idnits 2.17.1 draft-ietf-mmusic-msid-07.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 (October 14, 2014) is 3483 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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-26) exists of draft-ietf-rtcweb-jsep-07 ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-12 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--).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 October 14, 2014 5 Expires: April 17, 2015 7 WebRTC MediaStream Identification in the Session Description Protocol 8 draft-ietf-mmusic-msid-07 10 Abstract 12 This document specifies a Session Description Protocol (SDP) Grouping 13 mechanism for RTP media streams that can be used to specify relations 14 between media streams. 16 This mechanism is used to signal the association between the SDP 17 concept of "m-line" and the WebRTC concept of "MediaStream" / 18 "MediaStreamTrack" using SDP signaling. 20 This document is a work item of the MMUSIC WG, whose discussion list 21 is mmusic@ietf.org. 23 Requirements Language 25 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 26 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 27 document are to be interpreted as described in RFC 2119 [RFC2119]. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on April 17, 2015. 46 Copyright Notice 48 Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . 5 68 3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 6 69 4. Generic SDP Offer/Answer Procedures . . . . . . . . . . . . . 6 70 4.1. Generating the Initial Offer . . . . . . . . . . . . . . 7 71 4.2. Answerer Processing of the Offer . . . . . . . . . . . . 7 72 4.3. Generating the Answer . . . . . . . . . . . . . . . . . . 7 73 4.4. Offerer Processing of the Answer . . . . . . . . . . . . 7 74 5. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . 7 75 5.1. Handling of non-signalled tracks . . . . . . . . . . . . 8 76 5.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 9 77 5.2.1. Generating the initial offer . . . . . . . . . . . . 10 78 5.2.2. Parsing the initial offer . . . . . . . . . . . . . . 10 79 5.2.3. Generating the answer . . . . . . . . . . . . . . . . 10 80 5.2.4. Offerer processing of the answer . . . . . . . . . . 10 81 5.2.5. Modifying the session . . . . . . . . . . . . . . . . 11 82 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 84 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 85 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 86 9.1. Normative References . . . . . . . . . . . . . . . . . . 13 87 9.2. Informative References . . . . . . . . . . . . . . . . . 13 88 Appendix A. Design considerations, rejected alternatives . . . . 14 89 Appendix B. Usage with multiple MediaStreams per M-line . . . . 14 90 B.1. Mechanism design with multiple SSRCs . . . . . . . . . . 15 91 B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 16 92 Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 16 93 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 16 94 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 16 95 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 16 96 C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 16 97 C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 17 98 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 17 99 C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 17 100 C.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 17 101 C.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 17 102 C.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 18 103 C.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 18 104 C.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 18 105 C.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 18 106 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 108 1. Introduction 110 1.1. Structure Of This Document 112 This document adds a new Session Description Protocol (SDP) 113 Grouping[RFC5888] relation between SDP m-lines [RFC4566] that can 114 associate application layer identifiers with the binding between 115 media streams, attaching identifiers to the media streams and 116 attaching identifiers to the groupings they form. 118 Section 1.2 gives the background on why a new mechanism is needed. 120 Section 2 gives the definition of the new mechanism. 122 Section 3 gives the definition of the msid-semantic field, which 123 gives the possibility of using MSIDs with different semantics in the 124 same SDP message. 126 Section 5 gives the application of the new mechanism for providing 127 necessary semantic information for the association of 128 MediaStreamTracks to MediaStreams in the WebRTC 129 API[W3C.WD-webrtc-20120209]. 