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Alvestrand 3 Internet-Draft Google 4 Intended status: Standards Track October 15, 2012 5 Expires: April 18, 2013 7 Cross Session Stream Identification in the Session Description Protocol 8 draft-alvestrand-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 within 14 different RTP sessions. 16 This mechanism is used to signal the association between the RTP 17 concept of SSRC and the WebRTC concept of "media stream" / "media 18 stream track" using SDP signalling. 20 This document is an input document for discussion. It should be 21 discussed in the MMUSIC WG list, 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 April 18, 2013. 46 Copyright Notice 48 Copyright (c) 2012 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. Application to the WEBRTC MediaStream . . . . . . . . . . 4 67 2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . . 4 68 3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 5 69 4. Applying Msid to WebRTC Media Streams . . . . . . . . . . . . 6 70 4.1. Handling of non-signalled tracks . . . . . . . . . . . . . 7 71 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 73 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 76 8.2. Informative References . . . . . . . . . . . . . . . . . . 10 77 Appendix A. Design considerations, open questions and and 78 alternatives . . . . . . . . . . . . . . . . . . . . 10 79 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 11 80 B.1. Changes from rtcweb-msid-00 to -01 . . . . . . . . . . . . 11 81 B.2. Changes from rtcweb-msid-01 to -02 . . . . . . . . . . . . 11 82 B.3. Changes from rtcweb-msid-02 to mmusic-msid-00 . . . . . . 11 83 B.4. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . . 12 84 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 86 1. Introduction 88 1.1. Structure Of This Document 90 This document extends the SSRC grouping framework [RFC5888] by adding 91 a new grouping relation that can cross RTP session boundaries. 93 Section 1.2 gives the background on why a new mechanism is needed. 95 Section 2 gives the definition of the new mechanism. 97 Section 4 gives the application of the new mechanism for providing 98 necessary semantic information for the association of 99 MediaStreamTracks to MediaStreams in the WebRTC API . 101 1.2. Why A New Mechanism Is Needed 103 When media is carried by RTP [RFC3550], each RTP media stream is 104 distinguished inside an RTP session by its SSRC; each RTP session is 105 distinguished from all other RTP sessions by being on a different 106 transport association (strictly speaking, 2 transport associations, 107 one used for RTP and one used for RTCP, unless RTCP multiplexing 108 [RFC5761] is used). 110 There exist cases where an application using RTP and SDP needs to 111 signal some relationship between RTP media streams that may be 112 carried in either the same RTP session or different RTP sessions. 113 For instance, there may be a need to signal a relationship between a 114 video track in one RTP session and an audio track in another RTP 115 session. In traditional SDP, it is not possible to signal that these 116 two tracks should be carried in one session, so they are carried in 117 different RTP sessions. 119 The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to 120 associate RTP media streams when those RTP media streams are part of 121 the same RTP session. The semantics of this mechanism prevent the 122 association of RTP media streams that are spread across different RTP 123 sessions. 125 The SDP grouping framework [RFC5888] can be used to group RTP 126 sessions. When an RTP session carries one and only one RTP media 127 stream, it is possible to associate RTP media streams across 128 different RTP sessions. However, if an RTP session has multiple RTP 129 media streams, using multiple SSRCs, the SDP grouping framework 130 cannot be used for this purpose. 132 There are use cases (some of which are discussed in 133 [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of 134 these approaches is appropriate; In those cases, a new mechanism is 135 needed. 137 In addition, there is sometimes the need for an application to 138 specify some application-level information about the association 139 between the SSRC and the group. This is not possible using either of 140 the frameworks above. 142 1.3. Application to the WEBRTC MediaStream 144 The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies 145 that communication between WebRTC entities is done via MediaStreams, 146 which contain MediaStreamTracks. A MediaStreamTrack is generally 147 carried using a single SSRC in an RTP session (forming an RTP media 148 stream. The collision of terminology is unfortunate.) There might 149 possibly be additional SSRCs, possibly within additional RTP 150 sessions, in order to support functionality like forward error 151 correction or simulcast. This complication is ignored below. 