unified-plan], each m-line describes 122 exactly one media source, and if mulitple media sources are carried 123 in an RTP session, this is signalled using BUNDLE 124 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each 125 media source is carried in its own RTP session. 127 There exist cases where an application using RTP and SDP needs to 128 signal some relationship between RTP media streams that may be 129 carried in either the same RTP session or different RTP sessions. 130 For instance, there may be a need to signal a relationship between a 131 video track and an audio track, and where the generator of the SDP 132 does not yet know if they will be carried in the same RTP session or 133 different RTP sessions. 135 The SDP grouping framework [RFC5888] can be used to group m-lines. 136 However, there is sometimes the need for an application to specify 137 some application-level information about the association between the 138 SSRC and the group. This is not possible using the SDP grouping 139 framework. 141 1.3. Application to the WEBRTC MediaStream 143 The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies 144 that communication between WebRTC entities is done via MediaStreams, 145 which contain MediaStreamTracks. A MediaStreamTrack is generally 146 carried using a single SSRC in an RTP session (forming an RTP media 147 stream. The collision of terminology is unfortunate.) There might 148 possibly be additional SSRCs, possibly within additional RTP 149 sessions, in order to support functionality like forward error 150 correction or simulcast. This complication is ignored below. 152 In the RTP specification, media streams are identified using the SSRC 153 field. Streams are grouped into RTP Sessions, and also carry a 154 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 155 Therefore, the association of an RTP media stream to MediaStreams 156 need to be explicitly signaled. 158 The WebRTC work has come to agreement (documented in 159 [I-D.roach-mmusic-unified-plan]) that one M-line is used to describe 160 each MediaStreamTrack, and that the BUNDLE mechanism 161 [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to group 162 MediaStreamTracks into RTP sessions. Therefore, the need is to 163 specify the ID of a MediaStreamTrack and its containing MediaStream 164 for each M-line, which can be accomplished with a media-level 165 attribute. 167 This usage is described in Section 4. 169 2. The Msid Mechanism 171 This document registers a new SDP [RFC4566] media-level "msid" 172 attribute. This new attribute allows endpoints to associate RTP 173 media streams that are carried in the same or different m-lines, as 174 well as allowing application-specific information to the association. 176 The value of the "msid" attribute consists of an identifier and 177 optional application-specific data, according to the following ABNF 178 [RFC5234] grammar: 180 ; "attribute" is defined in RFC 4566. 181 attribute =/ msid-attr 182 msid-attr = "msid:" identifier [ " " appdata ] 183 identifier = token 184 appdata = token 186 An example MSID value for a group with the identifier "examplefoo" 187 and application data "examplebar" might look like this: 188 msid:examplefoo examplebar 190 The identifier is a string of ASCII characters chosen from 0-9, a-z, 191 A-Z and - (hyphen), consisting of between 1 and 64 characters. It 192 MUST be unique among the identifier values used in the same SDP 193 session. It is RECOMMENDED that is generated using a random-number 194 generator. 196 Application data is carried on the same line as the identifier, 197 separated from the identifier by a space. 199 The identifier uniquely identifies a group within the scope of an SDP 200 description. 202 There may be multiple msid attributes on a single m-line. There may 203 also be multiple m-lines that have the same value for identifier and 204 application data. 206 Endpoints can update the associations between SSRCs as expressed by 207 msid attributes at any time; the semantics and restrictions of such 208 grouping and ungrouping are application dependent. 210 3. The Msid-Semantic Attribute 212 A session-level attribute is defined for signaling the semantics 213 associated with an msid grouping. This allows msid groupings with 214 different semantics to coexist. 216 This OPTIONAL attribute gives the group identifier and its group 217 semantic; it carries the same meaning as the ssrc-group-attr of RFC 218 5576 section 4.2, but uses the identifier of the group rather than a 219 list of SSRC values. 