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The spec permits a user or user agent to switch between tracks at any time: Unless and until a new set of constraints is provided, the user agent MAY change its choice of track at any point, provided that 1) the new choice does not violate given user permissions, and 2) it notifies the application code by raising an event. The event is undefined, but that's not the concern. As a user, I don't want this to happen. Therefore, as an application developer, I don't want this to happen. If this is possible (it sounds like a bad idea to me), there needs to be a way to prevent it from happening. sourceId constraints are not sufficient.
Proposed change (by Martin T): OLD: Unless and until a new set of constraints is provided, the user agent MAY change its choice of track at any point, provided that 1) the new choice does not violate given user permissions, and 2) it notifies the application code by raising an event. It may wish to do this, for example, if the user interface or network congestion changes. Note that no such change will have an effect on the presence or absence of each type of track, merely the contents. NEW: Once selected, the source for a MediaStreamTrack MUST NOT change.
Proposed change: https://github.com/fluffy/webrtc-w3c/commit/e1e11e5ffe641b409e9393f4d7e7c770ec406142
Fixed in Ed's draft December 25th 2013: http://dev.w3.org/2011/webrtc/editor/archives/20131225/getusermedia.html