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RTCWEB Working Group                                       H. Alvestrand
Internet-Draft                                                    Google
Intended status: Informational                            April 11, 2019
Expires: October 13, 2019


                    Using SSRC with WebRTC Simulcast
               draft-alvestrand-mmusic-simulcast-ssrc-00

Abstract

   This document describes a convention for sending "a=ssrc" attributes
   in SDP together with "a=simulcast" attributes.  This allows SFUs that
   need SSRC information to have this info easily accessible.

   Given that it is intended as an interim measure, it does not aim for
   being published as an RFC.

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
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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 October 13, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  How to represent SSRC information . . . . . . . . . . . . . .   3
   4.  How to request that SSRC information be included  . . . . . .   3
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Sunsetting the interim measure  . . . . . . . . . . . . . . .   4
   7.  Open questions  . . . . . . . . . . . . . . . . . . . . . . .   4
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   5
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   In developing the WebRTC specification, the IETF decided on a form of
   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
   header extensions (RTPStreamId) [I-D.ietf-avtext-rid] to describe the
   mapping between simulcast layers and RTP streams.

   The SDP format is described in [I-D.ietf-mmusic-sdp-simulcast].

   This posed a problem for some SFUs, which required information on
   what SSRCs the incoming streams were going to appear on in order to
   be configured correctly.

   This document gives a convention for adding information about SSRCs
   to the SDP produced by conformant WebRTC implementations in order to
   make this information available.

   This document does not specify an Internet standard.  It is an
   interim measure, intended to be useful in the time between the
   introduction of RID-based simulcast in browsers and the full support
   of RID-based simulcast by SFUs.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP



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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  How to represent SSRC information

   The syntax for representing SSRC information is taken from [RFC5576].

   Each media section in the SDP contains one a=ssrc attribute per
   simulcast stream, formatted as "a=ssrc:<ssrc> cname:<cname>".  The
   cname is the sender's single cname as defined in
   [I-D.ietf-rtcweb-rtp-usage]; carrying some attribute is required by
   the "a=ssrc" syntax, and sending the cname is compatible with what
   has been done in other instances.

   The list of SSRCs used is declared in an attribute with the FID (Flow
   Identification) semantic, as defined in [RFC5888].

   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 a=simulcast: attribute.

   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
   and a=ssrc-group attributes.

   It is RECOMMENDED not to use RTX with this configuration, since the
   inclusion of the required declarations for associating RTX SSRCs with
   their main SSRCs would make the SDP unwieldy and hard to interpret
   correctly.

4.  How to request that SSRC information be included

   If an SFU wishes to request that a browser send SSRC information, it
   should send an offer containing the line "a=x-please-send-ssrcs",
   together with a line requesting simulcast:

   m=video
   a=simulcast:recv a,b,c
   a=please-send-ssrcs

   The SFU can detect whether the request has been honored by looking
   for a=ssrc attributes in the responding answer.

   If a Javascript application wishes to request that the browser
   generate offers containing SSRC, it can include the non-standard
   attribute "showSsrcInSimulcastOffer" in the RTCPeerConnection
   constructor:




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   pc = new RTCPeerConnection({showSsrcInSimulcastOffer: true})

   It is possible to verify that the request is understood by checking
   for the presence of this attribute in the RTCPeerConnection
   parameters:

   if (showSsrcInSimulcastOffer in pc.getConfiguration) {
      // the request has been understood correctly
   }

   Formally, this amounts to changing the API of a W3C specification,
   but adding nonstandard attributes to an initialization dictionary has
   been done before in other contexts; it seems like a relatively
   harmless thing to do, but should be reviewed in the W3C WEBRTC WG
   anyway.

5.  Example

   m=video
   a=simulcast:send hi,mid,low
   a=rid:hi send
   a=rid:mid send
   a=rid:low send
   a=ssrc-group:FID 123 456 789
   a=ssrc:123 cname:foo
   a=ssrc:456 cname:foo
   a=ssrc:789 cname:foo

6.  Sunsetting the interim measure

   This specification is intended to give SFU authors time to convert to
   the new mechanism.  Since the invocation of this mechanism is
   explicit, it is easy to check on what the usage is, and emit
   deprecation warnings; those should probably be emitted from day 1.

   Once enough time has passed, this mechanism can be removed.

7.  Open questions

   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:
   line with new tag fields either on the a=ssrc: lines or the a=rid:



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   lines, thus giving explicit correlation.  This, however, breaks the
   standard format of those lines.  Inventing new syntax for an interim
   solution seems like a Bad Thing.

   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
   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
   publication is not sought.

8.  Security Considerations

   This document describes two existing mechanisms: a=simulcast and
   a=ssrc-group.  Each of these is defined in an RFC with security
   considerations.

   The only added attack surface here is the ability to create
   mismatches between the two lists of simulcast RTP streams, causing
   different implementations to choose different streams to display.
   This is a special instance of the general rule that "people who can
   modify your SDP can mess things up"; normal precautions when passing
   SDP around should be adequate.

9.  IANA Considerations

   This document has no IANA actions.

   If it were to be published, this section would have to request IANA
   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
   registered.

   If the document succeeds in being transitory in nature, registration
   may not be needed.

10.  References

10.1.  Normative References

   [I-D.ietf-avtext-rid]
              Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
              Identifier Source Description (SDES)", draft-ietf-avtext-
              rid-09 (work in progress), October 2016.

   [I-D.ietf-mmusic-rid]
              Roach, A., "RTP Payload Format Restrictions", draft-ietf-
              mmusic-rid-15 (work in progress), May 2018.




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   [I-D.ietf-mmusic-sdp-simulcast]
              Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
              "Using Simulcast in SDP and RTP Sessions", draft-ietf-
              mmusic-sdp-simulcast-14 (work in progress), March 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <https://www.rfc-editor.org/info/rfc5576>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <https://www.rfc-editor.org/info/rfc5888>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [I-D.ietf-rtcweb-rtp-usage]
              Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
              Communication (WebRTC): Media Transport and Use of RTP",
              draft-ietf-rtcweb-rtp-usage-26 (work in progress), March
              2016.

Acknowledgments

   Many thanks to Amit Hilbuch, Adam Roach, Bernard Aboba, Philip Hancke
   and many others who have given input to the design of this mechanism.

Author's Address

   Harald Alvestrand
   Google

   Email: harald@alvestrand.no








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