131 1.2. Why A New Mechanism Is Needed 133 When media is carried by RTP [RFC3550], each RTP media stream is 134 distinguished inside an RTP session by its SSRC; each RTP session is 135 distinguished from all other RTP sessions by being on a different 136 transport association (strictly speaking, 2 transport associations, 137 one used for RTP and one used for RTCP, unless RTP/RTCP multiplexing 138 [RFC5761] is used). 140 SDP gives a description based on m-lines. According to the model 141 used in [I-D.ietf-rtcweb-jsep], each m-line describes exactly one 142 media source, and if mulitple media sources are carried in an RTP 143 session, this is signalled using BUNDLE 144 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each 145 media source is carried in its own RTP session. 147 There exist cases where an application using RTP and SDP needs to 148 signal some relationship between RTP media streams that may be 149 carried in either the same RTP session or different RTP sessions. 150 For instance, there may be a need to signal a relationship between a 151 video track and an audio track, and where the generator of the SDP 152 does not yet know if they will be carried in the same RTP session or 153 different RTP sessions. 155 The SDP grouping framework [RFC5888] can be used to group m-lines. 156 However, there is sometimes the need for an application to specify 157 some application-level information about the association between the 158 m-line and the group. This is not possible using the SDP grouping 159 framework. 161 1.3. Application to the WEBRTC MediaStream 163 The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies 164 that communication between WebRTC entities is done via MediaStreams, 165 which contain MediaStreamTracks. A MediaStreamTrack is generally 166 carried using a single SSRC in an RTP session (forming an RTP media 167 stream. The collision of terminology is unfortunate.) There might 168 possibly be additional SSRCs, possibly within additional RTP 169 sessions, in order to support functionality like forward error 170 correction or simulcast. This complication is ignored below. 172 In the RTP specification, media streams are identified using the SSRC 173 field. Streams are grouped into RTP Sessions, and also carry a 174 CNAME. Neither CNAME nor RTP session correspond to a MediaStream. 175 Therefore, the association of an RTP media stream to MediaStreams 176 need to be explicitly signaled. 178 WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where 179 one SDP m-line is used to describe each MediaStreamTrack, and that 180 the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is used 181 to group MediaStreamTracks into RTP sessions. Therefore, the need is 182 to specify the ID of a MediaStreamTrack and its associated 183 MediaStream for each m-line, which can be accomplished with a media- 184 level SDP attribute. 186 This usage is described in Section 5. 188 2. The Msid Mechanism 190 This document defines a new SDP [RFC4566] media-level "msid" 191 attribute. This new attribute allows endpoints to associate RTP 192 media streams that are carried in the same or different m-lines. The 193 attribute also allows application-specific information to the 194 association. 196 The value of the "msid" attribute consists of an identifier and 197 optional application-specific data, according to the following ABNF 198 [RFC5234] grammar: 200 ; "attribute" is defined in RFC 4566. 201 attribute =/ msid-attr 202 msid-attr = "msid:" msid-id [ SP msid-appdata ] 203 msid-id = 1*64token-char ; see RFC 4566 204 msid-appdata = 1*64token-char ; see RFC 4566 206 An example msid value for a group with the identifier "examplefoo" 207 and application data "examplebar" might look like this: 209 msid:examplefoo examplebar 211 The identifier is a string of ASCII characters that are legal in a 212 "token", consisting of between 1 and 64 characters. It MUST be 213 unique among the identifier values used in the same SDP session. It 214 is RECOMMENDED that it is generated using a random-number generator. 216 Application data is carried on the same line as the identifier, 217 separated from the identifier by a space. 219 The identifier uniquely identifies a group within the scope of an SDP 220 description. 222 There may be multiple msid attributes in a single media description. 223 There may also be multiple media descriptions that have the same 224 value for identifier and application data. 226 Endpoints can update the associations between RTP media streams as 227 expressed by msid attributes at any time; the semantics and 228 restrictions of such grouping and ungrouping are application 229 dependent. 231 3. The Msid-Semantic Attribute 233 A session-level attribute is defined for signaling the semantics 234 associated with an msid grouping. This allows msid groupings with 235 different semantics to coexist. 