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Re: Moving forward with SDP control

From: Harald Alvestrand <harald@alvestrand.no>
Date: Thu, 18 Jul 2013 16:48:54 +0200
Message-ID: <51E80056.6080604@alvestrand.no>
To: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
CC: "public-webrtc@w3.org" <public-webrtc@w3.org>
On 07/18/2013 02:21 PM, Stefan Håkansson LK wrote:
> I have two "Why+What's" and one "How":
> 1. Pause/resume sending of a MediaStreamTrack
> ---------------------------------------------
> Why: There are many scenarios where it would make sense to be able ask
> the browser to send a MediaStreamTrack (while keeping RTCP going). One
> example is in a multiparty service using simulcast. If the large scale
> version of your video is not shown for any of the other participants in
> the session, it makes sense to pause the sending (but be able to quickly
> resume if a talker switch means your video will be shown).
> Currently we have "disable" on MediaStreamTrack level, but that
> corresponds to sending blackness /silence.
> Note also that the very first use-case in [1] talks about the
> possibility to pause sending of audio and video.

Just to make certain: Do you mean "sender-initiated pause and resume" here?
We have had discussions in the past where people talked about both 
pause and receiver-initiated pause (sometimes called "on hold").

> What: This depends on how we signal pause/resume. *If* this is done by
> using sendonly/recvonly/inactive in the SDP this is what I think should
> be available in an API. (I still think RTCP signaling for this makes
> more sense.)

Another question is whether we need to signal it at all in version 1.0 - 
when I try it with Chrome, the bandwidth of a disabled audio/video call 
seems to be ~13 Kbits/sec (down from 300+ for a non-disabled call).

Blackness compresses well.

> 2. Setting BW for a MediaStreamTrack
> ------------------------------------
> Why: There are situations where a suitable start bit-rate can be known,
> or guessed. If this knowledge could be used the perceived end-user
> quality could be improved (since a higher quality is available from
> start since there is no need to start at a really low bit-rate).
> There are also situations where it could be beneficial if min and max
> bit-rates to be used can be influenced.
> * The app developer may know that below a certain bit-rate the quality
> is so bad that the browser could stop sending it, and likewise there may
> be knowledge about a bit-rate above which the quality does not improve.
> * There are situations when there is an agreement between the service
> provider and the connectivity provider about min and max bit-rates.
> What: Again, this depends on how much BW info is included in the SDP.
> But my understanding is that there should be some (since RTCP rates to
> be used are based on this info IIUC).

SDP has the b=AS: number, which can be specified at the m-line level or 
at the session level.
Now that we have one m-line per MediaStreamTrack, it seems logical that 
we can use that to signal the desired bandwidth.

However .... the sender can just send at the desired bandwidth, no need 
for signalling. The receiver ... will have to use signalling, either in 
SDP or outside SDP.

Which one were you thinking of?

> (3. Other stuff, but not signaled
> --------------------------------
> There are other parameters on how media (per MediaStreamTrack) is
> handled when to be sent over the network that do not need any signaling
> - such as priority, type of audio (to determine if AGC should be used or
> not), etc., mentioned in [1]. While not signaled, it could make sense to
> have the same API surface for this kind of settings.)
> How:
> ----
> I think that what I proposed a long time ago [2] is not completely
> broken. I mimics the DTMFSender pattern, it re-uses the constraint model
> that we already have in "Media Capture and Streams" [3], it offers one
> surface to control parameters related to sending a MediaStreamTrack over
> the network. It also does not break any existing applications since the
> use of it is not needed. Using certain constraints or methods would lead
> to a "negotiationneeded" event since the SDP is affected, others would
> not (since they will not need any signaling).
> But I'm sure we will see better proposals.
> Stefan
> [1]
> http://datatracker.ietf.org/doc/draft-ietf-rtcweb-use-cases-and-requirements/?include_text=1
> [2]
> http://lists.w3.org/Archives/Public/public-webrtc/2013Jan/att-0005/PrioAPI.pdf
> [3] http://dev.w3.org/2011/webrtc/editor/getusermedia.html
> On 7/16/13 2:19 PM, Harald Alvestrand wrote:
>> Hi all,
>> Recently there has been a lot of discussion (primarily in the
>> IETF/rtcweb space though, but this topic really belongs here) about the
>> desire to meet most use-cases without having to parse, modify or
>> construct SDP.
>> This was discussed already as part of the discussion on whether
>> PeerConnection and SDP should be maintained or not last year [1].
>> In the meantime, a number of API extensions have been created, notably
>> the constraints setting and modification interfaces, which seem likely
>> to be useful in achieving the goals people seek to achieve by SDP mangling.
>> However, this work has not progressed very quickly, or very comprehensively.
>> It may be time for a more structured approach.
>> We think it makes sense to divide the information needed into subcategories:
>> * Define the use cases for which SDP mangling is currently thought to be
>> required - the "why" of the SDP tweaking.
>> * Propose what parameters one should be able to control/influence
>> without having to do SDP mangling. A proposal should describe what the
>> current API specification produces, what the needed mangling is, and
>> what the desired effect of the mangling is - the "what" of the SDP tweaking.
>> * Propose suitable API surfaces to control/influence how media is
>> encoded and transported over the network - the "how" of the SDP
>> tweaking. We think that a requirement should be that working
>> applications do not break when adding this surface - if it is not used
>> things should work as today.
>> Someone may make a proposal encompassing all 3 pieces of information
>> (why, what and how) - or just some of the first ones - or a proposal for
>> a latter one that builds upon others' proposals (a "how" building on
>> someone else's "why" and "what"). But we would not want to consider a
>> "how" without a "what", or a "what" without a "why" - it just becomes
>> impossible to figure out whether the original requirement is satisfied
>> if we don't build all 3 layers of the proposals.
>> Does this sound like a way we could move forward?
>> Harald for the chairs.
>> [1] http://lists.w3.org/Archives/Public/public-webrtc/2012Sep/0098.html
Received on Thursday, 18 July 2013 14:49:25 UTC

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