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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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-03 == Outdated reference: A later version (-03) exists of draft-westerlund-avtcore-multiplex-architecture-02 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment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 August 13, 2013 5 Expires: February 14, 2014 7 Cross Session Stream Identification in the Session Description Protocol 8 draft-ietf-mmusic-msid-01 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 February 14, 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 9 74 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . . . 11 80 B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 12 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 . . . . . . 13 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 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 93 1. Introduction 95 1.1. Structure Of This Document 97 This document extends the SSRC grouping framework [RFC5888] by adding 98 a new grouping relation that can cross RTP session boundaries if 99 needed. 101 Section 1.2 gives the background on why a new mechanism is needed. 103 Section 2 gives the definition of the new mechanism. 105 Section 4 gives the application of the new mechanism for providing 106 necessary semantic information for the association of 107 MediaStreamTracks to MediaStreams in the WebRTC API . 109 1.2. Why A New Mechanism Is Needed 111 When media is carried by RTP [RFC3550], each RTP media stream is 112 distinguished inside an RTP session by its SSRC; each RTP session is 113 distinguished from all other RTP sessions by being on a different 114 transport association (strictly speaking, 2 transport associations, 115 one used for RTP and one used for RTCP, unless RTCP multiplexing 116 [RFC5761] is used). 118 SDP gives a description based on m-lines. According to the model 119 used in [I-D.roach-mmusic-unified-plan], each m-line describes 120 exactly one media source, and if mulitple media sources are carried 121 in an RTP session, this is signalled using BUNDLE 122 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each 123 media source is carried in its own RTP session. 125 There exist cases where an application using RTP and SDP needs to 126 signal some relationship between RTP media streams that may be 127 carried in either the same RTP session or different RTP sessions. 128 For instance, there may be a need to signal a relationship between a 129 video track and an audio track, and where the generator of the SDP 130 does not yet know if they will be carried in the same RTP session or 131 different RTP sessions. 133 The SDP grouping framework [RFC5888] can be used to group m-lines. 134 However, there is sometimes the need for an application to specify 135 some application-level information about the association between the 136 SSRC and the group. This is not possible using the SDP grouping 137 framework. 139 1.3. Application to the WEBRTC MediaStream 141 The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies 142 that communication between WebRTC entities is done via MediaStreams, 143 which contain MediaStreamTracks. A MediaStreamTrack is generally 144 carried using a single SSRC in an RTP session (forming an RTP media 145 stream. The collision of terminology is unfortunate.) There might 146 possibly be additional SSRCs, possibly within additional RTP 147 sessions, in order to support functionality like forward error 148 correction or simulcast. This complication is ignored below. 150 In the RTP specification, media streams are identified using the SSRC 151 field. Streams are grouped into RTP Sessions, and also carry a 152 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 153 Therefore, the association of an RTP media stream to MediaStreams 154 need to be explicitly signaled. 156 The WebRTC work has come to agreement (documented in 157 [I-D.roach-mmusic-unified-plan]) that one M-line is used to describe 158 each MediaStreamTrack, and that the BUNDLE mechanism 159 [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to group 160 MediaStreamTracks into RTP sessions. Therefore, the need is to 161 specify the ID of a MediaStreamTrack and its containing MediaStream 162 for each M-line, which can be accomplished with a media-level 163 attribute. 165 This usage is described in Section 4. 167 2. The Msid Mechanism 169 This document registers a new SDP [RFC4566] media-level "msid" 170 attribute. This new attribute allows endpoints to associate RTP 171 media streams that are carried in the same or different m-lines, as 172 well as allowing application-specific information to the association. 