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Re: ACTION-95: Constraints usage in the WebRTC spec

From: Harald Alvestrand <harald@alvestrand.no>
Date: Tue, 26 Nov 2013 16:33:21 +0100
Message-ID: <5294BF41.2050407@alvestrand.no>
To: public-webrtc@w3.org
Thanks for putting together this analysis!

On 11/26/2013 03:33 PM, Adam Bergkvist wrote:
> Constraints in the WebRTC specification.
>
> The spec currently uses constraints in three ways:
> 1. Constructor argument
> 2. Arugment to method modifying an object (e.g. updateIce())
> 3. Arugment to method performing an action on an object (e.g. 
> addStream())
>
> The first two usages seem to work OK. 1) is similar to constructing a 
> Constrainable object and then calling applyConstraints() and 2) is 
> pretty much applyConstraints() with a different name. 3) is trickier. 
> Even though the call is made on a Constrainable object, the 
> constraints are associated with an operation made on the Constrainable 
> object and not the object itself. I think we should avoid this kind of 
> usage.

To my mind, the operations in 3) should be grouped into two groups:

- Those that "really" apply the constraint to a newly created object 
that has, up to now, been hidden. In the case of addStream, I think we 
"found" the newly created object at our last meeting.

- Those that don't have a constrainable object (either hidden or overt) 
to apply the constraint to. In these cases, I question the value of this 
usage, and agree that options dictionaries are likely to be better.

>
> To evaluate each constraint, I've used the following two questions. If 
> the answer is yes to both of these questions then a settable property 
> should not be a constraint.
>
> 1. Is there a set of possible values that are the same for all browsers?
> 2. Will this settable property succeed if applied at the correct 
> state? I.e., it shouldn't fail because a value is out of the range of 
> what the browser is capable of.
>
> Even though the answer to question 2 may be "no", there might still be 
> a reason to use a regular settings dictionary.
>
> # Constraints
>
> ## IceTransports
> Used with PeerConnection() Constructor and updateIce()
>
> Controls which types of candidates the ICE agent is allowed to use.
>
> The possible values are "none", "relay" and "all". If a browser, for 
> some special reason, is configured to not support all these values, 
> that might be a reason to have it as a constraint.
>
> Proposal: Move to RTCConfiguration?
>
> ## OfferToReceiveVideo/Audio
> Used with createOffer()
>
> Used to express a preference for receiving a media type (even though 
> the media line is not added as a result of addStream()).
>
> Proposal: Move to CreateOfferOptions dictionary
>
> ## IceRestart
> Used with createOffer()
>
> Force generation of new ICE credentials.
>
> Proposal: Move to a CreateOfferOptions dictionary
>
> ## VoiceActivityDetection
> No usage documented.
>
> Enables VAD, if available.
>
> Proposal: Should be a constraint on the DooHickey object.
>
> ## RequestIdentity
> Used with PeerConnection() Constructor, createOffer() and createAnswer()
>
> Indicates whether an identity should be requested.
>
> Proposal: Move to RTCConfiguration and CreateOfferAnswerOptions 
> dictionary
>
> # addStream() Method
>
> I think we should remove the MediaConstraints argument here since it's 
> an operation of type 3) mentioned above. It's also a question about 
> what precision we can have on the constraints since a MediaStream can 
> contain several tracks and it's likely that the script would like to 
> address constraints to individual MediaStreamTrack objects. We're 
> probably better off with having constraints on the DooHickey object.

I think it would make sense to have addStream constrain the DooHickey 
(whatever we choose to call it), in the same way that getUserMedia() 
constrains the resulting MediaStreamTracks.

>
> /Adam
>
>
Received on Tuesday, 26 November 2013 15:33:55 UTC

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