< draft-alvestrand-mmusic-simulcast-ssrc-00.txt   draft-alvestrand-mmusic-simulcast-ssrc-01.txt >
RTCWEB Working Group H. Alvestrand RTCWEB Working Group H. Alvestrand
Internet-Draft Google Internet-Draft Google
Intended status: Informational April 11, 2019 Intended status: Informational July 06, 2019
Expires: October 13, 2019 Expires: January 7, 2020
Using SSRC with WebRTC Simulcast Using SSRC with WebRTC Simulcast
draft-alvestrand-mmusic-simulcast-ssrc-00 draft-alvestrand-mmusic-simulcast-ssrc-01
Abstract Abstract
This document describes a convention for sending "a=ssrc" attributes This document describes a convention for sending "a=ssrc" attributes
in SDP together with "a=simulcast" attributes. This allows SFUs that in SDP together with "a=simulcast" attributes. This allows SFUs that
need SSRC information to have this info easily accessible. need SSRC information to have this info easily accessible.
Given that it is intended as an interim measure, it does not aim for Given that it is intended as an interim measure, it does not aim for
being published as an RFC. being published as an RFC.
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Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. How to represent SSRC information . . . . . . . . . . . . . . 3 3. How to represent SSRC information . . . . . . . . . . . . . . 3
4. How to request that SSRC information be included . . . . . . 3 4. How to request that SSRC information be included . . . . . . 3
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Sunsetting the interim measure . . . . . . . . . . . . . . . 4 6. Sunsetting the interim measure . . . . . . . . . . . . . . . 4
7. Open questions . . . . . . . . . . . . . . . . . . . . . . . 4 7. Open questions . . . . . . . . . . . . . . . . . . . . . . . 4
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 5 10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 6 10.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6 Change log . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 A.1. Changes from version -00 to -01 . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
In developing the WebRTC specification, the IETF decided on a form of In developing the WebRTC specification, the IETF decided on a form of
simulcast that doesn't require fixed SSRC allocation, but rather used simulcast that doesn't require fixed SSRC allocation, but rather used
a combination of SDP tags (a=rid) [I-D.ietf-mmusic-rid] and RTP a combination of SDP tags (a=rid) [I-D.ietf-mmusic-rid] and RTP
header extensions (RTPStreamId) [I-D.ietf-avtext-rid] to describe the header extensions (RTPStreamId) [I-D.ietf-avtext-rid] to describe the
mapping between simulcast layers and RTP streams. mapping between simulcast layers and RTP streams.
The SDP format is described in [I-D.ietf-mmusic-sdp-simulcast]. The SDP format is described in [I-D.ietf-mmusic-sdp-simulcast].
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The syntax for representing SSRC information is taken from [RFC5576]. The syntax for representing SSRC information is taken from [RFC5576].
Each media section in the SDP contains one a=ssrc attribute per Each media section in the SDP contains one a=ssrc attribute per
simulcast stream, formatted as "a=ssrc:<ssrc> cname:<cname>". The simulcast stream, formatted as "a=ssrc:<ssrc> cname:<cname>". The
cname is the sender's single cname as defined in cname is the sender's single cname as defined in
[I-D.ietf-rtcweb-rtp-usage]; carrying some attribute is required by [I-D.ietf-rtcweb-rtp-usage]; carrying some attribute is required by
the "a=ssrc" syntax, and sending the cname is compatible with what the "a=ssrc" syntax, and sending the cname is compatible with what
has been done in other instances. has been done in other instances.
The list of SSRCs used is declared in an attribute with the FID (Flow The list of SSRCs used is declared in an attribute with the SIM
Identification) semantic, as defined in [RFC5888]. (Simulcast) semantic, which is registered by this memo.
The order of SSRCs in the a=ssrc-group attribute MUST match the order The order of SSRCs in the a=ssrc-group attribute MUST match the order
of the rid attributes in the corresponding streams in the "send" part of the rid attributes in the corresponding streams in the "send" part
of the a=simulcast: attribute. of the a=simulcast: attribute.