174 The value of the "msid" attribute consists of an identifier and 175 optional application-specific data, according to the following ABNF 176 [RFC5234] grammar: 178 ; "attribute" is defined in RFC 4566. 179 attribute =/ msid-attr 180 msid-attr = "msid:" identifier [ " " appdata ] 181 identifier = token 182 appdata = token 184 An example MSID value for a group with the identifier "examplefoo" 185 and application data "examplebar" might look like this: 186 msid:examplefoo examplebar 188 The identifier is a string of ASCII characters chosen from 0-9, a-z, 189 A-Z and - (hyphen), consisting of between 1 and 64 characters. It 190 MUST be unique among the identifier values used in the same SDP 191 session. It is RECOMMENDED that is generated using a random-number 192 generator. 194 Application data is carried on the same line as the identifier, 195 separated from the identifier by a space. 197 The identifier uniquely identifies a group within the scope of an SDP 198 description. 200 There may be multiple msid attributes on a single m-line. There may 201 also be multiple m-lines that have the same value for identifier and 202 application data. 204 Endpoints can update the associations between SSRCs as expressed by 205 msid attributes at any time; the semantics and restrictions of such 206 grouping and ungrouping are application dependent. 208 3. The Msid-Semantic Attribute 210 In order to fully reproduce the semantics of the SDP and SSRC 211 grouping frameworks, a session-level attribute is defined for 212 signaling the semantics associated with an msid grouping. 214 This OPTIONAL attribute gives the group identifier and its group 215 semantic; it carries the same meaning as the ssrc-group-attr of RFC 216 5576 section 4.2, but uses the identifier of the group rather than a 217 list of SSRC values. 219 An empty list of identifiers is an indication that the sender 220 understands the indicated semantic, but has no msid groupings of the 221 given type in the present SDP. 223 The ABNF of msid-semantic is: 225 attribute =/ msid-semantic-attr 226 msid-semantic-attr = "msid-semantic:" token (" " identifier)* 227 token = 229 The semantic field may hold values from the IANA registries 230 "Semantics for the "ssrc-group" SDP Attribute" and "Semantics for the 231 "group" SDP Attribute". 233 An example msid-semantic might look like this: 234 a=msid-semantic:LS xyzzy forolow 236 This means that the SDP description has two lip sync groups, with the 237 group identifiers xyzzy and forolow, respectively. 239 4. Applying Msid to WebRTC MediaStreams 241 This section creates a new semantic for use with the framework 242 defined in Section 2, to be used for associating SSRCs representing 243 MediaStreamTracks within MediaStreams as defined in 244 [W3C.WD-webrtc-20120209]. 246 The semantic token for this semantic is "WMS" (short for WebRTC Media 247 Stream). 249 The value of the msid corresponds to the "id" attribute of a 250 MediaStream. 252 In a WebRTC-compatible SDP description, all SSRCs intending to be 253 sent from one peer will be identified in the SDP generated by that 254 entity. 256 The appdata for a WebRTC MediaStreamTrack consists of the "id" 257 attribute of a MediaStreamTrack. 259 If two different SSRCs have the same value for identifier and 260 appdata, it means that these two SSRCs are both intended for the same 261 MediaStreamTrack. This may occur if the sender wishes to use 262 simulcast or forward error correction, or if the sender intends to 263 switch between multiple codecs on the same MediaStreamTrack. 265 When an SDP description is updated, a specific msid continues to 266 refer to the same MediaStream. Once negotiation has completed on a 267 session, there is no memory; an msid value that appears in a later 268 negotiation will be taken to refer to a new MediaStream. 270 The following are the rules for handling updates of the list of 271 m-lines and their msid values. 273 o When a new msid value occurs in the description, the recipient can 274 signal to its application that a new MediaStream has been added. 276 o When a description is updated to have more SSRCs with the same 277 msid value, but different appdata values, the recipient can signal 278 to its application that new MediaStreamTracks have been added to 279 the media stream. 281 o When a description is updated to no longer list the msid value on 282 a specific ssrc, the recipient can signal to its application that 283 the corresponding media stream track has been closed. 285 o When a description is updated to no longer list the msid value on 286 any ssrc, the recipient can signal to its application that the 287 media stream has been closed. 