221 This attribute MUST be present if "a=msid" is used. 223 An empty list of identifiers is an indication that the sender 224 understands the indicated semantic, but has no msid groupings of the 225 given type in the present SDP. 227 The ABNF of msid-semantic is: 229 attribute =/ msid-semantic-attr 230 msid-semantic-attr = "msid-semantic:" token (" " identifier)* 231 token = 233 The semantic field may hold values from the IANA registriy "Semantics 234 for the msid-semantic SDP attribute" (which is defined by this memo). 236 An example msid-semantic might look like this, if a semantic LS was 237 registered by IANA for the same purpose as the existing LS grouping 238 semantic: 239 a=msid-semantic:LS xyzzy forolow 241 This means that the SDP description has two lip sync groups, with the 242 group identifiers xyzzy and forolow, respectively. 244 4. Applying Msid to WebRTC MediaStreams 246 This section creates a new semantic for use with the framework 247 defined in Section 2, to be used for associating SSRCs representing 248 MediaStreamTracks within MediaStreams as defined in 249 [W3C.WD-webrtc-20120209]. 251 The semantic token for this semantic is "WMS" (short for WebRTC Media 252 Stream). 254 The value of the msid corresponds to the "id" attribute of a 255 MediaStream. 257 In a WebRTC-compatible SDP description, all SSRCs intending to be 258 sent from one peer will be identified in the SDP generated by that 259 entity. 261 The appdata for a WebRTC MediaStreamTrack consists of the "id" 262 attribute of a MediaStreamTrack. 264 If two different m-lines have MSID attributes with the same value for 265 identifier and appdata, it means that these two m-lines are both 266 intended for the same MediaStreamTrack. So far, no semantic for such 267 a mixture have been defined, but this specification does not forbid 268 the practice. 270 When an SDP description is updated, a specific msid continues to 271 refer to the same MediaStream. Once negotiation has completed on a 272 session, there is no memory; an msid value that appears in a later 273 negotiation will be taken to refer to a new MediaStream. 275 The following are the rules for handling updates of the list of 276 m-lines and their msid values. 278 o When a new msid value occurs in the description, the recipient can 279 signal to its application that a new MediaStream has been added. 281 o When a description is updated to have more m-lines with the same 282 msid value, but different appdata values, the recipient can signal 283 to its application that new MediaStreamTracks have been added to 284 the media stream. 286 o When a description is updated to no longer list the msid value on 287 a specific m-line, the recipient can signal to its application 288 that the corresponding media stream track has been closed. 290 o When a description is updated to no longer list the msid value on 291 any m-line, the recipient can signal to its application that the 292 media stream has been closed. 294 In addition to signaling that the track is closed when it disappears 295 from the SDP, the track will also be signaled as being closed when 296 all associated SSRCs have disappeared by the rules of [RFC3550] 297 section 6.3.4 (BYE packet received) and 6.3.5 (timeout). 299 The association between SSRCs and m-lines is specified in 300 [I-D.roach-mmusic-unified-plan]. 302 4.1. Handling of non-signalled tracks 304 Non-WebRTC entities will not send msid. This means that there will 305 be some incoming RTP packets that the recipient has no predefined 306 MediaStream id value for. 308 Handling will depend on whether or not any MSIDs are signaled in the 309 relevant m-line(s). There are two cases: 311 o No msid-semantic:WMS attribute is present. The SDP session is 312 assumed to be a backwards-compatible session. All incoming media, 313 on all m-lines that are part of the SDP session, are assumed to 314 belong to independent media streams, each with one track. The 315 identifier of this media stream and of the media stream track is a 316 randomly generated string; the label of this media stream will be 317 set to "Non-WMS stream". 319 o An msid-semantic:WMS attribute is present. In this case, the 320 session is WebRTC compatible, and the packets are either caused by 321 a bug or by timing skew between the arrival of the media packets 322 and the SDP description. These packets MAY be discarded, or they 323 MAY be buffered for a while in order to allow immediate startup of 324 the media stream when the SDP description is updated. The arrival 325 of media packets MUST NOT cause a new MediaStreamTrack to be 326 signaled. 