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Re: Simulcast V1

From: Emil Ivov <emcho@jitsi.org>
Date: Sun, 6 Sep 2015 13:53:48 -0500
Message-ID: <CAPvvaaL6FjGEiG64k3OHaDhp5vga39q=SowNVTasPF5PsBOyYg@mail.gmail.com>
To: Adam Roach <adam@nostrum.com>
Cc: Adam Roach <abr@mozilla.com>, "public-webrtc@w3.org" <public-webrtc@w3.org>
On Sun, Sep 6, 2015 at 1:52 PM, Adam Roach <adam@nostrum.com> wrote:
> It's made it on the agenda to discuss this week; I would imagine we'll have
> a better picture by the end of Thursday.

Excellent! I sincerely hope we do!

Thanks,
Emil

> On 9/6/15 1:47 PM, Emil Ivov wrote:
>>
>> 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
>>
>>
>



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Received on Sunday, 6 September 2015 18:54:37 UTC

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