289 In addition to signaling that the track is closed when it disappears 290 from the SDP, the track will also be signaled as being closed when 291 all associated SSRCs have disappeared by the rules of [RFC3550] 292 section 6.3.4 (BYE packet received) and 6.3.5 (timeout). 294 4.1. Handling of non-signalled tracks 296 Pre-WebRTC entities will not send msid. This means that there will 297 be some incoming RTP packets with SSRCs where the recipient does not 298 know about a corresponding MediaStream id. 300 Handling will depend on whether or not any MSIDs are signaled in the 301 relevant m-line(s). There are two cases: 303 o No msid-semantic:WMS attribute is present. The SDP session is 304 assumed to be a backwards-compatible session. All incoming SSRCs, 305 on all m-lines that are part of the SDP session, are assumed to 306 belong to independent media streams, each with one track. The 307 identifier of this media stream and of the media stream track is a 308 randomly generated string; the label of this media stream will be 309 set to "Non-WMS stream". 311 o An msid-semantic:WMS attribute is present. In this case, the 312 session is WebRTC compatible, and the newly arrived SSRCs are 313 either caused by a bug or by timing skew between the arrival of 314 the media packets and the SDP description. These packets MAY be 315 discarded, or they MAY be buffered for a while in order to allow 316 immediate startup of the media stream when the SDP description is 317 updated. The arrival of media packets MUST NOT cause a new 318 MediaStreamTrack to be signaled. 320 If a WebRTC entity sends a description, it MUST include the msid- 321 semantic:WMS attribute, even if no media streams are sent. This 322 allows us to distinguish between the case of no media streams at the 323 moment and the case of legacy SDP generation. 325 It follows from the above that the WebRTC entity must have the SDP of 326 the other party before it can decide correctly whether or not a 327 "default" MediaStream should be created. RTP media packets that 328 arrive before the remote party's SDP MUST be buffered or discarded, 329 and MUST NOT cause a new MediaStreamTrack to be signalled. 331 It follows from the above that media stream tracks in the "default" 332 media stream cannot be closed by signaling; the application must 333 instead signal these as closed when the SSRC disappears according to 334 the rules of RFC 3550 section 6.3.4 and 6.3.5. 336 NOTE IN DRAFT: Previous versions of this memo suggested adding all 337 incoming SSRCs to a single MediaStream. This is problematic because 338 we do not know if the SSRCs are synchronized or not before we learn 339 the CNAME of the SSRCs, which only happens when an RTCP packet 340 arrives. How to identify a non-WMS stream is still open for 341 discussion - including whether it's necessary to do so. Using the 342 stream label seems like an easy thing to do for debuggability - it's 343 not signalled, and is intended for human consumption anyway. 345 5. IANA Considerations 347 This document requests IANA to register the "msid" attribute in the 348 "att-field (media level only)" registry within the SDP parameters 349 registry, according to the procedures of [RFC4566] 351 The required information is: 353 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 354 successor address designated by IESG 356 o Attribute name: msid 358 o Long-form attribute name: Media stream group Identifier 360 o The attribute value contains only ASCII characters, and is 361 therefore not subject to the charset attribute. 363 o The attribute gives an association over a set of m-lines. It can 364 be used to signal the relationship between a WebRTC MediaStream 365 and a set of SSRCs. 367 o The details of appropriate values are given in RFC XXXX. 369 This document requests IANA to create a new registry called 370 "Semantics for the msid-semantic SDP attribute", which should have 371 exactly the same rules as for the "Semantics for the ssrc-group SDP 372 attribute" registry (Expert Review), and to register the "WMS" 373 semantic within this new registry. 375 The required information is: 377 o Description: WebRTC Media Stream, as given in RFC XXXX. 379 o Token: WMS 381 o Standards track reference: RFC XXXX 383 IANA is requested to replace "RFC XXXX" with the RFC number of this 384 document upon publication. 386 6. Security Considerations 388 An adversary with the ability to modify SDP descriptions has the 389 ability to switch around tracks between media streams. This is a 390 special case of the general security consideration that modification 391 of SDP descriptions needs to be confined to entities trusted by the 392 application. 394 If implementing buffering as mentioned in section Section 4.1, the 395 amount of buffering should be limited to avoid memory exhaustion 396 attacks. 