328 If a WebRTC entity sends a description, it MUST include the msid- 329 semantic:WMS attribute, even if no media streams are sent. This 330 allows us to distinguish between the case of no media streams at the 331 moment and the case of legacy SDP generation. 333 It follows from the above that the WebRTC entity must have the SDP of 334 the other party before it can decide correctly whether or not a 335 "default" MediaStream should be created. RTP media packets that 336 arrive before the remote party's SDP MUST be buffered or discarded, 337 and MUST NOT cause a new MediaStreamTrack to be signalled. 339 It follows from the above that media stream tracks in the "default" 340 media stream cannot be closed by signaling; the application must 341 instead signal these as closed when the SSRC disappears according to 342 the rules of RFC 3550 section 6.3.4 and 6.3.5. 344 NOTE IN DRAFT: Previous versions of this memo suggested adding all 345 incoming SSRCs to a single MediaStream. This is problematic because 346 we do not know if the SSRCs are synchronized or not before we learn 347 the CNAME of the SSRCs, which only happens when an RTCP packet 348 arrives. How to identify a non-WMS stream is still open for 349 discussion - including whether it's necessary to do so. Using the 350 stream label seems like an easy thing to do for debuggability - it's 351 not signalled, and is intended for human consumption anyway. 353 Another alternative is to group the incoming media streams based on 354 CNAME; this preseerves the synchronization semantics of CNAME, but 355 means that one cannot signal the MediaStreamTrack before the CNAME of 356 the SSRC is known (which will happen only on arrival of the relevant 357 RTCP packet). 359 5. IANA Considerations 361 This document requests IANA to register the "msid" attribute in the 362 "att-field (media level only)" registry within the SDP parameters 363 registry, according to the procedures of [RFC4566] 365 The required information is: 367 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 368 successor address designated by IESG 370 o Attribute name: msid 371 o Long-form attribute name: Media stream group Identifier 373 o The attribute value contains only ASCII characters, and is 374 therefore not subject to the charset attribute. 376 o The attribute gives an association over a set of m-lines. It can 377 be used to signal the relationship between a WebRTC MediaStream 378 and a set of SSRCs. 380 o The details of appropriate values are given in RFC XXXX. 382 This document requests IANA to create a new registry called 383 "Semantics for the msid-semantic SDP attribute", which should have 384 exactly the same rules as for the "Semantics for the ssrc-group SDP 385 attribute" registry (Expert Review), and to register the "WMS" 386 semantic within this new registry. 388 The required information is: 390 o Description: WebRTC Media Stream, as given in RFC XXXX. 392 o Token: WMS 394 o Standards track reference: RFC XXXX 396 IANA is requested to replace "RFC XXXX" with the RFC number of this 397 document upon publication. 399 6. Security Considerations 401 An adversary with the ability to modify SDP descriptions has the 402 ability to switch around tracks between media streams. This is a 403 special case of the general security consideration that modification 404 of SDP descriptions needs to be confined to entities trusted by the 405 application. 407 If implementing buffering as mentioned in section Section 4.1, the 408 amount of buffering should be limited to avoid memory exhaustion 409 attacks. 411 No other attacks that are relevant to the browser's security have 412 been identified that depend on this mechanism. 414 7. Acknowledgements 416 This note is based on sketches from, among others, Justin Uberti and 417 Cullen Jennings. 419 Special thanks to Miguel Garcia and Paul Kyzivat for their work in 420 reviewing this draft, with many specific language suggestions. 422 8. References 424 8.1. Normative References 426 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 427 Requirement Levels", BCP 14, RFC 2119, March 1997. 429 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 430 Jacobson, "RTP: A Transport Protocol for Real-Time 431 Applications", STD 64, RFC 3550, July 2003. 