Network Working Group H. Alvestrand Internet-Draft Google Intended status: Standards Track January 25, 2012 Expires: July 28, 2012 Signalling Media Stream ID in the Session Description Protocol draft-alvestrand-rtcweb-msid-01 Abstract This document specifies how the association between the RTP concept of SSRC and the WebRTC concept of "media stream" / "media stream track" is carried using SDP signalling. This document is an input document for discussion. It should be discussed in the RTCWEB WG list, rtcweb@ietf.org. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 28, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents Alvestrand Expires July 28, 2012 [Page 1] Internet-Draft MSID in SDP January 2012 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Basic mechanism . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Signalling muted and non-desired tracks . . . . . . . . . . . . 5 4. Handling of non-signalled tracks . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Design considerations, open questions and and alternatives . . . . . . . . . . . . . . . . . . . . . 8 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . . 8 B.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Alvestrand Expires July 28, 2012 [Page 2] Internet-Draft MSID in SDP January 2012 1. Introduction The W3C WebRTC API specification [W3C.WD-webrtc-20111027] specifies that communication between WebRTC entities is done via MediaStreams, which contain MediaStreamTracks. A MediaStreamTrack is generally carried using a single SSRC in an RTP session. There might possibly with additional SSRCs, possibly within additional RTP sessions, in order to support functionality like forward error correction or simulcast. This complication is ignored below. One SSRC may utilize multiple codecs, such as an audio track switching between audio, comfort noise and DTMF. An SSRC MUST NOT switch between media types (no switching between audio and video, for instance) In the RTP specification, media streams are identified using the SSRC field. Streams are grouped into RTP Sessions, and also carry a CNAME. Neither CNAME nor RTP session correspond to a MediaStream. Therefore, the association of an RTP media stream to MediaStreams need to be explicitly signalled. The marking needs to be on a per-SSRC basis, since one RTP session can carry media from multiple MediaStreams, and one MediaStream can have media in multiple RTP sessions. This means that the [RFC4574] "label" attribute, which is used to label RTP sessions, is not usable for this purpose. The marking needs to also carry the unique identifier of the RTP media stream as a MediaStreamTrack within the media stream; this is done using a single letter to identify whether it belongs in the video or audio track list, and the MediaStreamTrack's position within that array. 2. Basic mechanism Association of the MediaStream to the SSRC is done via an "msid" attribute attached to the SSRC in the SDP description, using the "Source Specific Media Attribute" mechanism [RFC5576]: a=ssrc:1234 msid:examplefoo v1 The msid is a string of ASCII characters chosen from 0-9, a-z, A-Z and - (hyphen), consisting of between 1 and 64 characters. The value "default" (all lower case) has special meaning, and MUST NOT be generated. Values starting with "example" (all lower case) are reserved for documentation, and MUST NOT be generated by an implementation. Alvestrand Expires July 28, 2012 [Page 3] Internet-Draft MSID in SDP January 2012 After the msid, there is a track identifier, denoting the index at which this track appears in the respective track list. The track identifier is prefixed with "a" for audio, or "v" for video. ABNF[RFC5234] grammar: msidattribute = "msid:" identifier " " trackid identifier = 1*64 ("0".."9" / "a".."z" / "-") trackid = ("v" / "a" "1".."9" *8("0".."9") (Note: one possible generation algorithm is to generate 6 random bytes, base64 encode them (giving 8 bytes), and prefixing with a letter that is neither "d" nor "e". Another possibility is using some form of UUID.) The msid uniquely identifies a media stream within the scope of an SDP description. When an SDP description is updated, a specific msid continues to refer to the same media stream; an msid value MUST NOT be reused for another media stream within an SDP session's lifetime. The value of the msid corresponds to the "id" attribute of a MediaStream. (note: as of Jan 11, 2012, this is called "label". The word "label" means many other things, so the same word should not be used.) In a WebRTC-compatible SDP description, all SSRCs intending to be sent from one peer will be identified in the SDP generated by that entity. The following are the rules for handling updates of the list of SSRCs and their msid values. o When a new msid value occurs in the description, the recipient can signal to its application that a new media stream has been added. o When a description is updated to have more SSRCs with the same msid value, the recipient can signal to its application that new media stream tracks have been added to the media stream. o When a description is updated to no longer list the msid value on a specific ssrc, the recipient can signal to its application that the corresponding media stream track has been closed o When a description is updated to no longer list the msid value on any ssrc, the recipient can signal to its application that the media stream track has been closed. Alvestrand Expires July 28, 2012 [Page 4] Internet-Draft MSID in SDP January 2012 OPEN ISSUE: Exactly when should the recipient signal that the track is closed? When the msid value disappears from the description, when a BYE packet is received, when a timeout passes with no media, any of the above, or some combination of the above? 3. Signalling muted and non-desired tracks In addition to identifying the track, there is a need for specifying whether or not a track is muted by the sender (that is, the track still exists, but is not sending data at the moment), whether the track is muted by the recipient (that is, the track is OK to receive, but it is useless to send data on it now because the recipient is not listening, or rejected by the recipient (that is, for some reason the recipient is unwilling or unable to receive the track). This is accomplished using the mechanisms from [I-D.lennox-mmusic-sdp-source-selection], in particular these constructs: o a=remote-ssrc: recv:off - recipient does not wish to receive data o a=SSRC: sending:off - sender is not currently sending data o If some sources are present with a=remote-ssrc: attributes, section 8 of the referenced draft states that the other SSRCs SHOULD be regarded as rejected by the receiver. An implementation compliant with this specification MUST also comply with all requirements of [I-D.lennox-mmusic-sdp-source-selection]. 