153 In the RTP specification, media streams are identified using the SSRC 154 field. Streams are grouped into RTP Sessions, and also carry a 155 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 156 Therefore, the association of an RTP media stream to MediaStreams 157 need to be explicitly signalled. 159 The marking needs to be on a per-SSRC basis, since one RTP session 160 can carry media from multiple MediaStreams, and one MediaStream can 161 have media in multiple RTP sessions. This means that the [RFC4574] 162 "label" attribute, which is used to label RTP sessions, is not usable 163 for this purpose. 165 The marking needs to also carry the unique identifier of the RTP 166 media stream as a MediaStreamTrack within the media stream; this is 167 done using a single letter to identify whether it belongs in the 168 video or audio track list, and the MediaStreamTrack's position within 169 that array. 171 This usage is described in Section 4. 173 2. The Msid Mechanism 175 This document extends the Source-Specific Media Attributes framework 176 [RFC5576] by adding a new "msid" attribute that can be used with the 177 "a=ssrc" SDP attribute. This new attribute allows endpoints to 178 associate RTP media streams that are carried in different RTP 179 sessions, as well as allowing application-specific information to the 180 association. 182 The value of the "msid" attribute consists of an identifier and 183 optional application-specific data, according to the following ABNF 184 [RFC5234] grammar: 186 ; "attribute" is defined in RFC 4566. 187 ; This attribute should be used with the ssrc-attr from RFC 5576. 188 attribute =/ msid-attr 189 msid-attr = "msid:" identifier [ " " appdata ] 190 identifier = 1*64 ("0".."9" / "a".."z" / "-") 191 appdata = 1*64 ("0".."9" / "a".."z" / "-") 193 An example MSID value for the SSRC 1234 might look like this: 194 a=ssrc:1234 msid:examplefoo v1 196 The identifier is a string of ASCII characters chosen from 0-9, a-z, 197 A-Z and - (hyphen), consisting of between 1 and 64 characters. It 198 MUST be unique among the identifier values used in the same SDP 199 session. It is RECOMMENDED that is generated using a random-number 200 generator. 202 Application data is carried on the same line as the identifier, 203 separated from the identifier by a space. 205 The identifier uniquely identifies a group within the scope of an SDP 206 description. 208 There may be multiple msid attributes on a single SSRC. There may 209 also be multiple SSRCs that have the same value for identifier and 210 application data. 212 Endpoints can update the associations between SSRCs as expressed by 213 msid attributes at any time; the semantics and restrictions of such 214 grouping and ungrouping are application dependent. 216 3. The Msid-Semantic Attribute 218 In order to fully reproduce the semantics of the SDP and SSRC 219 grouping frameworks, a session-level attribute is defined for 220 signalling the semantics associated with an msid grouping. 222 This OPTIONAL attribute gives the group identifier and its group 223 semantic; it carries the same meaning as the ssrc-group-attr of RFC 224 5576 section 4.2, but uses the identifier of the group rather than a 225 list of SSRC values. 227 The ABNF of msid-semantic is: 229 attribute =/ msid-semantic-attr 230 msid-semantic-attr = "msid-semantic:" " " identifier token 231 token = 233 The semantic field may hold values from the IANA registries 234 "Semantics for the "ssrc-group" SDP Attribute" and "Semantics for the 235 "group" SDP Attribute". 237 An example msid-semantic might look like this: 238 a=msid-semantic: examplefoo LS 240 4. Applying Msid to WebRTC Media Streams 242 This section creates a new semantic for use with the framework 243 defined in Section 2, to be used for associating SSRCs representing 244 media stream tracks with media streams as defined in 245 [W3C.WD-webrtc-20120209]. 247 The semantic token for WebRTC Media Streams is "WMS". 249 The value of the msid corresponds to the "id" attribute of a 250 MediaStream. (note: as of Jan 11, 2012, this is called "label". The 251 word "label" means many other things, so the same word should not be 252 used.) 254 In a WebRTC-compatible SDP description, all SSRCs intending to be 255 sent from one peer will be identified in the SDP generated by that 256 entity. 258 The appdata for a WebRTC MediaStreamTrack consists of the track type 259 and the track number; the track type is encoded as the single letter 260 "a" (audio) or "v" (video), and the track number is encoded as a 261 decimal integer with no leading zeros. The first track is track 262 zero, and is identified as "a0" for audio, and "v0" for video. 264 If two different SSRCs have the same value for identifier and 265 appdata, it means that these two SSRCs are both intended for the same 266 MediaStreamTrack. This may occur if the sender wishes to use 267 simulcast or forward error correction, or if the sender intends to 268 switch between multiple codecs on the same MediaStreamTrack. 