237 This OPTIONAL attribute gives the group identifier and its group 238 semantic; it carries the same meaning as the ssrc-group-attr of RFC 239 5576 section 4.2, but uses the identifier of the group rather than a 240 list of SSRC values. 242 This attribute MUST be present if "a=msid" is used. 244 An empty list of identifiers is an indication that the sender 245 supports the indicated semantic, but has no msid groupings of the 246 given type in the present SDP. 248 An identifier of "*" is an indication that all "a=msid" lines in the 249 SDP have this specific semantic. 251 The ABNF of msid-semantic is: 253 attribute =/ msid-semantic-attr 254 msid-semantic-attr = "msid-semantic:" msid-semantic msid-list 255 msid-semantic = token ; see RFC 4566 256 msid-list = *(" " msid-id) / " *" 258 The semantic field holds values from the IANA registriy "Semantics 259 for the msid-semantic SDP attribute" (which is defined in Section 6). 261 An example msid-semantic might look like this, if a semantic LS was 262 registered by IANA for the same purpose as the existing LS grouping 263 semantic: 265 a=msid-semantic:LS xyzzy forolow 267 This means that the SDP description has two lip sync groups, with the 268 group identifiers xyzzy and forolow, respectively. 270 The msid-semantic attribute can occur more than once, but MUST NOT 271 occur more than once with the same msid-semantic value. 273 4. Generic SDP Offer/Answer Procedures 275 In accordance with guidance on definitions of SDP extensions, this 276 section gives the generic procedures that have to be followed by all 277 implementations of Msid, independent of which semantics they support. 279 Note that the use of msid is not negotiated; each side declares what 280 it uses. 282 4.1. Generating the Initial Offer 284 An entity implmementing an MSID semantic MUST add one or more "msid- 285 semantic" attributes to its session level attributes, indicating the 286 MSID semantic it supports. 288 4.2. Answerer Processing of the Offer 290 If an "msid-semantic" attribute is present in the answer, and the 291 offerer supports the indicated semantic, the offerer MUST follow the 292 procedures described for that semantic. 294 4.3. Generating the Answer 296 An entity implmementing an MSID semantic MUST add a "msid-semantic" 297 attribute to its session level attributes, indicating the MSID 298 semantic it supports. 300 4.4. Offerer Processing of the Answer 302 If an "msid-semantic" attribute is present in the answer, and the 303 offerer supports the indicated semantic, the offerer MUST follow the 304 procedures described for that semantic. 306 5. Applying Msid to WebRTC MediaStreams 308 This section creates a new semantic for use with the framework 309 defined in Section 2, to be used for associating m-lines representing 310 MediaStreamTracks within MediaStreams as defined in 311 [W3C.WD-webrtc-20120209]. 313 In the Javascript API, each MediaStream and MediaStreamTrack has an 314 "id" attribute, which is a DOMString. 316 The semantic token for this semantic is "WMS" (short for WebRTC Media 317 Stream). 319 The value of the "identifier" field in the msid consists of the "id" 320 attribute of a MediaStream, as defined in its WebIDL specification. 322 The value of the "appdata" field in the msid consists of the "id" 323 attribute of a MediaStreamTrack, as defined in its WebIDL 324 specification. 326 If two different m-lines have MSID attributes with the same value for 327 identifier and appdata, it means that these two m-lines are both 328 intended for the same MediaStreamTrack. So far, no semantic for such 329 a mixture have been defined, but this specification does not forbid 330 the practice. 332 When an SDP description is updated, a specific msid "identifier" 333 continues to refer to the same MediaStream, and a specific "appdata" 334 to the same MediaStreamTrack. Once negotiation has completed on a 335 session, there is no memory apart from the currently valid SDP 336 descriptions; an msid "identifier" value that appears in a later 337 negotiation will be taken to refer to a new MediaStream. 339 The following are the rules for handling updates of the list of 340 m-lines and their msid values. 342 o When a new msid "identifier" value occurs in the description, the 343 recipient can signal to its application that a new MediaStream has 344 been added. 346 o When a description is updated to have more media sections with the 347 same msid "identifier" value, but different "appdata" values, the 348 recipient can signal to its application that new MediaStreamTracks 349 have been added to the MediaStream. 351 o When a description is updated to no longer list the msid attribute 352 on a specific media description, the recipient can signal to its 353 application that the corresponding MediaStreamTrack has ended. 