Offer without m-section allowed

[nils-ohlmeier]

I failed to find any text in webrtc-pc or JSEP which indicates that the createOffer() call should fail if no RtpTransceiver or data channel has been added to a PeerConnection.

Webrtc-pc contains the following:

5. If this inspection failed for any reason, reject p with a newly created OperationError and abort these steps.

But section 5.2.1. Initial Offers does not seem to contain any wording indicating to treat the absence of any RtpTransceiver and DataChannel as such an failure.

I think such a scenario should be treated as a failure, as I fail to see any value in generating an empty offer.

[juberti]

Good observation. Personally, I'm inclined to allow this, unless it's forbidden by 3264 - we would just generate the session-level values with no m= sections.

[juberti]

3264 indicates that this is OK:

 The offer will contain zero or more media streams (each media stream
 is described by an "m=" line and its associated attributes).  Zero
 media streams implies that the offerer wishes to communicate, but
 that the streams for the session will be added at a later time
 through a modified offer.

I think we should explicitly mention that this is allowed, but otherwise take no action.