270 When an SDP description is updated, a specific msid continues to 271 refer to the same media stream; an msid value MUST NOT be reused for 272 another media stream within a PeerConnection's lifetime. 274 The following are the rules for handling updates of the list of SSRCs 275 and their msid values. 277 o When a new msid value occurs in the description, the recipient can 278 signal to its application that a new media stream has been added. 280 o When a description is updated to have more SSRCs with the same 281 msid value, the recipient can signal to its application that new 282 media stream tracks have been added to the media stream. 284 o When a description is updated to no longer list the msid value on 285 a specific ssrc, the recipient can signal to its application that 286 the corresponding media stream track has been closed. 288 o When a description is updated to no longer list the msid value on 289 any ssrc, the recipient can signal to its application that the 290 media stream has been closed. 292 OPEN ISSUE: Exactly when should the recipient signal that the track 293 is closed? When the msid value disappears from the description, when 294 the SSRC disappears by the rules of [RFC3550] section 6.3.4 (BYE 295 packet received) and 6.3.5 (timeout), any of the above, or some 296 combination of the above? 298 4.1. Handling of non-signalled tracks 300 Pre-WebRTC entities will not send msid. This means that there will 301 be some incoming RTP packets with SSRCs where the recipient does not 302 know about a corresponding MediaStream id. 304 Handling will depend on whether or not any SSRCs are signalled in the 305 relevant RTP session. There are two cases: 307 o No SSRC is signalled with an msid attribute. The SDP session is 308 assumed to be a backwards-compatible session. All incoming SSRCs, 309 on all RTP sessions that are part of the SDP session, are assumed 310 to belong to a single media stream. The identifier of this media 311 stream is "default". 313 o Some SSRCs are signalled with an msid attribute. In this case, 314 the session is WebRTC compatible, and the newly arrived SSRCs are 315 either caused by a bug or by timing skew between the arrival of 316 the media packets and the SDP description. These packets MAY be 317 discarded, or they MAY be buffered for a while in order to allow 318 immediate startup of the media stream when the SDP description is 319 updated. The arrival of media packets MUST NOT cause a new 320 MediaStreamTrack to be signalled. 322 Note: This means that it is wise to include at least one a=ssrc: line 323 with an msid attribute, even when no media streams are yet attached 324 to the session. (Alternative: Mark the RTP session explicitly as "I 325 will signal the media stream tracks explicitly"). 327 It follows from the above that media stream tracks in the "default" 328 media stream cannot be closed by signalling; the application must 329 instead signal these as closed when the SSRC disappears according to 330 the rules of RFC 3550 section 6.3.4 and 6.3.5. 332 5. IANA Considerations 334 This document requests IANA to register the "msid" attribute in the 335 "att-field (source level)" registry within the SDP parameters 336 registry, according to the procedures of [RFC5576] 338 The required information is: 340 o Contact name, email: IETF, contacted via rtcweb@ietf.org, or a 341 successor address designated by IESG 343 o Attribute name: msid 345 o Long-form attribute name: Media stream group Identifier 347 o The attribute value contains only ASCII characters, and is 348 therefore not subject to the charset attribute. 350 o The attribute gives an association over a set of SSRCs, 351 potentially in different RTP sessions. It can be used to signal 352 the relationship between a WebRTC MediaStream and a set of SSRCs. 354 o The details of appropriate values are given in RFC XXXX. 356 This document requests IANA to register the "WMS" semantic within the 357 "Semantics for the "ssrc-group" SDP Attribute" registry within the 358 SDP parameters registry. 360 The required information is: 362 o Description: WebRTC Media Stream, as given in RFC XXXX. 364 o Token: WMS 366 o Standards track reference: RFC XXXX 368 IANA is requested to replace "RFC XXXX" with the RFC number of this 369 document upon publication. 371 6. Security Considerations 373 An adversary with the ability to modify SDP descriptions has the 374 ability to switch around tracks between media streams. This is a 375 special case of the general security consideration that modification 376 of SDP descriptions needs to be confined to entities trusted by the 377 application. 379 No attacks that are relevant to the browser's security have been 380 identified that depend on this mechanism. 