It is RECOMMENDED that both the a=rid: attributes and the a=ssrc: It is RECOMMENDED that both the a=rid: attributes and the a=ssrc:
attributes appear in the same order as the order in the a=simulcast attributes appear in the same order as the order in the a=simulcast
and a=ssrc-group attributes. and a=ssrc-group attributes.
It is RECOMMENDED not to use RTX with this configuration, since the It is RECOMMENDED not to use RTX with this configuration, since the
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generate offers containing SSRC, it can include the non-standard generate offers containing SSRC, it can include the non-standard
attribute "showSsrcInSimulcastOffer" in the RTCPeerConnection attribute "showSsrcInSimulcastOffer" in the RTCPeerConnection
constructor: constructor:
pc = new RTCPeerConnection({showSsrcInSimulcastOffer: true}) pc = new RTCPeerConnection({showSsrcInSimulcastOffer: true})
It is possible to verify that the request is understood by checking It is possible to verify that the request is understood by checking
for the presence of this attribute in the RTCPeerConnection for the presence of this attribute in the RTCPeerConnection
parameters: parameters:
if (showSsrcInSimulcastOffer in pc.getConfiguration) { if ('showSsrcInSimulcastOffer' in pc.getConfiguration()) {
// the request has been understood correctly // the request has been understood correctly
} }
Formally, this amounts to changing the API of a W3C specification, Formally, this amounts to changing the API of a W3C specification,
but adding nonstandard attributes to an initialization dictionary has but adding nonstandard attributes to an initialization dictionary has
been done before in other contexts; it seems like a relatively been done before in other contexts; it seems like a relatively
harmless thing to do, but should be reviewed in the W3C WEBRTC WG harmless thing to do, but should be reviewed in the W3C WEBRTC WG
anyway. anyway. Publishing a separate extension document in the W3C seems
like bureaucratic overkill.
5. Example 5. Example
m=video m=video
a=simulcast:send hi,mid,low a=simulcast:send hi,mid,low
a=rid:hi send a=rid:hi send
a=rid:mid send a=rid:mid send
a=rid:low send a=rid:low send
a=ssrc-group:FID 123 456 789 a=ssrc-group:SIM 123 456 789
a=ssrc:123 cname:foo a=ssrc:123 cname:foo
a=ssrc:456 cname:foo a=ssrc:456 cname:foo
a=ssrc:789 cname:foo a=ssrc:789 cname:foo
6. Sunsetting the interim measure 6. Sunsetting the interim measure
This specification is intended to give SFU authors time to convert to This specification is intended to give SFU authors time to convert to
the new mechanism. Since the invocation of this mechanism is the new mechanism. Since the invocation of this mechanism is
explicit, it is easy to check on what the usage is, and emit explicit, it is easy to check on what the usage is, and emit
deprecation warnings; those should probably be emitted from day 1. deprecation warnings; those should probably be emitted from day 1.
Once enough time has passed, this mechanism can be removed. Once enough time has passed, this mechanism can be removed.
7. Open questions 7. Open questions
NOTE IN DRAFT: The goal is to make this section empty. NOTE IN DRAFT: The goal is to make this section empty.
The SSRC-group "FID" was picked because it seemed to have the right
semantic, but it's not clear what it's been used for elsewhere.
Chrome has been using the group "SIM" without registering it; this
might be a better choice.
It's been suggested that it's better to replace the a=ssrc-group: It's been suggested that it's better to replace the a=ssrc-group:
line with new tag fields either on the a=ssrc: lines or the a=rid: line with new tag fields either on the a=ssrc: lines or the a=rid:
lines, thus giving explicit correlation. This, however, breaks the lines, thus giving explicit correlation. This, however, breaks the
standard format of those lines. Inventing new syntax for an interim standard format of those lines. Inventing new syntax for an interim
solution seems like a Bad Thing. solution seems like a Bad Thing.
People have asked whether and how this document should be published. People have asked whether and how this document should be published.
