W3C home > Mailing lists > Public > public-webrtc@w3.org > September 2015

Re: Simulcast V1

From: Emil Ivov <emcho@jitsi.org>
Date: Sun, 6 Sep 2015 13:47:41 -0500
To: Adam Roach <abr@mozilla.com>, "public-webrtc@w3.org" <public-webrtc@w3.org>
Message-ID: <55EC8A4D.7090509@jitsi.org>
Hey all,

I was wondering if we reached any semblance of consensus here.

Regardless of the specific solution we choose, is it fair to say that we 
agree browser-to-server simulcast is important for 1.0 but we are still 
need to converge on an acceptable minimum hassle approach?

Emil

On 13.08.15 18:03, Adam Roach wrote:
> I've been involved in a number of recent conversations around simulcast
> for WebRTC, and a several implementors have indicated that it's an
> important feature for the initial release of WebRTC.
>
> As I understand the state of play:
>
>   * Chrome has a form of simulcasting implemented using undocumented SDP
>     mangling
>   * Firefox has no simulcasting implemented, but will soon
>   * The WebRTC 1.0 API has no simulcast-related controls whatsoever
>   * The IETF MMUSIC working group is nearing completion on a document
>     (draft-ietf-mmusic-sdp-simulcast-01) that allows negotiation of
>     simulcast in SDP
>
>
> I also understand and sympathize with the goal to stop adding any
> non-trivial modifications to the existing WebRTC spec, so that we can
> finally publish an initial version of the document.
>
> At the same time, the vast majority of the use cases that make sense for
> simulcast involve browsers talking to an MCU (or similar server),
> sending multiple encodings per track in the browser-to-MCU direction,
> but receiving only one encoding per track in the MCU-to-browser direction.
>
> This is interesting, because it means that we don't really require any
> controls that indicate the desire for a browser to /receive/ simulcast
> -- all we need is the ability to indicate a willingness to send it. At
> the same time, the MCU will know what resolutions (and other variations)
> it wants to receive, and can inform the browser of this information via SDP.
>
> Based on the foregoing, then, I propose that we instead add a trivial
> control to the existing RTCRtpSender objects. My strawman proposal would
> be something like:
>
>
> ------------------------------------------------------------------------
>
> partial interface RTCRtpSender {
>    attribute unsigned short maxSimulcastCount;
> };
>
> maxSimulcastCount of type unsigned short
>
>     This attribute controls the number of simulcast streams that will be
>     offered for the specific RTCRtpSender. The actual number of streams
>     used for this sender will depend on the answer that is passed to
>     setRemoteDescription.
>
> ------------------------------------------------------------------------
>
> Here's how that would work (I'm going to use simulcast with two
> encodings for my examples, but extrapolating use for more streams than
> that should be obvious).
>
> If the browser is the entity creating the offer, the script driving its
> side of stuff would (for any streams it wants to support simulcast) set:
>
>    rtpSender.maxSimulcastCount = 2;
>
> The SDP that it gets from a subsequent createOffer would include two
> simulcast PTs. Both would have identical imageattrs, indicating the
> range of encodings supported for simulcast. Only one would be supported
> for recv (this is just the resulting m-line):
>
>     m=video 49300 RTP/AVP 97 98
>     a=rtpmap:97 H264/90000
>     a=rtpmap:98 H264/90000
>     a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000
>     a=fmtp:98 profile-level-id=42c00b; max-fs=3600; max-mbps=108000
>     a=imageattr:97 send [x=[128:16:1280],y=[72:9:720]] recv [x=[128:16:1280],y=[72:9:720]]
>     a=imageattr:98 send [x=[128:16:1280],y=[72:9:720]]
>     a=simulcast send 97;98 recv 97
>
>
> The MCU would then communicate actual desired resolutions using imagattr
> "recv" in its answer:
>
>     m=video 49674 RTP/AVP 97 98
>     a=rtpmap:97 H264/90000
>     a=rtpmap:98 H264/90000
>     a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000
>     a=fmtp:98 profile-level-id=42c00b; max-fs=240; max-mbps=3600
>     a=imageattr:97 send [x=[320:16:1280],y=[180:9:720]] recv [x=1280,y=720]
>     a=imageattr:98 recv [x=320,y=180]
>     a=simulcast recv 97;98 send 97
>
>
> ------------------------------------------------------------------------
>
> Conversely, if the MCU were creating the offer, it would include the
> simulcast resolutions in the offer:
>
>     m=video 49674 RTP/AVP 97 98
>     a=rtpmap:97 H264/90000
>     a=rtpmap:98 H264/90000
>     a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000
>     a=fmtp:98 profile-level-id=42c00b; max-fs=240; max-mbps=3600
>     a=imageattr:97 send [x=[320:16:1280],y=[180:9:720]] recv [x=1280,y=720]
>     a=imageattr:98 recv [x=320,y=180]
>     a=simulcast recv 97;98 send 97
>
>
> When the receiving JavaScript calls setRemoteDescription, the
> maxSimulcastCount on the corresponding sender(s) would be automatically
> updated according to the number of encodings indicated for each video
> m-line. And, of course, the answer created by createAnswer would
> similarly contain simulcast information matching the number of desired
> encodings from the offer:
>
>     m=video 49300 RTP/AVP 97 98
>     a=rtpmap:97 H264/90000
>     a=rtpmap:98 H264/90000
>     a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000
>     a=fmtp:98 profile-level-id=42c00b; max-fs=3600; max-mbps=108000
>     a=imageattr:97 send [x=1280,y=720] recv [x=[320:16:1280],y=[180:9:720]]
>     a=imageattr:98 send [x=320,y=180]
>     a=simulcast send 97;98 recv 97
>
>
> ------------------------------------------------------------------------
>
> I think this satisfies a broad range of simulcast use cases with very
> little impact on the 1.0 API. I'll also note that this is intended to be
> a first-pass of simulcast implementation; if we find that other use
> cases arise that would benefit from more granular controls, we could
> easily add them in post-1.0 systems in a way that I believe could easily
> be backwards compatible with the scheme I describe above.
>
> --
> Adam Roach
> Principal Platform Engineer
> abr@mozilla.com
> +1 650 903 0800 x863

-- 
https://jitsi.org
Received on Sunday, 6 September 2015 18:48:11 UTC

This archive was generated by hypermail 2.3.1 : Monday, 23 October 2017 15:19:46 UTC