398 No other attacks that are relevant to the browser's security have 399 been identified that depend on this mechanism. 401 7. Acknowledgements 403 This note is based on sketches from, among others, Justin Uberti and 404 Cullen Jennings. 406 Special thanks to Miguel Garcia and Paul Kyzivat for their work in 407 reviewing this draft, with many specific language suggestions. 409 8. References 411 8.1. Normative References 413 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 414 Requirement Levels", BCP 14, RFC 2119, March 1997. 416 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 418 Jacobson, "RTP: A Transport Protocol for Real-Time 419 Applications", RFC 3550, July 2003. 421 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 422 Description Protocol", RFC 4566, July 2006. 424 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 425 Specifications: ABNF", RFC 5234, January 2008. 427 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 428 Media Attributes in the Session Description Protocol 429 (SDP)", RFC 5576, June 2009. 431 [W3C.WD-webrtc-20120209] 432 Bergkvist, A., Burnett, D., Narayanan, A., and C. 433 Jennings, "WebRTC 1.0: Real-time Communication Between 434 Browsers", World Wide Web Consortium WD WD-webrtc- 435 20120209, February 2012, 436 . 438 8.2. Informative References 440 [I-D.ietf-mmusic-sdp-bundle-negotiation] 441 Holmberg, C., Alvestrand, H., and C. Jennings, 442 "Multiplexing Negotiation Using Session Description 443 Protocol (SDP) Port Numbers", 444 draft-ietf-mmusic-sdp-bundle-negotiation-03 (work in 445 progress), February 2013. 447 [I-D.roach-mmusic-unified-plan] 448 Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for 449 Using SDP with Large Numbers of Media Flows", 450 draft-roach-mmusic-unified-plan-00 (work in progress), 451 July 2013. 453 [I-D.westerlund-avtcore-multiplex-architecture] 454 Westerlund, M., Burman, B., Perkins, C., and H. 455 Alvestrand, "Guidelines for using the Multiplexing 456 Features of RTP", 457 draft-westerlund-avtcore-multiplex-architecture-02 (work 458 in progress), July 2012. 460 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 461 Control Packets on a Single Port", RFC 5761, April 2010. 463 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 464 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 466 Appendix A. Design considerations, open questions and and alternatives 468 This appendix should be deleted before publication as an RFC. 470 One suggested mechanism has been to use CNAME instead of a new 471 attribute. This was abandoned because CNAME identifies a 472 synchronization context; one can imagine both wanting to have tracks 473 from the same synchronization context in multiple MediaStreams and 474 wanting to have tracks from multiple synchronization contexts within 475 one MediaStream (but the latter is impossible, since a MediaStream is 476 defined to impose synchronization on its members). 478 Another suggestion has been to put the msid value within an attribute 479 of RTCP SR (sender report) packets. This doesn't offer the ability 480 to know that you have seen all the tracks currently configured for a 481 media stream. 483 There has been a suggestion that this mechanism could be used to mute 484 tracks too. This is not done at the moment. 486 Discarding of incoming data when the SDP description isn't updated 487 yet (section 3) may cause clipping. However, the same issue exists 488 when crypto keys aren't available. Input sought. 490 There's been a suggestion that acceptable SSRCs should be signaled in 491 a response, giving a recipient the ability to say "no" to certain 492 SSRCs. This is not supported in the current version of this 493 document. 495 Appendix B. Usage with multiple MediaStreams per M-line 497 This appendix is included to document the usage of msid as a source- 498 specific attribute. Prior to the acceptance of the Unified Plan 499 document, some implementations used this mechanism to distinguish 500 between multiple MediaStreamTracks that were carried in the same 501 M-line. 503 It reproduces some of the original justification text for this 504 mechanism that is not relevant when Unified Plan is used. 506 B.1. Mechanism design with multiple SSRCs 508 When media is carried by RTP [RFC3550], each RTP media stream is 509 distinguished inside an RTP session by its SSRC; each RTP session is 510 distinguished from all other RTP sessions by being on a different 511 transport association (strictly speaking, 2 transport associations, 512 one used for RTP and one used for RTCP, unless RTCP multiplexing 514 [RFC5761] is used). 