355 In addition to signaling that the track is closed when its msid 356 attribute disappears from the SDP, the track will also be signaled as 357 being closed when all associated SSRCs have disappeared by the rules 358 of [RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout), 359 and when the corresponding media section is disabled by setting the 360 port number to zero. Changing the direction of the media section to 361 "recvonly" will not close the MediaStreamTrack. 363 The association between SSRCs and m-lines is specified in 364 [I-D.ietf-rtcweb-jsep]. 366 5.1. Handling of non-signalled tracks 368 Entities that do not implement the WMS semantic will not send "msid- 369 semantic:WMS". This means that there will be some incoming RTP 370 packets that the recipient has no predefined MediaStream id value 371 for. 373 Note that this handling is triggered by incoming RTP packets, not by 374 SDP negotiation. 376 Handling will depend on whether or not the msid-semantic:WMS 377 attribute is present. There are two cases: 379 o No "msid-semantic:WMS" attribute is present. The SDP session is 380 assumed to be a backwards-compatible session. All incoming media, 381 on all m-lines that are part of the SDP session, are assumed to 382 belong to independent media streams, each with one track. The 383 identifier of this media stream and of the media stream track is a 384 randomly generated string; the WebIDL "label" attribute of this 385 media stream will be set to "Non-WMS stream". 387 o An "msid-semantic:WMS" attribute is present. In this case, the 388 sender implements the WMS semantic, and the packets are either 389 caused by a bug or by timing skew between the arrival of the media 390 packets and the SDP description. These packets MAY be discarded, 391 or they MAY be buffered for a while in order to allow immediate 392 startup of the media stream when the SDP description is updated. 393 The arrival of media packets MUST NOT cause a new MediaStreamTrack 394 to be signaled. 396 If an entity implementing the WMS semantic sends a description, it 397 MUST include the msid-semantic:WMS attribute, even if no media 398 streams are sent. This allows us to distinguish between the case of 399 no media streams at the moment and the case of legacy SDP generation. 401 It follows from the above that the media receiver implmementing the 402 WMS semantic must have the SDP of the other party before it can 403 decide correctly which of the two cases described above applies. RTP 404 media packets that arrive before the remote party's SDP MUST be 405 buffered or discarded, and MUST NOT cause a new MediaStreamTrack to 406 be signalled. 408 It follows from the above that media stream tracks in the "default" 409 media stream cannot be closed by removing the msid attribute; the 410 application must instead signal these as closed when the SSRC 411 disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5 412 or by disabling the m-line by setting its port to zero. 414 5.2. Detailed Offer/Answer Procedures 416 These procedures are given in terms of RFC 3264-recommended sections. 417 They describe the actions to be taken in terms of MediaStreams and 418 MediaStreamTracks; they do not include event signalling inside the 419 application, which is described in JSEP. 421 They are specifically applicable to the WMS semantic; other semantics 422 will have their own consideration. 424 5.2.1. Generating the initial offer 426 For each media section in the offer, if there is an associated 427 MediaStreamTrack, the offerer adds one "a=msid" attribute to the 428 section for each MediaStream with which the MediaStreamTrack is 429 associated. The "identifier" field of the attribute is set to the 430 WebIDL "id" attribute of the MediaStream, and the "appdata" field is 431 set to the WebIDL "id" attribute of the MediaStreamTrack. 433 The offerer adds an "msid-semantic:WMS" field to the session-level 434 headers, and appends to it either a list of all the identifiers used 435 in the offer, or the single character "*". 437 5.2.2. Parsing the initial offer 439 For each media section in the offer, and for each "a=msid" attribute 440 in the media section, the receiver of the offer will perform the 441 following steps: 443 o Extract the "appdata" field of the "a=msid" attribute 445 o Check if a MediaStreamTrack with the same WebIDL "id" attribute as 446 the "appdata" field already exists, and is not in the "ended" 447 state. If it is not found, create it. 449 o Extract the "identifier" field of the "a=msid" attribte. 451 o Check if a MediaStream with the same WebIDL "id" attribute already 452 exists. If not, create it. 454 o Add the MediaStreamTrack to the MediaStream 456 5.2.3. Generating the answer 458 The answer is generated in exactly the same manner as the offer. 