382 7. Acknowledgements 384 This note is based on sketches from, among others, Justin Uberti and 385 Cullen Jennings. 387 Special thanks to Miguel Garcia for his work in reviewing this draft, 388 with many specific language suggestions. 390 8. References 392 8.1. Normative References 394 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 395 Requirement Levels", BCP 14, RFC 2119, March 1997. 397 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 398 Jacobson, "RTP: A Transport Protocol for Real-Time 399 Applications", STD 64, RFC 3550, July 2003. 401 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 402 Specifications: ABNF", STD 68, RFC 5234, January 2008. 404 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 405 Media Attributes in the Session Description Protocol 406 (SDP)", RFC 5576, June 2009. 408 [W3C.WD-webrtc-20120209] 409 Bergkvist, A., Burnett, D., Narayanan, A., and C. 410 Jennings, "WebRTC 1.0: Real-time Communication Between 411 Browsers", World Wide Web Consortium WD WD-webrtc- 412 20120209, February 2012, 413 . 415 8.2. Informative References 417 [I-D.westerlund-avtcore-multiplex-architecture] 418 Westerlund, M., Burman, B., Perkins, C., and H. 419 Alvestrand, "Guidelines for using the Multiplexing 420 Features of RTP", 421 draft-westerlund-avtcore-multiplex-architecture-02 (work 422 in progress), July 2012. 424 [RFC4574] Levin, O. and G. Camarillo, "The Session Description 425 Protocol (SDP) Label Attribute", RFC 4574, August 2006. 427 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 428 Control Packets on a Single Port", RFC 5761, April 2010. 430 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 431 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 433 Appendix A. Design considerations, open questions and and alternatives 435 This appendix should be deleted before publication as an RFC. 437 One suggested mechanism has been to use CNAME instead of a new 438 attribute. This was abandoned because CNAME identifies a 439 synchronization context; one can imagine both wanting to have tracks 440 from the same synchronization context in multiple media streams and 441 wanting to have tracks from multiple synchronization contexts within 442 one media stream. 444 Another suggestion has been to put the msid value within an attribute 445 of RTCP SR (sender report) packets. This doesn't offer the ability 446 to know that you have seen all the tracks currently configured for a 447 media stream. 449 There has been a suggestion that this mechanism could be used to mute 450 tracks too. This is not done at the moment. 452 The special value "default" and the reservation of "example*" seems 453 bothersome; apart from that, it's a random string. It's uncertain 454 whether "example" has any benefit. 456 An alternative to the "default" media stream is to let each new media 457 stream track without a msid attribute create its own media stream. 458 Input on this question is sought. 460 Discarding of incoming data when the SDP description isn't updated 461 yet (section 3) may cause clipping. However, the same issue exists 462 when crypto keys aren't available. Input sought. 464 There's been a suggestion that acceptable SSRCs should be signalled 465 in a response, giving a recipient the ability to say "no" to certain 466 SSRCs. This is not supported in the current version of this 467 document. 469 This specification reuses the ssrc-group semantics registry for this 470 semantic, on the argument that the WMS purpose is more similar to an 471 SSRC grouping than a session-level grouping, and allows values from 472 both registries, on the argument that some semantics (like LS) are 473 well defined for MSID. Input sought. 475 Appendix B. Change log 477 This appendix should be deleted before publication as an RFC. 479 B.1. Changes from rtcweb-msid-00 to -01 481 Added track identifier. 483 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 484 for track muting. 486 Some rewording. 488 B.2. Changes from rtcweb-msid-01 to -02 490 Split document into sections describing a generic grouping mechanism 491 and sections describing the application of this grouping mechanism to 492 the WebRTC MediaStream concept. 494 Removed the mechanism for muting tracks, since this is not central to 495 the MSID mechanism. 497 B.3. Changes from rtcweb-msid-02 to mmusic-msid-00 499 Changed the draft name according to the wishes of the MMUSIC group 500 chairs. 502 Added text indicting cases where it's appropriate to have the same 503 appdata for multiple SSRCs. 505 Minor textual updates. 507 B.4. Changes from mmusic-msid-00 to -01 509 Increased the amount of explanatory text, much based on a review by 510 Miguel Garcia. 512 Removed references to BUNDLE, since that spec is under active 513 discussion. 515 Removed distinguished values of the MSID identifier. 517 Author's Address 519 Harald Alvestrand 520 Google 521 Kungsbron 2 522 Stockholm, 11122 523 Sweden 525 Email: harald@alvestrand.no