Contact emails
kr...@chromium.org, orp...@chromium.org
Design doc/Spec
Summary
Add a boolean property RTCConfiguration.offerExtmapAllowMixed to enable the attribute extmap-allow-mixed in WebRTC SDP offers.
The SDP attribute extmap-allow-mixed, as defined in RFC8285, will be included in the SDP offer if this property is set to true. The SDP attribute extmap-allow-mixed is supported from Chrome M-71, but due to backward compatibility problems with older Chrome releases it is not included in the SDP offer by default for now.
Motivation
New features in the future will require the use of 2-bytes RTP header extensions and the matching signaling in SDP offers, but we can’t create any offer right now including them as valid inputs would be rejected by our old SDP parser and prevent negotiation.
This property is added to help with the backward compatibility problems and make it possible to start using 2-bytes RTP header extensions for some specific applications.
This property would be purely transitional, first off by default, switched to on by default when enough clients have updated to a compatible Chrome release and later removed. It is similar to the RTCConfiguration.sdpSemantics transitional property added to help the transition to Unified-Plan SDP.
Risks
Interoperability and Compatibility
Edge: No signals
Firefox: Not needed
Safari: Not needed
Web / Framework developers: No signals
Safari is actually impacted by the same issue since we share the same SDP parser, but they don’t plan on exposing this property, even with an updated native WebRTC parser. Since they don’t currently use any 2-bytes RTP header extensions, the matching native property would be off by default, at least in the next version.
Most Web developers will probably not need to use this feature and rely on the default behavior. Other than early adopters of 2-bytes RTP header extensions, it will only be relevant to future clients with the feature enabled by default, trying to create an SDP offer for older versions of Chrome (M70 and below) or current versions of Safari.
Ergonomics
The underlying feature will be optionally used by the future RTP extensions for HDR rendering and WebRTC simulcast API.
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?
Yes
Is this feature fully tested by web-platform-tests?
No, but WPT tests will be written to ensure browsers can properly negotiate SDP offers using the underlying feature, and Chrome specific tests will be written to test this property.
Link to entry on the feature dashboard
https://www.chromestatus.com/features/6269234631933952
Requesting approval to ship?
Yes
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