460 This includes adding a "msid-semantic:WMS" attribute in the session- 461 level headers, independent of whether or not such a header was 462 present in the offer. 464 5.2.4. Offerer processing of the answer 466 The answer is processed in exactly the same manner as the offer. 468 5.2.5. Modifying the session 470 On subsequent exchanges, precisely the same procedure as for the 471 initial offer/answer is followed, but with one additional step in the 472 parsing of the offer and answer: 474 o For each MediaStreamTrack that has been created as a result of 475 previous offer/answer exchanges, and is not in the "ended" state, 476 check to see if there is still an "a=msid" attribute in the 477 present SDP whose "appdata" field is the same as the WebIDL "id" 478 attribute of the track. 480 o If no such attribute is found, close the MediaStreamTrack. This 481 will set its state to "ended". 483 6. IANA Considerations 485 This document requests IANA to register the "msid" attribute in the 486 "att-field (media level only)" registry within the SDP parameters 487 registry, according to the procedures of [RFC4566] 489 The required information for "msid" is: 491 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 492 successor address designated by IESG 494 o Attribute name: msid 496 o Long-form attribute name: Media stream group Identifier 498 o Subject to charset: The attribute value contains only ASCII 499 characters, and is therefore not subject to the charset attribute. 501 o Purpose: The attribute gives an association over a set of m-lines. 502 It can be used to signal the relationship between a WebRTC 503 MediaStream and a set of m-lines. 505 o Appropriate values: The details of appropriate values are given in 506 RFC XXXX. 508 This document requests IANA to register the "msid-semantic" attribute 509 in the "att-field (session level) registry within the SDP parameters 510 registry, according to the same procedures. 512 The required information is: 514 o Contact name, email: IETF, contacted via mmusic@ietf.org, or a 515 successor address designated by IESG 517 o Attribute name: msid-semantic 519 o Long-form attribute name: Msid group semantic identifier 521 o Subject to charset: The attribute value contains only ASCII 522 characters, and is therefore not subject to the charset attribute. 524 o Purpose: The attribute gives the semantics of an association over 525 a set of m-lines. 527 o Appropriate values: The details are given in RFC XXXX. 529 This document requests IANA to create a new registry called 530 "Semantics for the msid-semantic SDP attribute", which should have 531 exactly the same rules as for the "Semantics for the ssrc-group SDP 532 attribute" registry (Expert Review), and to register the "WMS" 533 semantic within this new registry. 535 The required information is: 537 o Description: WebRTC Media Stream, as given in RFC XXXX. 539 o Token: WMS 541 o Standards track reference: RFC XXXX 543 IANA is requested to replace "RFC XXXX" with the RFC number of this 544 document upon publication. 546 7. Security Considerations 548 An adversary with the ability to modify SDP descriptions has the 549 ability to switch around tracks between media streams. This is a 550 special case of the general security consideration that modification 551 of SDP descriptions needs to be confined to entities trusted by the 552 application. 554 If implementing buffering as mentioned in Section 5.1, the amount of 555 buffering should be limited to avoid memory exhaustion attacks. 557 No other attacks have been identified that depend on this mechanism. 559 8. Acknowledgements 561 This note is based on sketches from, among others, Justin Uberti and 562 Cullen Jennings. 564 Special thanks to Flemming Andreassen, Miguel Garcia and Paul Kyzivat 565 for their work in reviewing this draft, with many specific language 566 suggestions. 568 9. References 570 9.1. Normative References 572 [I-D.ietf-rtcweb-jsep] 573 Uberti, J., Jennings, C., and E. Rescorla, "Javascript 574 Session Establishment Protocol", draft-ietf-rtcweb-jsep-07 575 (work in progress), July 2014. 577 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 578 Requirement Levels", BCP 14, RFC 2119, March 1997. 580 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 581 Jacobson, "RTP: A Transport Protocol for Real-Time 582 Applications", STD 64, RFC 3550, July 2003. 584 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 585 Description Protocol", RFC 4566, July 2006. 587 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 588 Specifications: ABNF", STD 68, RFC 5234, January 2008. 590 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 591 Media Attributes in the Session Description Protocol 592 (SDP)", RFC 5576, June 2009. 