4. Handling of non-signalled tracks Pre-WebRTC entities will not send msid. This means that there will be some incoming RTP packets with SSRCs where the recipient does not know about a corresponding MediaStream id. Handling will depend on whether or not any SSRCs are signalled in the relevant RTP session. There are two cases: o No SSRC is signalled with an msid attribute. The SDP session is assumed to be a backwards-compatible session. All incoming SSRCs, on all RTP sessions that are part of the SDP session, are assumed to belong to a single media stream. The ID of this media stream is "default". Alvestrand Expires July 28, 2012 [Page 5] Internet-Draft MSID in SDP January 2012 o Some SSRCs are signalled with an msid attribute. In this case, the session is WebRTC compatible, and the newly arrived SSRCs are either caused by a bug or by timing skew between the arrival of the media packets and the SDP description. These packets MAY be discarded, or they MAY be buffered for a while in order to allow immediate startup of the media stream when the SDP description is updated. The arrival of media packets MUST NOT cause a new MediaStreamTrack to be created. Note: This means that it is wise to include at least one a=ssrc: line with an msid attribute, even when no media streams are yet attached to the session. (Alternative: Mark the RTP session explicitly as "I will signal the media stream tracks explicitly"). It follows from the above that media stream tracks in the "default" media stream cannot be closed by signalling; the application must instead signal these as closed when either an RTCP BYE packet or the absence of media for a defined interval indicates that the stream is gone. 5. IANA Considerations This document requests IANA to register the "msid" attribute in the "att-field (source level)" registry within the SDP parameters registry, according to the procedures of [RFC5576] The required information is: o Contact name, email: IETF, contacted via rtcweb@ietf.org, or a successor address designated by IESG o Attribute name: msid o Long-form attribute name: Media Stream Identifier o The attribute value contains only ASCII characters, and is therefore not subject to the charset attribute. o The attribute gives an association between a WebRTC MediaStream and an SSRC. o The details of appropriate values are given in RFC XXXX. IANA is requested to replace "RFC XXXX" with the RFC number of this document upon publication. Alvestrand Expires July 28, 2012 [Page 6] Internet-Draft MSID in SDP January 2012 6. Security Considerations An adversary with the ability to modify SDP descriptions has the ability to switch around tracks between media streams. This is a special case of the general security consideration that modification of SDP descriptions needs to be confined to entities trusted by the application. No attacks that are relevant to the browser's security have been identified that depend on this mechanism. 7. Acknowledgements This note is based on sketches from, among others, Justin Uberti and Cullen Jennings. 8. References 8.1. Normative References [I-D.lennox-mmusic-sdp-source-selection] Lennox, J. and H. Schulzrinne, "Mechanisms for Media Source Selection in the Session Description Protocol (SDP)", draft-lennox-mmusic-sdp-source-selection-02 (work in progress), October 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, June 2009. [W3C.WD-webrtc-20111027] Burnett, D., Bergkvist, A., Jennings, C., and A. Narayanan, "WebRTC 1.0: Real-time Communication Between Browsers", World Wide Web Consortium WD WD-webrtc- 20111027, October 2011, . Alvestrand Expires July 28, 2012 [Page 7] Internet-Draft MSID in SDP January 2012 8.2. Informative References [RFC4574] Levin, O. and G. Camarillo, "The Session Description Protocol (SDP) Label Attribute", RFC 4574, August 2006. Appendix A. Design considerations, open questions and and alternatives This appendix should be deleted before publication as an RFC. One suggested mechanism has been to use CNAME instead of a new attribute. This was abandoned because CNAME identifies a synchronization context; one can imagine both wanting to have tracks from the same synchronization context in multiple media streams and wanting to have tracks from multiple synchronization contexts within one media stream. Another suggestion has been to put the msid value within an attribute of RTCP SR (sender report) packets. This doesn't offer the ability to know that you have seen all the tracks currently configured for a media stream. There has been a suggestion that this mechanism could be used to mute tracks too. This is not done at the moment. The special value "default" and the reservation of "example*" seems bothersome; apart from that, it's a random string. It's uncertain whether "example" has any benefit. An alternative to the "default" media stream is to let each new media stream track without a msid attribute create its own media stream. Input on this question is sought. Discarding of incoming data when the SDP description isn't updated yet (section 3) may cause clipping. However, the same issue exists when crypto keys aren't available. Input sought. There's been a suggestion that acceptable SSRCs should be signalled in a response, giving a recipient the ability to say "no" to certain SSRCs. This is not supported in the current version of this document. Appendix B. Change log This appendix should be deleted before publication as an RFC. Alvestrand Expires July 28, 2012 [Page 8] Internet-Draft MSID in SDP January 2012 B.1. Changes from -00 to -01 Added track identifier. Added inclusion-by-reference of draft-lennox-mmusic-source-selection for track muting. Some rewording. Author's Address Harald Alvestrand Google Kungsbron 2 Stockholm, 11122 Sweden Email: harald@alvestrand.no Alvestrand Expires July 28, 2012 [Page 9]