433 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 434 Description Protocol", RFC 4566, July 2006. 436 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 437 Specifications: ABNF", STD 68, RFC 5234, January 2008. 439 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 440 Media Attributes in the Session Description Protocol 441 (SDP)", RFC 5576, June 2009. 443 [W3C.WD-webrtc-20120209] 444 Bergkvist, A., Burnett, D., Jennings, C., and A. 445 Narayanan, "WebRTC 1.0: Real-time Communication Between 446 Browsers", World Wide Web Consortium WD WD-webrtc- 447 20120209, February 2012, 448 . 450 8.2. Informative References 452 [I-D.ietf-mmusic-sdp-bundle-negotiation] 453 Holmberg, C., Alvestrand, H., and C. Jennings, 454 "Multiplexing Negotiation Using Session Description 455 Protocol (SDP) Port Numbers", 456 draft-ietf-mmusic-sdp-bundle-negotiation-04 (work in 457 progress), June 2013. 459 [I-D.roach-mmusic-unified-plan] 460 Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for 461 Using SDP with Large Numbers of Media Flows", 462 draft-roach-mmusic-unified-plan-00 (work in progress), 463 July 2013. 465 [I-D.westerlund-avtcore-multiplex-architecture] 466 Westerlund, M., Perkins, C., and H. Alvestrand, 467 "Guidelines for using the Multiplexing Features of RTP", 468 draft-westerlund-avtcore-multiplex-architecture-03 (work 469 in progress), February 2013. 471 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 472 Control Packets on a Single Port", RFC 5761, April 2010. 474 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 475 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 477 Appendix A. Design considerations, open questions and and alternatives 479 This appendix should be deleted before publication as an RFC. 481 One suggested mechanism has been to use CNAME instead of a new 482 attribute. This was abandoned because CNAME identifies a 483 synchronization context; one can imagine both wanting to have tracks 484 from the same synchronization context in multiple MediaStreams and 485 wanting to have tracks from multiple synchronization contexts within 486 one MediaStream (but the latter is impossible, since a MediaStream is 487 defined to impose synchronization on its members). 489 Another suggestion has been to put the msid value within an attribute 490 of RTCP SR (sender report) packets. This doesn't offer the ability 491 to know that you have seen all the tracks currently configured for a 492 media stream. 494 There has been a suggestion that this mechanism could be used to mute 495 tracks too. This is not done at the moment. 497 Discarding of incoming data when the SDP description isn't updated 498 yet (section 3) may cause clipping. However, the same issue exists 499 when crypto keys aren't available. Input sought. 501 There's been a suggestion that acceptable SSRCs should be signaled in 502 a response, giving a recipient the ability to say "no" to certain 503 SSRCs. This is not supported in the current version of this 504 document. 506 Appendix B. Usage with multiple MediaStreams per M-line 508 This appendix is included to document the usage of msid as a source- 509 specific attribute. Prior to the acceptance of the Unified Plan 510 document, some implementations used this mechanism to distinguish 511 between multiple MediaStreamTracks that were carried in the same 512 M-line. 514 It reproduces some of the original justification text for this 515 mechanism that is not relevant when Unified Plan is used. 517 B.1. Mechanism design with multiple SSRCs 519 When media is carried by RTP [RFC3550], each RTP media stream is 520 distinguished inside an RTP session by its SSRC; each RTP session is 521 distinguished from all other RTP sessions by being on a different 522 transport association (strictly speaking, 2 transport associations, 523 one used for RTP and one used for RTCP, unless RTCP multiplexing 524 [RFC5761] is used). 526 There exist cases where an application using RTP and SDP needs to 527 signal some relationship between RTP media streams that may be 528 carried in either the same RTP session or different RTP sessions. 529 For instance, there may be a need to signal a relationship between a 530 video track in one RTP session and an audio track in another RTP 531 session. In traditional SDP, it is not possible to signal that these 532 two tracks should be carried in one session, so they are carried in 533 different RTP sessions. 