594 [W3C.WD-webrtc-20120209] 595 Bergkvist, A., Burnett, D., Jennings, C., and A. 596 Narayanan, "WebRTC 1.0: Real-time Communication Between 597 Browsers", World Wide Web Consortium WD WD- 598 webrtc-20120209, February 2012, 599 . 601 9.2. Informative References 603 [I-D.ietf-mmusic-sdp-bundle-negotiation] 604 Holmberg, C., Alvestrand, H., and C. Jennings, 605 "Negotiating Media Multiplexing Using the Session 606 Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- 607 negotiation-12 (work in progress), October 2014. 609 [I-D.roach-mmusic-unified-plan] 610 Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for 611 Using SDP with Large Numbers of Media Flows", draft-roach- 612 mmusic-unified-plan-00 (work in progress), July 2013. 614 [I-D.westerlund-avtcore-multiplex-architecture] 615 Westerlund, M., Perkins, C., and H. Alvestrand, 616 "Guidelines for using the Multiplexing Features of RTP", 617 draft-westerlund-avtcore-multiplex-architecture-03 (work 618 in progress), February 2013. 620 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 621 Control Packets on a Single Port", RFC 5761, April 2010. 623 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 624 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 626 Appendix A. Design considerations, rejected alternatives 628 This appendix should be deleted before publication as an RFC. 630 One suggested mechanism has been to use CNAME instead of a new 631 attribute. This was abandoned because CNAME identifies a 632 synchronization context; one can imagine both wanting to have tracks 633 from the same synchronization context in multiple MediaStreams and 634 wanting to have tracks from multiple synchronization contexts within 635 one MediaStream (but the latter is impossible, since a MediaStream is 636 defined to impose synchronization on its members). 638 Another suggestion has been to put the msid value within an attribute 639 of RTCP SR (sender report) packets. This doesn't offer the ability 640 to know that you have seen all the tracks currently configured for a 641 media stream. 643 Appendix B. Usage with multiple MediaStreams per M-line 645 This appendix is included to document the usage of msid as a source- 646 specific attribute. Prior to the acceptance of the Unified Plan 647 document, some implementations used this mechanism to distinguish 648 between multiple MediaStreamTracks that were carried in the same 649 M-line. 651 It reproduces some of the original justification text for this 652 mechanism that is not relevant when Unified Plan is used. 654 B.1. Mechanism design with multiple SSRCs 656 When media is carried by RTP [RFC3550], each RTP media stream is 657 distinguished inside an RTP session by its SSRC; each RTP session is 658 distinguished from all other RTP sessions by being on a different 659 transport association (strictly speaking, 2 transport associations, 660 one used for RTP and one used for RTCP, unless RTCP multiplexing 661 [RFC5761] is used). 663 There exist cases where an application using RTP and SDP needs to 664 signal some relationship between RTP media streams that may be 665 carried in either the same RTP session or different RTP sessions. 666 For instance, there may be a need to signal a relationship between a 667 video track in one RTP session and an audio track in another RTP 668 session. In traditional SDP, it is not possible to signal that these 669 two tracks should be carried in one session, so they are carried in 670 different RTP sessions. 672 Traditionally, SDP was used to describe the RTP sessions, with one 673 m-line being used to describe each RTP session. With the advent of 674 extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this 675 association may be more complex, with multiple m-lines being used to 676 describe one RTP session; the rest of this document therefore talks 677 about m-lines, not RTP sessions, when describing the signalling 678 mechanism. 680 The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to 681 associate RTP media streams when those RTP media streams are 682 described by the same m-line. The semantics of this mechanism 683 prevent the association of RTP media streams that are spread across 684 different m-lines. 686 The SDP grouping framework [RFC5888] can be used to group m-lines. 687 When an m-line describes one and only one RTP media stream, it is 688 possible to associate RTP media streams across different m-lines. 689 However, if an m-line has multiple RTP media streams, using multiple 690 SSRCs, the SDP grouping framework cannot be used for this purpose. 692 There are use cases (some of which are discussed in 693 [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of 694 these approaches is appropriate; In those cases, a new mechanism is 695 needed. 