indicates that a description MUST be treated as an
SDP answer, but not a final answer. A description used as an SDP
pranswer may be applied as a response to an SDP offer, or an update to
a previously sent SDP pranswer.
indicates that a description MUST be treated as an SDP
final answer, and the offer-answer exchange MUST be considered complete.
A description used as an SDP answer may be applied as a response to an
SDP offer or as an update to a previously sent SDP pranswer.
indicates that a description MUST be treated as
cancelling the current SDP negotiation and moving the SDP offer and
answer back to what it was in the previous stable state. Note the
local or remote SDP descriptions in the previous stable state could be
null if there has not yet been a successful offer-answer negotiation.