516 There exist cases where an application using RTP and SDP needs to 517 signal some relationship between RTP media streams that may be 518 carried in either the same RTP session or different RTP sessions. 519 For instance, there may be a need to signal a relationship between a 520 video track in one RTP session and an audio track in another RTP 521 session. In traditional SDP, it is not possible to signal that these 522 two tracks should be carried in one session, so they are carried in 523 different RTP sessions. 525 Traditionally, SDP was used to describe the RTP sessions, with one 526 m-line being used to describe each RTP session. With the advent of 527 extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this 528 association may be more complex, with multiple m-lines being used to 529 describe one RTP session; the rest of this document therefore talks 530 about m-lines, not RTP sessions, when describing the signalling 531 mechanism. 533 The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to 534 associate RTP media streams when those RTP media streams are 535 described by the same m-line. The semantics of this mechanism 536 prevent the association of RTP media streams that are spread across 537 different m-lines. 539 The SDP grouping framework [RFC5888] can be used to group m-lines. 540 When an m-line describes one and only one RTP media stream, it is 541 possible to associate RTP media streams across different m-lines. 542 However, if an m-line has multiple RTP media streams, using multiple 543 SSRCs, the SDP grouping framework cannot be used for this purpose. 545 There are use cases (some of which are discussed in 546 [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of 547 these approaches is appropriate; In those cases, a new mechanism is 548 needed. 550 In addition, there is sometimes the need for an application to 551 specify some application-level information about the association 552 between the SSRC and the group. This is not possible using either of 553 the frameworks above. 555 B.2. Usage with the SSRC attribute 557 When the MSID attribute was used with the SSRC attribute, it had to 558 be registered in the "Attribute names (source level)" registry rather 559 than the "Attribute names (media level only)" registry, and the msid 560 line was prefixed with "a=ssrc: ". Apart from that, usage of 561 the attribute with SSRC-bound flows was identical with the current 562 proposal. 564 Appendix C. Change log 566 This appendix should be deleted before publication as an RFC. 568 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 570 Added track identifier. 572 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 573 for track muting. 575 Some rewording. 577 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 579 Split document into sections describing a generic grouping mechanism 580 and sections describing the application of this grouping mechanism to 581 the WebRTC MediaStream concept. 583 Removed the mechanism for muting tracks, since this is not central to 584 the MSID mechanism. 586 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 588 Changed the draft name according to the wishes of the MMUSIC group 589 chairs. 591 Added text indicting cases where it's appropriate to have the same 592 appdata for multiple SSRCs. 594 Minor textual updates. 596 C.4. Changes from alvestrand-mmusic-msid-00 to -01 598 Increased the amount of explanatory text, much based on a review by 599 Miguel Garcia. 601 Removed references to BUNDLE, since that spec is under active 602 discussion. 604 Removed distinguished values of the MSID identifier. 606 C.5. Changes from alvestrand-mmusic-msid-01 to -02 608 Changed the order of the "msid-semantic: " attribute's value fields 609 and allowed multiple identifiers. This makes the attribute useful as 610 a marker for "I understand this semantic". 612 Changed the syntax for "identifier" and "appdata" to be "token". 614 Changed the registry for the "msid-semantic" attribute values to be a 615 new registry, based on advice given in Atlanta. 617 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 619 Updated terminology to refer to m-lines rather than RTP sessions when 620 discussing SDP formats and the ability of other linking mechanisms to 621 refer to SSRCs. 623 Changed the "default" mechanism to return independent streams after 624 considering the synchronization problem. 626 Removed the space from between "msid-semantic" and its value, to be 627 consistent with RFC 5576. 629 C.7. Changes from mmusic-msid-00 to -01 631 Reworked msid mechanism to be a per-m-line attribute, to align with 632 [I-D.roach-mmusic-unified-plan] 634 Author's Address 636 Harald Alvestrand 637 Google 638 Kungsbron 2 639 Stockholm, 11122 640 Sweden 642 Email: harald@alvestrand.no