697 In addition, there is sometimes the need for an application to 698 specify some application-level information about the association 699 between the SSRC and the group. This is not possible using either of 700 the frameworks above. 702 B.2. Usage with the SSRC attribute 704 When the MSID attribute was used with the SSRC attribute, it had to 705 be registered in the "Attribute names (source level)" registry rather 706 than the "Attribute names (media level only)" registry, and the msid 707 line was prefixed with "a=ssrc: ". Apart from that, usage of 708 the attribute with SSRC-bound flows was identical with the current 709 proposal. 711 Appendix C. Change log 713 This appendix should be deleted before publication as an RFC. 715 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 717 Added track identifier. 719 Added inclusion-by-reference of draft-lennox-mmusic-source-selection 720 for track muting. 722 Some rewording. 724 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 726 Split document into sections describing a generic grouping mechanism 727 and sections describing the application of this grouping mechanism to 728 the WebRTC MediaStream concept. 730 Removed the mechanism for muting tracks, since this is not central to 731 the MSID mechanism. 733 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 735 Changed the draft name according to the wishes of the MMUSIC group 736 chairs. 738 Added text indicting cases where it's appropriate to have the same 739 appdata for multiple SSRCs. 741 Minor textual updates. 743 C.4. Changes from alvestrand-mmusic-msid-00 to -01 745 Increased the amount of explanatory text, much based on a review by 746 Miguel Garcia. 748 Removed references to BUNDLE, since that spec is under active 749 discussion. 751 Removed distinguished values of the MSID identifier. 753 C.5. Changes from alvestrand-mmusic-msid-01 to -02 755 Changed the order of the "msid-semantic: " attribute's value fields 756 and allowed multiple identifiers. This makes the attribute useful as 757 a marker for "I understand this semantic". 759 Changed the syntax for "identifier" and "appdata" to be "token". 761 Changed the registry for the "msid-semantic" attribute values to be a 762 new registry, based on advice given in Atlanta. 764 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 766 Updated terminology to refer to m-lines rather than RTP sessions when 767 discussing SDP formats and the ability of other linking mechanisms to 768 refer to SSRCs. 770 Changed the "default" mechanism to return independent streams after 771 considering the synchronization problem. 773 Removed the space from between "msid-semantic" and its value, to be 774 consistent with RFC 5576. 776 C.7. Changes from mmusic-msid-00 to -01 778 Reworked msid mechanism to be a per-m-line attribute, to align with 779 [I-D.roach-mmusic-unified-plan] 781 C.8. Changes from mmusic-msid-01 to -02 783 Corrected several missed cases where the word "ssrc" was not changed 784 to "M-line". 786 Added pointer to unified-plan (which should be moved to point to 787 -jsep) 789 Removed suggestion that ssrc-group attributes can be used with "msid- 790 semantic", it is now only the msid-semantic registry. 792 C.9. Changes from mmusic-msid-02 to -03 794 Corrected even more cases where the word "ssrc" was not changed to 795 "M-line". 797 Added the functionality of using an asterisk (*) in the msid-semantic 798 line, in order to remove the need for listing all msids in the msid- 799 semantic line whne only one msid-semantic is in use. 801 Removed some now-unnecessary text. 803 C.10. Changes from mmusic-msid-03 to -04 805 Changed title to reflect focus on WebRTC MediaStreams 807 Added a section on receiver-side media stream control, using the 808 "msid-control" attribute. 810 C.11. Changes from -04 to -05 812 Removed the msid-control section after WG discussion. 814 Removed some text that seemed only to pertain to resolved issues. 816 C.12. Changes from -05 to -06 818 Addressed issues found in Fleming Andreassen's review 820 Referenced JSEP rather than unified-plan for the M-line mapping model 822 Relaxed MSID definition to allow "token-char" in values rather than 823 a-z 0-9 hyphen; tightened ABNF by adding length description to it. 825 Deleted discussion of abandoned alternatives, as part of preparing 826 for publication. 828 Added a "detailed procedures" section to the WMS semantics 829 description. 831 Added IANA registration of the "msid-semantic" attribute. 833 C.13. Changes from -06 to -07 835 Changed terminology from referring to "WebRTC device" to referring to 836 "entities that implement the WMS semantic". 838 Changed names for ABNF constructions based on a proposal by Paul 839 Kyzivat. 841 Included a section on generic offer/answer semantics. 843 Author's Address 845 Harald Alvestrand 846 Google 847 Kungsbron 2 848 Stockholm 11122 849 Sweden 851 Email: harald@alvestrand.no