If it makes sense to publish it as a historical record, it might make If it makes sense to publish it as a historical record, it might make
sense to publish as an RFC; it does not make sense to the author to sense to publish as an RFC; it does not make sense to the author to
ask for standards track publication. At the moment, it claims that ask for standards track publication. At the moment, it claims that
publication is not sought. publication is not sought.
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The only added attack surface here is the ability to create The only added attack surface here is the ability to create
mismatches between the two lists of simulcast RTP streams, causing mismatches between the two lists of simulcast RTP streams, causing
different implementations to choose different streams to display. different implementations to choose different streams to display.
This is a special instance of the general rule that "people who can This is a special instance of the general rule that "people who can
modify your SDP can mess things up"; normal precautions when passing modify your SDP can mess things up"; normal precautions when passing
SDP around should be adequate. SDP around should be adequate.
9. IANA Considerations 9. IANA Considerations
This document has no IANA actions. This document registers the "SIM" value of "ssrc-group" in the
registry titled "Semantics for the "ssrc-group" SDP Attribute",
according to the procedures of [RFC5576] section 12.3. That document
specifies that registration requires a standards-track document.
If it were to be published, this section would have to request IANA o Semantics: Simulcast group
to register the "please-send-ssrc" attribute, and if it mints a new
group semantic for a=ssrc-group, this will also have to be o Token: SIM
registered.
o Reference: RFC XXXX
This document registers the "please-send-ssrc" attribute in the
registry titled "att-field (media level only)" according to the
procedures of [RFC4566] section 8.2.4. That document specifies that
the registry is of type "specification required".
o Contact: Harald Alvestrand, harald@alvestrand.no, +47 73 53 43 26
o Attribute name: please-send-ssrc
o Long-form attribute name: Please send SSRC
o Type of attribute: Media level
o Subject to the charset attribute: No
o Purpose: Indicates that the sender wishes to receive SSRC
information in the form specified in RFC XXXX
o Appropriate values: Not relevant
RFC Editor: Please replace "RFC XXXX" with the RFC number of this
document, if published, and delete this paragraph.
If the document succeeds in being transitory in nature, registration If the document succeeds in being transitory in nature, registration
may not be needed. may not be needed.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.ietf-avtext-rid] [I-D.ietf-avtext-rid]
Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
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[I-D.ietf-mmusic-sdp-simulcast] [I-D.ietf-mmusic-sdp-simulcast]
Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty, Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
"Using Simulcast in SDP and RTP Sessions", draft-ietf- "Using Simulcast in SDP and RTP Sessions", draft-ietf-
mmusic-sdp-simulcast-14 (work in progress), March 2019. mmusic-sdp-simulcast-14 (work in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<https://www.rfc-editor.org/info/rfc5576>. <https://www.rfc-editor.org/info/rfc5576>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010, DOI 10.17487/RFC5888, June 2010,
<https://www.rfc-editor.org/info/rfc5888>. <https://www.rfc-editor.org/info/rfc5888>.
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May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References 10.2. Informative References
[I-D.ietf-rtcweb-rtp-usage] [I-D.ietf-rtcweb-rtp-usage]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP", Communication (WebRTC): Media Transport and Use of RTP",
draft-ietf-rtcweb-rtp-usage-26 (work in progress), March draft-ietf-rtcweb-rtp-usage-26 (work in progress), March
2016. 2016.
Change log
A.1. Changes from version -00 to -01
o Corrected example JS code
o Changed group semantic name from FID to SIM
o Added IANA registration information for SIM and please-send-ssrc
Acknowledgments Acknowledgments
Many thanks to Amit Hilbuch, Adam Roach, Bernard Aboba, Philip Hancke Many thanks to Amit Hilbuch, Adam Roach, Bernard Aboba, Philipp
and many others who have given input to the design of this mechanism. Hancke and many others who have given input to the design of this
mechanism.
Author's Address Author's Address
Harald Alvestrand Harald Alvestrand
Google Google
Email: harald@alvestrand.no Email: harald@alvestrand.no
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