535 Traditionally, SDP was used to describe the RTP sessions, with one 536 m-line being used to describe each RTP session. With the advent of 537 extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this 538 association may be more complex, with multiple m-lines being used to 539 describe one RTP session; the rest of this document therefore talks 540 about m-lines, not RTP sessions, when describing the signalling 541 mechanism. 543 The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to 544 associate RTP media streams when those RTP media streams are 545 described by the same m-line. The semantics of this mechanism 546 prevent the association of RTP media streams that are spread across 547 different m-lines. 549 The SDP grouping framework [RFC5888] can be used to group m-lines. 550 When an m-line describes one and only one RTP media stream, it is 551 possible to associate RTP media streams across different m-lines. 552 However, if an m-line has multiple RTP media streams, using multiple 553 SSRCs, the SDP grouping framework cannot be used for this purpose. 555 There are use cases (some of which are discussed in 556 [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of 557 these approaches is appropriate; In those cases, a new mechanism is 558 needed. 560 In addition, there is sometimes the need for an application to 561 specify some application-level information about the association 562 between the SSRC and the group. This is not possible using either of 563 the frameworks above. 565 B.2. Usage with the SSRC attribute 567 When the MSID attribute was used with the SSRC attribute, it had to 568 be registered in the "Attribute names (source level)" registry rather 569 than the "Attribute names (media level only)" registry, and the msid 570 line was prefixed with "a=ssrc: ". Apart from that, usage of 571 the attribute with SSRC-bound flows was identical with the current 572 proposal. 574 Appendix C. Change log 576 This appendix should be deleted before publication as an RFC. 578 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 580 Added track identifier. 582 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 583 for track muting. 585 Some rewording. 587 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 589 Split document into sections describing a generic grouping mechanism 590 and sections describing the application of this grouping mechanism to 591 the WebRTC MediaStream concept. 593 Removed the mechanism for muting tracks, since this is not central to 594 the MSID mechanism. 596 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 598 Changed the draft name according to the wishes of the MMUSIC group 599 chairs. 601 Added text indicting cases where it's appropriate to have the same 602 appdata for multiple SSRCs. 604 Minor textual updates. 606 C.4. Changes from alvestrand-mmusic-msid-00 to -01 608 Increased the amount of explanatory text, much based on a review by 609 Miguel Garcia. 611 Removed references to BUNDLE, since that spec is under active 612 discussion. 614 Removed distinguished values of the MSID identifier. 616 C.5. Changes from alvestrand-mmusic-msid-01 to -02 618 Changed the order of the "msid-semantic: " attribute's value fields 619 and allowed multiple identifiers. This makes the attribute useful as 620 a marker for "I understand this semantic". 622 Changed the syntax for "identifier" and "appdata" to be "token". 624 Changed the registry for the "msid-semantic" attribute values to be a 625 new registry, based on advice given in Atlanta. 627 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 629 Updated terminology to refer to m-lines rather than RTP sessions when 630 discussing SDP formats and the ability of other linking mechanisms to 631 refer to SSRCs. 633 Changed the "default" mechanism to return independent streams after 634 considering the synchronization problem. 636 Removed the space from between "msid-semantic" and its value, to be 637 consistent with RFC 5576. 639 C.7. Changes from mmusic-msid-00 to -01 641 Reworked msid mechanism to be a per-m-line attribute, to align with 642 [I-D.roach-mmusic-unified-plan] 644 C.8. Changes from mmusic-msid-01 to -02 646 Corrected several missed cases where the word "ssrc" was not changed 647 to "M-line". 649 Added pointer to unified-plan (which should be moved to point to 650 -jsep) 652 Removed suggestion that ssrc-group attributes can be used with "msid- 653 semantic", it is now only the msid-semantic registry. 655 Author's Address 657 Harald Alvestrand 658 Google 659 Kungsbron 2 660 Stockholm, 11122 661 Sweden 663 Email: harald@alvestrand.no