One document matched: draft-alvestrand-rtcweb-msid-00.txt
Network Working Group H. Alvestrand
Internet-Draft Google
Intended status: Standards Track January 11, 2012
Expires: July 14, 2012
Signalling Media Stream ID in the Session Description Protocol
draft-alvestrand-rtcweb-msid-00
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 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
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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 14, 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Basic mechanism . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Handling of non-signalled tracks . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Design considerations, open questions and and
alternatives . . . . . . . . . . . . . . . . . . . . . 6
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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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 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 largely 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 SSRC,
which is 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.
2. Basic mechanism
Association of the messagestream 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
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.
(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".)
The msid uniquely identifies a media stream within the scope of an
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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. It is an
IMPLEMENTATION CHOICE <OPEN ISSUE> whether the recipient waits
until an RTCP BYE packet is received on the media stream, or
whether the recipient waits for a timeout with no more media
arriving, before signalling that the media stream track is 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.
3. Handling of non-signalled tracks
Pre-WebRTC entities will not send msid. This means that there will
be some incoming RTP packets with SSRCs that are not signalled.
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
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is "default".
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 <what interval?> indicates
that the stream is gone.
4. 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, rtcweb@ietf.org (or a successor
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.
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5. 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.
6. Acknowledgements
This note is based on sketches from, among others, Justin Uberti and
Cullen Jennings.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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,
<http://www.w3.org/TR/2011/WD-webrtc-20111027>.
7.2. 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
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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 shoudl be deleted before publication as an RFC.
This is the first version.
Author's Address
Harald Alvestrand
Google
Email: harald@alvestrand.no
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